journal of Internal Medicine 1992: 231 : 161-168
Calcium absorption and calcium bioavailability P. CHARLES From the Aarhus Bone and Mineral Research Group, University Department of Endocrinology and Metabolism, Aarhus Amts Universitetshospital, Aarhus C. Denmark
Abstract. Charles P (Aarhus Bone and Mineral Research Group, University Department of Endocrinology and Metabolism, Aarhus Amts Universitetshospital, Aarhus C, Denmark). Calcium absorption and calcium bioavailability. journal of Internal Medicine, 1992; 231: 161-168. Calcium is important for bone health. It has been customary to focus on dietary calcium intake, but of central importance for the body needs in the individual patient is the actual calcium absorption. This absorption consists of an active vitamin D-mediated component and a passive diffusional component. A number of different methods are available for the evaluation of calcium absorption. At present the calcium absorption tests using calcium isotopes (radioactive or stable) appear to be the most reproducible way of determining calcium absorption. The major nutrient sources for calcium are milk and milk products, whereas some of the green vegetables have a low bioavailability of calcium. When deciding whether an increased calcium intake is advisable, the following questions must be answered. What is the calcium absorptive status of the patient? How should the calcium supplement be dispensed? What calcium salt should be used? When should calcium supplements be taken? What is the compliance of the patient? When should the treatment be evaluated? The calcium supplement might be taken as milk (or milk products) or, in patients with lactose intolerance, as calcium supplements. Quite a number of calcium supplements are available on the market, and many of them are marketed without proper knowledge of the bioavailability of the actual preparation. For the benefit of our patients it is now reasonable to demand such investigations before marketing calcium supplements. Keywords : bone metabolism, calcium absorption, calcium bioavailability, calcium supplements, dietary calcium.
Introduction Calcium is central to any discussion of bone health, since bone constitutes the major reservoir of calcium, containing 99% of the total body supply. In discussing the potential role of nutrients in the prevention and treatment of osteoporosis, it must first be realized that nutrition includes the study of the use of nutrients by the body. Utilization of dietary nutrients as calcium, in turn, involves intestinal absorption, transport, metabolic transformations and interconversion, storage and excretion [l].These considerations are particularly relevant to the issue of calcium and osteoporosis, inasmuch as many factors, such as disease, other nutrients, and certain drugs, may interfere in a clinically significant manner with
the utilization of dietary calcium, even when the dietary content of the latter may be adequate. When the diet is marginal or even deficient in calcium, then the adverse effects of these interfering factors become even more evident. Osteoporosis constitutes a n important health problem in modern Western countries, and is an increasing problem in the Far East. The clinical condition has no single basis, but many different factors have been suggested to contribute to its development, such as low peak bone mass, early menopause, sedentary lifestyle, cigarette smoking, slender figure, and various dietary components, such as daily calcium intake. The mean dietary consumption of calcium varies throughout the world from 150-200 mg d-' in the Far East to 5-600 mg d-' in the United States to 1000-1200 mg d-' in the 161
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Nordic countries. The dissimilar intake of calcium in different countries with the same incidence of osteoporotic fractures points to the fact that osteoporosis is a multifactorial disease, where calcium is one of the important contributing factors. The aim of this paper is to describe the mechanisms responsible for calcium absorption, and the different methods used to assess and evaluate the latter. The bioavailability of calcium from food constituents and different calcium supplements will then be evaluated. Finally, a strategy for calcium supplementation, be it dietary or pharmaceutical, will be suggested.
Calcium absorption Physiology
Intestinal calcium absorption is divided into an active saturable component, which is mediated by vitamin D and involves the calcium-binding protein (Ca-BP) [2, 31, and a passive component, which may be simple diffusion or facilitated (carrier-mediated) diffusion. The entire bowel is probably capable of absorbing calcium. However, it is generally believed that only the small intestine participates in calcium absorption under normal circumstances. The site at which the majority of calcium is absorbed is dependent not only on the absorptive capacity, but also on the length of the intestinal segment, the transit time, the calcium bioavailability and the intraluminal calcium concentration [3]. The duodenum has the greatest active absorption capacity per unit length, but the majority of calcium is absorbed in the jejunum, due to its greater total length [4, 51. An increase in dietary calcium will be followed by a proportional increase in the amount of calcium absorbed by diffusion, whereas the active absorption is saturable. Therefore the fraction of dietary calcium that is absorbed will decrease with increasing dietary calcium [2, 61. The active intestinal calcium absorption is primarily regulated by 1.25-dihydroxyvitamin D (1,25(OH),D). Other hormones may enhance (parathyroid hormone, growth hormone, oestrogen and progestagen) or suppress (glucocorticoids, excess of thyroid hormones and possibly calcitonin) the absorption of calcium, mainly via interaction with the renal conversion of 2 5-hydroxyvitamin D (25-OHD) to 1,25(OH),D, or the effect of 1,25(OH),D on the intestine. Moreover, a number of cations, anions, proteins, carbohydrates, fats and drugs have been shown to influence intestinal
calcium absorption. With the exception of fat and some drugs, these compounds are of limited practical importance in daily life [ 3 , 41. Absorbed calcium and phosphorus participate in the regulation of further calcium absorption through the effects of S-calcium on PTH secretion and of S-phosphate and S-PTH on the renal production of 1,25(OH),D [7]. These mechanisms form the physiological basis for the well-known adaptation of intestinal calcium absorption to low or high calcium intake [8]. The time period required for intestinal adaptation to a challenge in calcium intake has been investigated by Malm [8] by reducing calcium intake from an average value of 9 4 0 mg d -' to around 450mgd-' in 24 healthy male prisoners. The intestinal calcium absorption was significantly lower during the 6rst 2 weeks on the low calcium intake than during the following 2 weeks, suggesting that intestinal adaptation to a lower calcium intake took at least 2 weeks. Calcium is continuously lost into the intestinal lumen due to the secretion of digestive juice. Part of this digestive-juice calcium is reabsorbed, and the rest is excreted with the faeces, the endogenous faecal calcium. Calcium absorption-particularly the active component-declines with age [6, 91. This decrease in calcium absorption is probably more pronounced in women than in men, and may be caused by dietary vitamin D deficiency and a fall in endogenous production of vitamin D due to reduced exposure to the sun in the elderly population. Moreover, impairment of renal function with age and lack of oestrogen in postmenopausal women will reduce renal 1 ,25(OH),D production. Finally, the enhanced bone turnover in women just after the menopause may inhibit intestinal calcium absorption through mobilization of bone mineral, resulting in reduced PTH secretion and decreased 1-hydroxylation of 25OHD.
Methods of calcium absorption Several different methods for the determination of calcium absorption in man are available. Proper use and interpretation of the results require a n understanding of the inherent problems and/or advantages of each method.
CALCIUM BIOAVAILABILITY
Calcium balance and kinetic analysis [ l o ]
During the calcium balance study, the amount of calcium in diet, faeces and urine is determined. Simultaneously, a radiocalcium tracer is injected intravenously. Using a mathematical model, the kinetic analysis of counting data in serum, urine and faeces enables calculation of true calcium absorption, endogenous faecal calcium (the amount of digestivejuice calcium excreted with the faeces), and net calcium absorption. Balance studies are the traditional means of assessing calcium absorption. Balance studies combined with tracer kinetic studies were, until the introduction of the ' 1-day double stable isotope test ' by Eastell et al. [ l l ] , the only available method for quantification of the true calcium absorption. The method is considered to be the ' gold standard ' for the evaluation of different calcium absorption tests. It must be performed under steady-state conditions, and it then provides a measure of the actual calcium absorption, i.e. the long-term effect of a diet or a calcium supplement on calcium absorption and secretion. The method is tedious and expensive, and is not useful for testing the bioavailability of calcium from different food elements or calcium supplements. The precision of this method is not as high as for the calcium isotope tests, and it seems more appropriate for investigating a group of subjects than for evaluation of calcium absorption in a single individual. Calcium isotope techniques f11-151
A range of different techniques involving the administration of one (oral) or two (oral plus intravenous) isotopes are available. The measurement of calcium isotope in blood, urine or forearm following oral administration of the isotope is compared with that obtained after intravenous administration of the same or a different calcium isotope. The practical procedure for the test differs between centres. The double-isotope tracer methods have been shown to be highly reproducible and precise [ l l ] , compared to combined calcium balance and tracer kinetic studies. The simplified single-isotope version, e.g. the method introduced by Heaney et al. [15], appears to be almost as precise as the double-isotope techniques. These tests are suitable for the determination of calcium bioavailability from ditrerent food elements and calcium supplements by intrinsic labelling. The
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precision is high, they are suitable for multiple tests, and the results can be made quantitative. With the introduction of stable isotopes there is no radiation exposure, and the tests can be used in pregnant women and children. The methods are expensive, and the use of stable isotopes requires facilities for mass spectrometry. The simplified single-isotope technique [151 is relatively cheap, and can be performed on ambulatory patients. Faecal recovery of orally administered calcium tracer
This method involves counting the unabsorbed calcium tracer in faeces after oral administration. Its validity is dependent on the assumption that there is complete isotope dilution (between calcium tracer and stable calcium), and that the physiological handling of radiocalcium is similar to that of stable calcium. The method is most suitable for testing calcium absorption from liquid preparations in which the calcium tracer can be readily dispensed, requiring only one small oral dose of calcium isotope, and obviating the need for intravenous administration. The technique cannot be used to measure calcium absorption from pre-prepared calcium salt tablets, unless the salts have been prelabelled. A critical point is the completeness of faecal collection, yielding a full recovery of the isotope. Faecal markers such as chromium chloride should be used to correct for the faecal recovery of radiocalcium. Moreover, a visible faecal marker should be used to demarcate the faeces of interest. Other obvious limitations to the method include the fact that prolonged intestinal transit time and delay in faecal collection may produce error, and the need to handle faecal specimens. Furthermore, the cost of calcium isotopes is considerable. lntestinal washout technique
The faecal content of the entire gastrointestinal tract is first removed by drinking a calcium-free isoosmolar solution. A calcium load is then provided, followed by a standard low-calcium meal 4 h later. Twelve hours after the calcium load, the gastrointestinal tract is again cleaned. From the amount of calcium remaining, the absorbed calcium is calculated in order to derive a measure of net calcium absorption. This method appears to be unphysiological, and may potentially allow participation of the distal bowel in calcium absorption. It may therefore not be 7-2
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relevant to the physiological situation, in which the small intestine may be the principal site of calcium absorption, as mentioned above.
Segmental intestinal perfusion [ 1 6 ] In this method a triple-lumen tube is introduced into the desired segment of the gastrointestinal tract. A solution with a defined calcium content and electrolysed composition is perfused into the proximal port. Samples are collected at two distal ports located a fixed distance apart. The difference in calcium concentration at the two distal ports reflects a measure of calcium absorption over the defined segment. The use of calcium tracer with the perfusate allows calculation of bidirectional calcium fluxes. This method represents one of the best available techniques for examination of the physiology of calcium transport in man. It has enabled elucidation of the kinetics of calcium transport, of the participation of different parts of the gastrointestinal tract in the absorptive process, and of the intestinal action of 1,25-dihydroxyvitamin-D [17]. The test is not adapted for testing the calcium bioavailability of different calcium preparations, as it requires perfusion of calcium in a soluble form from well-defined solutions. It therefore ignores physicochemical factors of solubility in the gastric juice and of reciprocation upon neutralization by pancreatic secretion.
Calciuric response to oral calcium load [18. 191 This test was initially developed for the identification of patients with renal stones presenting with intestinal hyperabsorption of calcium. The test (which might be termed the ‘oral calcium tolerance test’ or ‘poor man’s calcium absorption test ’) has, in recent years, been used to measure calcium absorption from food or calcium supplements. It might be performed in two ways. First, during the fasting state the increase in urinary calcium following an oral calcium load provides a n indirect measure of intestinal calcium absorption. Secondly, one might use the increase in 24-h urinary calcium excretion after a n oral calcium load in addition to the usual diet. The principal advantages of this technique are its simplicity and its ease of performance without the need for calcium isotopes. It is therefore cheap and suitable for multiple tests. The major disadvantage of the test is that it has a poor reproducibility and, as it is merely quantitative,
it may divide patients into high and low absorbers. The results of the test may be invalid if the calcium load is given with a metabolizable carbohydrate, which may exert a calciuric action, or with certain anions, which are excreted in the urine and cause calcium trapping. Treatment with drugs that might alter the renal handling of calcium (thiazides, furosemide, and phosphate) may invalidate the results. The results of the test may be improved by extending the calcium load over, for instance, 3 d with concomitant collection of urine.
Calcium bioavailability from diet and calcium supplements Diet Discussion of calcium metabolism almost always concerns the dietary calcium intake. One should also be aware of the many factors that regulate calcium absorption, excretion and utilization. Instead of focusing on the exact number of milligrams of calcium that should be included in the diet, one should rather centre upon making sufficient efforts to maximize utilization of that calcium. Figure 1 illustrates the metabolism of calcium in 1 7 normal Danes [lo], showing the major excretory routes via urine, faeces and skin. It indicates that substantial quantities of calcium are excreted as what we term obligatory calcium losses each day. There are many points in this sequence at which other nutrients, drugs and diseases may have important effects on utilization of dietary calcium. A number of factors influence calcium absorption. As described previously, vitamin D is central to the regulation of calcium absorption. Vitamin-D levels decrease with age, due to both a decline in kidney function with a decrease in production of 1,25dihydroxyvitamin-D, and a decline in synthesis of vitamin D in the skin due to reduced exposure to the sun. In the Nordic countries it might therefore be advisable for the elderly population to take a vitaminD supplement with the diet. For many years lactose has been thought to promote the absorption of calcium. This has recently been questioned by Heaney (personal communication). Several other factors enhance or reduce the absorption of calcium as mentioned previously. Table 1 shows the major dietary sources of calcium. It can be seen that milk and milk products are the main sources of calcium in the western
CALCIUM BIO AV AIL ABILITY
Controls
+
D:31 (4)
I
Bone
e: 3.4 (0.2) B: -2.2 (0.6)
b: 5 (1)
F: 26 (3)
Table I . Major dietary sources of calcium (approximate values) Nutrient
Ca (mmol d-')
m: 4.9 (0.6)
a: 8 (1)
Fig. 1. Calcium metabolism in 1 7 normal Danes. Mean values are expressed as mmol Ca d-I. D = dietary calcium : a = true absorbed calcium; e = endogenous faecal calcium: b = net absorbed calcium: F = faecal calcium : U = urinary excretion of calcium: d = dermal calcium loss: m = bone mineralization rate : r = bone resorption rate: B = calcium balance (copyright Peder Charles, 1987: used with permission).
1 65
U: 5.5 (0.6)
Table 2. Bioavailability of selected food calcium sources'
Ca I cium mg 100 g-I
Primary food sources of calcium Milk and milk products Milk Whole Skimmed Yoghurt Cheese Ice cream
111 124 134 600-1000 120
Secondary food sources of calcium Beans Nuts Almonds Salmon, canned with bones Sardines, canned with bones Broccoli, cooked Spinach, cooked Rhubarb. cooked Kale. cooked Parsley
65 75 250 200 300 130 160 300 200 100
countries. The bioavailability estimated by fractional absorption of some of the major food elements is shown in Table 2 [20]. While certain vegetables have a significant calcium content, the presence of oxalate significantly reduces the bioavailability of calcium in these food items. Oxalate is also an important component of many kidney stones and, when considering the possible hazards of high doses of calcium,
d: 1.6 (0.1)
Source
Fractional absorption with meal
Spinach Bone meal/substance Milk (2%) Kale
0.05 0.32 0.32 0.41
*Modified from Heaney et nl. [20].
it should be borne in mind that, it may be even more important to restrict the intake of food containing high amounts of oxalate. Vitamin C, or ascorbic acid, is degraded directly to oxalate, and when consumed in large amounts results in the elevation of urinary oxalate excretion [21]. A diet that is high in fat [3] will decrease the bioavailability of dietary calcium. At a time when we are concerned that a high-fat diet may be a significant factor in the development of heart disease, stroke and certain forms of cancer, attention should also be paid to its possible role in producing calcium deficiency and osteoporosis. Dietary fibre [22], when consumed in large amounts, interferes with calcium bioavailability. Zinc will also interfere with calcium absorption [ 2 11. Megadoses of vitamin A [23] may produce disturbances in calcium metabolism, the exact mechanisms of which require further investigation. Caffeine in-
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Table 3. Some calcium supplement sources
Drug
Calcium per tablet (mg)
Tablets d-'
Calcium phosphate Calcium lactate Calcium gluconate Calcium Sandozm Calcium carbonate
116 65 45 500 500
8 16 24 2 2
~~
*Required to provide c. 1 g of elemental calcium d-'
creases the urinary excretion of calcium, and alcohol [24] affects calcium and vitamin-D metabolism, probably at several sites. Furthermore, there is some evidence to suggest that under certain circumstances iron may interfere with calcium absorption [2 51. Therefore the consumption of a diet that is highly unbalanced, and particularly one that includes irrational megadose supplements of vitamins and minerals, may have important adverse effects on bone status, and in particular on bone health in general [2 11. Moreover, a diet that is high in protein [26] tends to increase the urinary excretion of calcium. This effect may be less marked with animal protein than with vegetable protein, possibly because the former often has a high phosphate content. Phosphate [2 71 tends to decrease urinary calcium excretion and increases faecal calcium excretion. However, the net effect of a diet high in phosphate is probably not to influence the calcium balance significantly. Increasing dietary sodium [28] will result in an increase in urinary calcium. Dietary sodium might even have a greater effect on urinary calcium loss than calcium itself.
Calcium supplements A review of the literature reveals the use of a wide variety of calcium supplements in osteoporosis, without uniformity in dosage or in the frequency of administration. This practice has prevailed despite apparent differences in bioavailability among calcium salts and the well-known dependence of calcium absorption on the dose of calcium supplement. A number of different calcium salts are listed in Table 3. To supply 1 g of elemental calcium, one must administer 1 6 tablets of calcium lactate per day, but only 2 tablets of calcium Sandoz (calcium carbonate plus calcium lactogluconate). Many different calcium
salts are available on the over-the-counter market, most of which have never been tested for calcium absorbability, and which might therefore not be absorbed merely due to the formulation of the tablet.
Strategy for calcium supplementation Having made the decision that calcium supplementation is desirable for an individual, a number of factors must be considered. What is the calcium absorptive status of the subject? In a group of postmenopausal women ingesting 800 mg calcium d-' investigated by Heaney et al. [6]. a substantial inter-individual variation in fractional calcium absorption, ranging from 0.13-0.43, was found, whereas the intra-individual variation was found to be only 0.055 [15]. Moreover, the absorption capacity of calcium is consistent for the patient with regard to load, substance and interval. For example, as shown by Heaney et al. [29] in a group of postmenopausal women, highly significant correlations of absorption fraction within individuals were found, when the same calcium substance, but at different load levels, was tested three times over a n 8-week period. This was even observed over intervals as long as 5 years. Due to this wide range of absorption efficiencies, fractional calcium absorption should be assessed in patients prior to calcium supplementation. How should the calcium supplement be dispensed? It has to be decided whether the patient should have more calcium with the diet or as a calcium salt. A change in the diet is advisable for patients who also require protein and calories, and in this instance both milk and milk products are advisable. In obese subjects skimmed milk or a calcium supplement might be used, and in patients who are allergic to milk, a calcium supplement would be preferable. What calcium salt should be used? In deciding what calcium product should be prescribed, it is necessary for the physician to be familiar with that particular calcium drug. First, there is a limited role of solubility in vitro [20]. which means that even though a calcium supplement appears to be quite insoluble in vitro, it might be well absorbed in vivo. Only calcium salts that have been tested with regard to their bioavailability should be used, and the drug should be obtained from the same supplier. Calcium density shows great variation between different calcium salts, a factor that must be taken into consideration (Table 3).
CALCIUM B I 0 A V A IL A B ILI T Y
When should calcium supplements be taken? To improve calcium absorption, the calcium supplement should be given in divided doses, as this increases the fractional calcium absorption [30]. Two grams of calcium in the morning might give a fractional calcium absorption of 14%,while the same calcium salt given in four doses would increase the fractional calcium absorption to 29%. Patients do not always wish to take tablets four times a day, so even if this might increase fractional calcium absorption, it might be better to give one dose of calcium supplement with breakfast and another at bedtime. This will increase the compliance of the patient. The bioavailability of a calcium supplement is increased when it is taken with a meal [31], whereas a calcium supplement taken at bedtime on an empty stomach will decrease the absorptive efficiency. The rationale for taking a calcium supplement at bedtime is that PTH-stimulated bone resorption is decreased during the night [32]. How is the compliance of the patient controlled? Heaney [33] has recently shown that the calcium density of a random faecal sample can be used to control patient compliance. When should the treatment be evaluated? In order to evaluate the effect of the calcium supplement, bone mineral content might be measured before the start and after each year during treatment.
Conclusions Our knowledge of calcium absorption and bioavailability has greatly improved in recent years. As a result of the increasing concern about osteoporosis, the use of calcium supplements has greatly increased, particularly in the United States. Many of these calcium supplements are marketed without proper knowledge of the bioavailability of the actual preparation. For the benefit of our patients it is now reasonable to demand such investigations before marketing these calcium supplements.
References 1 Rivlin RS. Summary and concluding statement: evidence relating selected vitamins and minerals to health and disease in the elderly population in the United States. Am J Clin Nutr 1 9 8 2 ; 36: 1083-6. 2 Wilkinson R. Absorption of calcium, phosphorus and magnesium. In: Nordin BEC. ed. Calcium. Phosphate and Magnesium Metabolism. Edinburgh, London. New York: Churchill Livingstone, 1976: 36-1 12. 3 Kenny AD. Intestinal Calcium Absorption and its Regulation. Boca Raton. Florida: CRC Press, Inc. 1 9 8 2 : 1-152.
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4 Birge SJ, Peck WA. Berman M. Whedon GI). Study of calcium absorption in man: a kinetic analysis and physiologic model. J Clin Jnvest 1 9 6 9 : 4 8 : 1705-13. 5 Wensel RH. Rich C, Brown AC, Volwiler W. Absorption of calcium measured by intubation and perfusion of the intact human small intestine. J Clin Invest 7969: 4 8 : 3768-75. 6 Heaney RP. Recker RR, Stegman MR. Moy AJ. Calcium absorption in women: relationships to calcium intake, estrogen status, and age. J Bone Miner Res 1 9 8 9 ; 4:,469-75. 7 Tanaka Y. DeLuca HF. The control of 25-hydroxyvitamin D metabolism by inorganic phosphorus. Arch Biochem Biophys 1 9 7 3 : 154: 566-74. 8 Malm OJ. Calcium requirement and adaptation in adult men. Scand J Lab Clin Invest 1958: 10 (Suppl. 36): 1-152. 9 Avioli LV. McDonald JE. Lee SW. The influence of age on the intestinal absorption of "Ca in women and its relation to "Ca absorption in postmenopausal osteoporosis. J Clin Invest 1965 : 44: 1960-7. 10 Charles P. Metabolic bone disease evaluated by a combined calcium balance and tracer kinetic study. Dan Med Bull 1 9 8 9 ; 36: 463-78. 11 Eastell R. Vieira NE. Yergey. Riggs BL. One-day test using stable isotopes to measure true fractional calcium absorption. J Bone Miner Res 1989: 4: 463-8. 12 Bronner F. Experimental studies of calclum absorption in man. Bib1 Nutr Dieta 1962; 3: 22-31. 13 DeGrazia ]A. Rich C. Studies of intestinal absorption of 45calcium in man. Metabolism 1 9 6 4 ; 13: 650-60. 1 4 Birge SJ,Peck WA. Berman M, Whedon GD. Studies of calcium absorption in man : a kinetic analysis and physiologic model. J Clin Invest 1969: 4 8 : 1705-13. 15 Heaney RP. Recker RR. Estimation of true calcium absorption. Ann Intern Med 1985: 103: 516-21. 16 Wensel RH. Rich C. Brown AC. Volwiler W. Absorption of calcium measured by intubation and perfusion of the intact human small intestine. I Clin Invest 1969; 4 8 : 1768-75. 17 Pak CYC. Avioli LV. Editorial. Factors affecting absorbability of calcium from calcium salts and food. Calcif Tissue Int 1 9 8 8 ; 4 3 : 55-60. 1 8 Broadus AE. Dominguez M. Bartter FC. Pathophysiological studies in idiopathic hypercalciuria : use of oral calcium tolerance test to characterize distinctive hypercalciuric subgroups. J Clin Endocrinol Metab 1978: 4 7 : 751-6. 19 Nicar MJ, Pak CYC. Calcium bionvailability from calcium carbonate and calcium citrate. J Clin Endocrinol Metub 1985 ; 61: 391-3. 2 0 Heaney RP. Recker RR. Weaver CM. Absorbability of calcium sources: the limited role of solubility. Calcif Tissue Int 1 9 9 0 : 4 6 : 300-4. 21 Rivlin RS. Women's Health: Osteoporosis: Nutrition. Public Health Reports Supplementary 1986 ; July-August : 1 3 1-5. 2 2 Cummings JH, Hill MJ, Jivrai T. Houston H. Branch WJ, Jenkins DJA. The effect of meat protein and dietary fiber on colonic function and metabolism. I. Changes in bowel habit, bile acid excretion and calcium absorption. Am I Clin Nutr 1979; 32: 2086-93. 23 Navia JM. Harris SS. Vitamin A influences on calcium metabolism and calcification. Ann NY Acad Sci 1980: 3 5 5 : 45-57. 2 4 Vodoz JF. Luisiez M. Donath A, Courvoisier B. Jarcia B. Diminution de I'absorption intcstinale de 4 7 calcium dans I'alcoolisme chronique. Schweiz Med Wocherrschr 1977; 107: 1525-29.
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25 Kelsay JL. Behall KM, Prather ES. Effect of fiber from fruits and vegetables on metabolic responses of human subjects. 11. Calcium, magnesium, iron and silicon balances. Am ] Clin Nutr 1979, 32: 1876-80. 2 6 Schuette SA. Hegsted M. Zemel MB. Linkswiler HM. Renal acid, urinary cyclic A M P , and hydroxyproline excretion as affected by levels of protein, sulphur amino acid, and phosphorus intake. ] Nutr 1 9 8 1 : 1 1 1 : 2106-16. 2 7 Heaney RP. Recker RR. Effects of nitrogen, phosphorus, and caffeine on calcium balance in women. Lab Clin Med 1 9 8 2 : 99: 46-55. 28 Nordin BEC. Polley KJ. Metabolic consequences of the menopause. Calcif Tissue Int 1 9 8 7 : 41 : S-1-S-59. 29 Heaney RP. Weaver CM. Fitzsimmons ML. Recker RR. Calcium absorptive consistency. ] Bone Miner Res 1990: 11 : 1139-42. 3 0 Heaney RP. Weaver CM. Fitzsimmons ML. The influence of
calcium load on absorption fraction. ] Bone Miner Res 1 9 9 0 : 11: 1135-8. 31 Heaney RP. Smith KT. Recker RR. Hinders SM. Meal effects on calcium absorption. Am ] Clin Nutr 1 9 8 9 : 49: 372-6. 32 Horowitz M. Need AG, Philcox JC, Nordin BEC. Biochemical effects of a calcium supplement in osteoporotic postmenopausal women with normal absorption and malabsorption of calcium. Miner Electrolyte Metab 1 9 8 7 : 13: 112-6. 3 3 Heaney RP. Fecal calcium density: a measure of calcium compliance. ] Bone Miner Res 1 9 9 1 : 6 (5): 469-71. Received 1 9 July 1991. accepted 2 October 7991. Correspondence ;Peder Charles. MD PhD, Medicinsk endokrinologisk afdeling. Aarhus Amts Universitetshospital. Tage Hansengade. DK-8000 Aarhus C, Denmark.
Calcium absorption and calcium bioavailability P. CHARLES From the Aarhus Bone and Mineral Research Group, University Department of Endocrinology and Metabolism, Aarhus Amts Universitetshospital, Aarhus C. Denmark
Abstract. Charles P (Aarhus Bone and Mineral Research Group, University Department of Endocrinology and Metabolism, Aarhus Amts Universitetshospital, Aarhus C, Denmark). Calcium absorption and calcium bioavailability. journal of Internal Medicine, 1992; 231: 161-168. Calcium is important for bone health. It has been customary to focus on dietary calcium intake, but of central importance for the body needs in the individual patient is the actual calcium absorption. This absorption consists of an active vitamin D-mediated component and a passive diffusional component. A number of different methods are available for the evaluation of calcium absorption. At present the calcium absorption tests using calcium isotopes (radioactive or stable) appear to be the most reproducible way of determining calcium absorption. The major nutrient sources for calcium are milk and milk products, whereas some of the green vegetables have a low bioavailability of calcium. When deciding whether an increased calcium intake is advisable, the following questions must be answered. What is the calcium absorptive status of the patient? How should the calcium supplement be dispensed? What calcium salt should be used? When should calcium supplements be taken? What is the compliance of the patient? When should the treatment be evaluated? The calcium supplement might be taken as milk (or milk products) or, in patients with lactose intolerance, as calcium supplements. Quite a number of calcium supplements are available on the market, and many of them are marketed without proper knowledge of the bioavailability of the actual preparation. For the benefit of our patients it is now reasonable to demand such investigations before marketing calcium supplements. Keywords : bone metabolism, calcium absorption, calcium bioavailability, calcium supplements, dietary calcium.
Introduction Calcium is central to any discussion of bone health, since bone constitutes the major reservoir of calcium, containing 99% of the total body supply. In discussing the potential role of nutrients in the prevention and treatment of osteoporosis, it must first be realized that nutrition includes the study of the use of nutrients by the body. Utilization of dietary nutrients as calcium, in turn, involves intestinal absorption, transport, metabolic transformations and interconversion, storage and excretion [l].These considerations are particularly relevant to the issue of calcium and osteoporosis, inasmuch as many factors, such as disease, other nutrients, and certain drugs, may interfere in a clinically significant manner with
the utilization of dietary calcium, even when the dietary content of the latter may be adequate. When the diet is marginal or even deficient in calcium, then the adverse effects of these interfering factors become even more evident. Osteoporosis constitutes a n important health problem in modern Western countries, and is an increasing problem in the Far East. The clinical condition has no single basis, but many different factors have been suggested to contribute to its development, such as low peak bone mass, early menopause, sedentary lifestyle, cigarette smoking, slender figure, and various dietary components, such as daily calcium intake. The mean dietary consumption of calcium varies throughout the world from 150-200 mg d-' in the Far East to 5-600 mg d-' in the United States to 1000-1200 mg d-' in the 161
7
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162
P. CHARLES
Nordic countries. The dissimilar intake of calcium in different countries with the same incidence of osteoporotic fractures points to the fact that osteoporosis is a multifactorial disease, where calcium is one of the important contributing factors. The aim of this paper is to describe the mechanisms responsible for calcium absorption, and the different methods used to assess and evaluate the latter. The bioavailability of calcium from food constituents and different calcium supplements will then be evaluated. Finally, a strategy for calcium supplementation, be it dietary or pharmaceutical, will be suggested.
Calcium absorption Physiology
Intestinal calcium absorption is divided into an active saturable component, which is mediated by vitamin D and involves the calcium-binding protein (Ca-BP) [2, 31, and a passive component, which may be simple diffusion or facilitated (carrier-mediated) diffusion. The entire bowel is probably capable of absorbing calcium. However, it is generally believed that only the small intestine participates in calcium absorption under normal circumstances. The site at which the majority of calcium is absorbed is dependent not only on the absorptive capacity, but also on the length of the intestinal segment, the transit time, the calcium bioavailability and the intraluminal calcium concentration [3]. The duodenum has the greatest active absorption capacity per unit length, but the majority of calcium is absorbed in the jejunum, due to its greater total length [4, 51. An increase in dietary calcium will be followed by a proportional increase in the amount of calcium absorbed by diffusion, whereas the active absorption is saturable. Therefore the fraction of dietary calcium that is absorbed will decrease with increasing dietary calcium [2, 61. The active intestinal calcium absorption is primarily regulated by 1.25-dihydroxyvitamin D (1,25(OH),D). Other hormones may enhance (parathyroid hormone, growth hormone, oestrogen and progestagen) or suppress (glucocorticoids, excess of thyroid hormones and possibly calcitonin) the absorption of calcium, mainly via interaction with the renal conversion of 2 5-hydroxyvitamin D (25-OHD) to 1,25(OH),D, or the effect of 1,25(OH),D on the intestine. Moreover, a number of cations, anions, proteins, carbohydrates, fats and drugs have been shown to influence intestinal
calcium absorption. With the exception of fat and some drugs, these compounds are of limited practical importance in daily life [ 3 , 41. Absorbed calcium and phosphorus participate in the regulation of further calcium absorption through the effects of S-calcium on PTH secretion and of S-phosphate and S-PTH on the renal production of 1,25(OH),D [7]. These mechanisms form the physiological basis for the well-known adaptation of intestinal calcium absorption to low or high calcium intake [8]. The time period required for intestinal adaptation to a challenge in calcium intake has been investigated by Malm [8] by reducing calcium intake from an average value of 9 4 0 mg d -' to around 450mgd-' in 24 healthy male prisoners. The intestinal calcium absorption was significantly lower during the 6rst 2 weeks on the low calcium intake than during the following 2 weeks, suggesting that intestinal adaptation to a lower calcium intake took at least 2 weeks. Calcium is continuously lost into the intestinal lumen due to the secretion of digestive juice. Part of this digestive-juice calcium is reabsorbed, and the rest is excreted with the faeces, the endogenous faecal calcium. Calcium absorption-particularly the active component-declines with age [6, 91. This decrease in calcium absorption is probably more pronounced in women than in men, and may be caused by dietary vitamin D deficiency and a fall in endogenous production of vitamin D due to reduced exposure to the sun in the elderly population. Moreover, impairment of renal function with age and lack of oestrogen in postmenopausal women will reduce renal 1 ,25(OH),D production. Finally, the enhanced bone turnover in women just after the menopause may inhibit intestinal calcium absorption through mobilization of bone mineral, resulting in reduced PTH secretion and decreased 1-hydroxylation of 25OHD.
Methods of calcium absorption Several different methods for the determination of calcium absorption in man are available. Proper use and interpretation of the results require a n understanding of the inherent problems and/or advantages of each method.
CALCIUM BIOAVAILABILITY
Calcium balance and kinetic analysis [ l o ]
During the calcium balance study, the amount of calcium in diet, faeces and urine is determined. Simultaneously, a radiocalcium tracer is injected intravenously. Using a mathematical model, the kinetic analysis of counting data in serum, urine and faeces enables calculation of true calcium absorption, endogenous faecal calcium (the amount of digestivejuice calcium excreted with the faeces), and net calcium absorption. Balance studies are the traditional means of assessing calcium absorption. Balance studies combined with tracer kinetic studies were, until the introduction of the ' 1-day double stable isotope test ' by Eastell et al. [ l l ] , the only available method for quantification of the true calcium absorption. The method is considered to be the ' gold standard ' for the evaluation of different calcium absorption tests. It must be performed under steady-state conditions, and it then provides a measure of the actual calcium absorption, i.e. the long-term effect of a diet or a calcium supplement on calcium absorption and secretion. The method is tedious and expensive, and is not useful for testing the bioavailability of calcium from different food elements or calcium supplements. The precision of this method is not as high as for the calcium isotope tests, and it seems more appropriate for investigating a group of subjects than for evaluation of calcium absorption in a single individual. Calcium isotope techniques f11-151
A range of different techniques involving the administration of one (oral) or two (oral plus intravenous) isotopes are available. The measurement of calcium isotope in blood, urine or forearm following oral administration of the isotope is compared with that obtained after intravenous administration of the same or a different calcium isotope. The practical procedure for the test differs between centres. The double-isotope tracer methods have been shown to be highly reproducible and precise [ l l ] , compared to combined calcium balance and tracer kinetic studies. The simplified single-isotope version, e.g. the method introduced by Heaney et al. [15], appears to be almost as precise as the double-isotope techniques. These tests are suitable for the determination of calcium bioavailability from ditrerent food elements and calcium supplements by intrinsic labelling. The
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precision is high, they are suitable for multiple tests, and the results can be made quantitative. With the introduction of stable isotopes there is no radiation exposure, and the tests can be used in pregnant women and children. The methods are expensive, and the use of stable isotopes requires facilities for mass spectrometry. The simplified single-isotope technique [151 is relatively cheap, and can be performed on ambulatory patients. Faecal recovery of orally administered calcium tracer
This method involves counting the unabsorbed calcium tracer in faeces after oral administration. Its validity is dependent on the assumption that there is complete isotope dilution (between calcium tracer and stable calcium), and that the physiological handling of radiocalcium is similar to that of stable calcium. The method is most suitable for testing calcium absorption from liquid preparations in which the calcium tracer can be readily dispensed, requiring only one small oral dose of calcium isotope, and obviating the need for intravenous administration. The technique cannot be used to measure calcium absorption from pre-prepared calcium salt tablets, unless the salts have been prelabelled. A critical point is the completeness of faecal collection, yielding a full recovery of the isotope. Faecal markers such as chromium chloride should be used to correct for the faecal recovery of radiocalcium. Moreover, a visible faecal marker should be used to demarcate the faeces of interest. Other obvious limitations to the method include the fact that prolonged intestinal transit time and delay in faecal collection may produce error, and the need to handle faecal specimens. Furthermore, the cost of calcium isotopes is considerable. lntestinal washout technique
The faecal content of the entire gastrointestinal tract is first removed by drinking a calcium-free isoosmolar solution. A calcium load is then provided, followed by a standard low-calcium meal 4 h later. Twelve hours after the calcium load, the gastrointestinal tract is again cleaned. From the amount of calcium remaining, the absorbed calcium is calculated in order to derive a measure of net calcium absorption. This method appears to be unphysiological, and may potentially allow participation of the distal bowel in calcium absorption. It may therefore not be 7-2
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P. CHARLES
relevant to the physiological situation, in which the small intestine may be the principal site of calcium absorption, as mentioned above.
Segmental intestinal perfusion [ 1 6 ] In this method a triple-lumen tube is introduced into the desired segment of the gastrointestinal tract. A solution with a defined calcium content and electrolysed composition is perfused into the proximal port. Samples are collected at two distal ports located a fixed distance apart. The difference in calcium concentration at the two distal ports reflects a measure of calcium absorption over the defined segment. The use of calcium tracer with the perfusate allows calculation of bidirectional calcium fluxes. This method represents one of the best available techniques for examination of the physiology of calcium transport in man. It has enabled elucidation of the kinetics of calcium transport, of the participation of different parts of the gastrointestinal tract in the absorptive process, and of the intestinal action of 1,25-dihydroxyvitamin-D [17]. The test is not adapted for testing the calcium bioavailability of different calcium preparations, as it requires perfusion of calcium in a soluble form from well-defined solutions. It therefore ignores physicochemical factors of solubility in the gastric juice and of reciprocation upon neutralization by pancreatic secretion.
Calciuric response to oral calcium load [18. 191 This test was initially developed for the identification of patients with renal stones presenting with intestinal hyperabsorption of calcium. The test (which might be termed the ‘oral calcium tolerance test’ or ‘poor man’s calcium absorption test ’) has, in recent years, been used to measure calcium absorption from food or calcium supplements. It might be performed in two ways. First, during the fasting state the increase in urinary calcium following an oral calcium load provides a n indirect measure of intestinal calcium absorption. Secondly, one might use the increase in 24-h urinary calcium excretion after a n oral calcium load in addition to the usual diet. The principal advantages of this technique are its simplicity and its ease of performance without the need for calcium isotopes. It is therefore cheap and suitable for multiple tests. The major disadvantage of the test is that it has a poor reproducibility and, as it is merely quantitative,
it may divide patients into high and low absorbers. The results of the test may be invalid if the calcium load is given with a metabolizable carbohydrate, which may exert a calciuric action, or with certain anions, which are excreted in the urine and cause calcium trapping. Treatment with drugs that might alter the renal handling of calcium (thiazides, furosemide, and phosphate) may invalidate the results. The results of the test may be improved by extending the calcium load over, for instance, 3 d with concomitant collection of urine.
Calcium bioavailability from diet and calcium supplements Diet Discussion of calcium metabolism almost always concerns the dietary calcium intake. One should also be aware of the many factors that regulate calcium absorption, excretion and utilization. Instead of focusing on the exact number of milligrams of calcium that should be included in the diet, one should rather centre upon making sufficient efforts to maximize utilization of that calcium. Figure 1 illustrates the metabolism of calcium in 1 7 normal Danes [lo], showing the major excretory routes via urine, faeces and skin. It indicates that substantial quantities of calcium are excreted as what we term obligatory calcium losses each day. There are many points in this sequence at which other nutrients, drugs and diseases may have important effects on utilization of dietary calcium. A number of factors influence calcium absorption. As described previously, vitamin D is central to the regulation of calcium absorption. Vitamin-D levels decrease with age, due to both a decline in kidney function with a decrease in production of 1,25dihydroxyvitamin-D, and a decline in synthesis of vitamin D in the skin due to reduced exposure to the sun. In the Nordic countries it might therefore be advisable for the elderly population to take a vitaminD supplement with the diet. For many years lactose has been thought to promote the absorption of calcium. This has recently been questioned by Heaney (personal communication). Several other factors enhance or reduce the absorption of calcium as mentioned previously. Table 1 shows the major dietary sources of calcium. It can be seen that milk and milk products are the main sources of calcium in the western
CALCIUM BIO AV AIL ABILITY
Controls
+
D:31 (4)
I
Bone
e: 3.4 (0.2) B: -2.2 (0.6)
b: 5 (1)
F: 26 (3)
Table I . Major dietary sources of calcium (approximate values) Nutrient
Ca (mmol d-')
m: 4.9 (0.6)
a: 8 (1)
Fig. 1. Calcium metabolism in 1 7 normal Danes. Mean values are expressed as mmol Ca d-I. D = dietary calcium : a = true absorbed calcium; e = endogenous faecal calcium: b = net absorbed calcium: F = faecal calcium : U = urinary excretion of calcium: d = dermal calcium loss: m = bone mineralization rate : r = bone resorption rate: B = calcium balance (copyright Peder Charles, 1987: used with permission).
1 65
U: 5.5 (0.6)
Table 2. Bioavailability of selected food calcium sources'
Ca I cium mg 100 g-I
Primary food sources of calcium Milk and milk products Milk Whole Skimmed Yoghurt Cheese Ice cream
111 124 134 600-1000 120
Secondary food sources of calcium Beans Nuts Almonds Salmon, canned with bones Sardines, canned with bones Broccoli, cooked Spinach, cooked Rhubarb. cooked Kale. cooked Parsley
65 75 250 200 300 130 160 300 200 100
countries. The bioavailability estimated by fractional absorption of some of the major food elements is shown in Table 2 [20]. While certain vegetables have a significant calcium content, the presence of oxalate significantly reduces the bioavailability of calcium in these food items. Oxalate is also an important component of many kidney stones and, when considering the possible hazards of high doses of calcium,
d: 1.6 (0.1)
Source
Fractional absorption with meal
Spinach Bone meal/substance Milk (2%) Kale
0.05 0.32 0.32 0.41
*Modified from Heaney et nl. [20].
it should be borne in mind that, it may be even more important to restrict the intake of food containing high amounts of oxalate. Vitamin C, or ascorbic acid, is degraded directly to oxalate, and when consumed in large amounts results in the elevation of urinary oxalate excretion [21]. A diet that is high in fat [3] will decrease the bioavailability of dietary calcium. At a time when we are concerned that a high-fat diet may be a significant factor in the development of heart disease, stroke and certain forms of cancer, attention should also be paid to its possible role in producing calcium deficiency and osteoporosis. Dietary fibre [22], when consumed in large amounts, interferes with calcium bioavailability. Zinc will also interfere with calcium absorption [ 2 11. Megadoses of vitamin A [23] may produce disturbances in calcium metabolism, the exact mechanisms of which require further investigation. Caffeine in-
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P. CHARLES
Table 3. Some calcium supplement sources
Drug
Calcium per tablet (mg)
Tablets d-'
Calcium phosphate Calcium lactate Calcium gluconate Calcium Sandozm Calcium carbonate
116 65 45 500 500
8 16 24 2 2
~~
*Required to provide c. 1 g of elemental calcium d-'
creases the urinary excretion of calcium, and alcohol [24] affects calcium and vitamin-D metabolism, probably at several sites. Furthermore, there is some evidence to suggest that under certain circumstances iron may interfere with calcium absorption [2 51. Therefore the consumption of a diet that is highly unbalanced, and particularly one that includes irrational megadose supplements of vitamins and minerals, may have important adverse effects on bone status, and in particular on bone health in general [2 11. Moreover, a diet that is high in protein [26] tends to increase the urinary excretion of calcium. This effect may be less marked with animal protein than with vegetable protein, possibly because the former often has a high phosphate content. Phosphate [2 71 tends to decrease urinary calcium excretion and increases faecal calcium excretion. However, the net effect of a diet high in phosphate is probably not to influence the calcium balance significantly. Increasing dietary sodium [28] will result in an increase in urinary calcium. Dietary sodium might even have a greater effect on urinary calcium loss than calcium itself.
Calcium supplements A review of the literature reveals the use of a wide variety of calcium supplements in osteoporosis, without uniformity in dosage or in the frequency of administration. This practice has prevailed despite apparent differences in bioavailability among calcium salts and the well-known dependence of calcium absorption on the dose of calcium supplement. A number of different calcium salts are listed in Table 3. To supply 1 g of elemental calcium, one must administer 1 6 tablets of calcium lactate per day, but only 2 tablets of calcium Sandoz (calcium carbonate plus calcium lactogluconate). Many different calcium
salts are available on the over-the-counter market, most of which have never been tested for calcium absorbability, and which might therefore not be absorbed merely due to the formulation of the tablet.
Strategy for calcium supplementation Having made the decision that calcium supplementation is desirable for an individual, a number of factors must be considered. What is the calcium absorptive status of the subject? In a group of postmenopausal women ingesting 800 mg calcium d-' investigated by Heaney et al. [6]. a substantial inter-individual variation in fractional calcium absorption, ranging from 0.13-0.43, was found, whereas the intra-individual variation was found to be only 0.055 [15]. Moreover, the absorption capacity of calcium is consistent for the patient with regard to load, substance and interval. For example, as shown by Heaney et al. [29] in a group of postmenopausal women, highly significant correlations of absorption fraction within individuals were found, when the same calcium substance, but at different load levels, was tested three times over a n 8-week period. This was even observed over intervals as long as 5 years. Due to this wide range of absorption efficiencies, fractional calcium absorption should be assessed in patients prior to calcium supplementation. How should the calcium supplement be dispensed? It has to be decided whether the patient should have more calcium with the diet or as a calcium salt. A change in the diet is advisable for patients who also require protein and calories, and in this instance both milk and milk products are advisable. In obese subjects skimmed milk or a calcium supplement might be used, and in patients who are allergic to milk, a calcium supplement would be preferable. What calcium salt should be used? In deciding what calcium product should be prescribed, it is necessary for the physician to be familiar with that particular calcium drug. First, there is a limited role of solubility in vitro [20]. which means that even though a calcium supplement appears to be quite insoluble in vitro, it might be well absorbed in vivo. Only calcium salts that have been tested with regard to their bioavailability should be used, and the drug should be obtained from the same supplier. Calcium density shows great variation between different calcium salts, a factor that must be taken into consideration (Table 3).
CALCIUM B I 0 A V A IL A B ILI T Y
When should calcium supplements be taken? To improve calcium absorption, the calcium supplement should be given in divided doses, as this increases the fractional calcium absorption [30]. Two grams of calcium in the morning might give a fractional calcium absorption of 14%,while the same calcium salt given in four doses would increase the fractional calcium absorption to 29%. Patients do not always wish to take tablets four times a day, so even if this might increase fractional calcium absorption, it might be better to give one dose of calcium supplement with breakfast and another at bedtime. This will increase the compliance of the patient. The bioavailability of a calcium supplement is increased when it is taken with a meal [31], whereas a calcium supplement taken at bedtime on an empty stomach will decrease the absorptive efficiency. The rationale for taking a calcium supplement at bedtime is that PTH-stimulated bone resorption is decreased during the night [32]. How is the compliance of the patient controlled? Heaney [33] has recently shown that the calcium density of a random faecal sample can be used to control patient compliance. When should the treatment be evaluated? In order to evaluate the effect of the calcium supplement, bone mineral content might be measured before the start and after each year during treatment.
Conclusions Our knowledge of calcium absorption and bioavailability has greatly improved in recent years. As a result of the increasing concern about osteoporosis, the use of calcium supplements has greatly increased, particularly in the United States. Many of these calcium supplements are marketed without proper knowledge of the bioavailability of the actual preparation. For the benefit of our patients it is now reasonable to demand such investigations before marketing these calcium supplements.
References 1 Rivlin RS. Summary and concluding statement: evidence relating selected vitamins and minerals to health and disease in the elderly population in the United States. Am J Clin Nutr 1 9 8 2 ; 36: 1083-6. 2 Wilkinson R. Absorption of calcium, phosphorus and magnesium. In: Nordin BEC. ed. Calcium. Phosphate and Magnesium Metabolism. Edinburgh, London. New York: Churchill Livingstone, 1976: 36-1 12. 3 Kenny AD. Intestinal Calcium Absorption and its Regulation. Boca Raton. Florida: CRC Press, Inc. 1 9 8 2 : 1-152.
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4 Birge SJ, Peck WA. Berman M. Whedon GI). Study of calcium absorption in man: a kinetic analysis and physiologic model. J Clin Jnvest 1 9 6 9 : 4 8 : 1705-13. 5 Wensel RH. Rich C, Brown AC, Volwiler W. Absorption of calcium measured by intubation and perfusion of the intact human small intestine. J Clin Invest 7969: 4 8 : 3768-75. 6 Heaney RP. Recker RR, Stegman MR. Moy AJ. Calcium absorption in women: relationships to calcium intake, estrogen status, and age. J Bone Miner Res 1 9 8 9 ; 4:,469-75. 7 Tanaka Y. DeLuca HF. The control of 25-hydroxyvitamin D metabolism by inorganic phosphorus. Arch Biochem Biophys 1 9 7 3 : 154: 566-74. 8 Malm OJ. Calcium requirement and adaptation in adult men. Scand J Lab Clin Invest 1958: 10 (Suppl. 36): 1-152. 9 Avioli LV. McDonald JE. Lee SW. The influence of age on the intestinal absorption of "Ca in women and its relation to "Ca absorption in postmenopausal osteoporosis. J Clin Invest 1965 : 44: 1960-7. 10 Charles P. Metabolic bone disease evaluated by a combined calcium balance and tracer kinetic study. Dan Med Bull 1 9 8 9 ; 36: 463-78. 11 Eastell R. Vieira NE. Yergey. Riggs BL. One-day test using stable isotopes to measure true fractional calcium absorption. J Bone Miner Res 1989: 4: 463-8. 12 Bronner F. Experimental studies of calclum absorption in man. Bib1 Nutr Dieta 1962; 3: 22-31. 13 DeGrazia ]A. Rich C. Studies of intestinal absorption of 45calcium in man. Metabolism 1 9 6 4 ; 13: 650-60. 1 4 Birge SJ,Peck WA. Berman M, Whedon GD. Studies of calcium absorption in man : a kinetic analysis and physiologic model. J Clin Invest 1969: 4 8 : 1705-13. 15 Heaney RP. Recker RR. Estimation of true calcium absorption. Ann Intern Med 1985: 103: 516-21. 16 Wensel RH. Rich C. Brown AC. Volwiler W. Absorption of calcium measured by intubation and perfusion of the intact human small intestine. I Clin Invest 1969; 4 8 : 1768-75. 17 Pak CYC. Avioli LV. Editorial. Factors affecting absorbability of calcium from calcium salts and food. Calcif Tissue Int 1 9 8 8 ; 4 3 : 55-60. 1 8 Broadus AE. Dominguez M. Bartter FC. Pathophysiological studies in idiopathic hypercalciuria : use of oral calcium tolerance test to characterize distinctive hypercalciuric subgroups. J Clin Endocrinol Metab 1978: 4 7 : 751-6. 19 Nicar MJ, Pak CYC. Calcium bionvailability from calcium carbonate and calcium citrate. J Clin Endocrinol Metub 1985 ; 61: 391-3. 2 0 Heaney RP. Recker RR. Weaver CM. Absorbability of calcium sources: the limited role of solubility. Calcif Tissue Int 1 9 9 0 : 4 6 : 300-4. 21 Rivlin RS. Women's Health: Osteoporosis: Nutrition. Public Health Reports Supplementary 1986 ; July-August : 1 3 1-5. 2 2 Cummings JH, Hill MJ, Jivrai T. Houston H. Branch WJ, Jenkins DJA. The effect of meat protein and dietary fiber on colonic function and metabolism. I. Changes in bowel habit, bile acid excretion and calcium absorption. Am I Clin Nutr 1979; 32: 2086-93. 23 Navia JM. Harris SS. Vitamin A influences on calcium metabolism and calcification. Ann NY Acad Sci 1980: 3 5 5 : 45-57. 2 4 Vodoz JF. Luisiez M. Donath A, Courvoisier B. Jarcia B. Diminution de I'absorption intcstinale de 4 7 calcium dans I'alcoolisme chronique. Schweiz Med Wocherrschr 1977; 107: 1525-29.
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25 Kelsay JL. Behall KM, Prather ES. Effect of fiber from fruits and vegetables on metabolic responses of human subjects. 11. Calcium, magnesium, iron and silicon balances. Am ] Clin Nutr 1979, 32: 1876-80. 2 6 Schuette SA. Hegsted M. Zemel MB. Linkswiler HM. Renal acid, urinary cyclic A M P , and hydroxyproline excretion as affected by levels of protein, sulphur amino acid, and phosphorus intake. ] Nutr 1 9 8 1 : 1 1 1 : 2106-16. 2 7 Heaney RP. Recker RR. Effects of nitrogen, phosphorus, and caffeine on calcium balance in women. Lab Clin Med 1 9 8 2 : 99: 46-55. 28 Nordin BEC. Polley KJ. Metabolic consequences of the menopause. Calcif Tissue Int 1 9 8 7 : 41 : S-1-S-59. 29 Heaney RP. Weaver CM. Fitzsimmons ML. Recker RR. Calcium absorptive consistency. ] Bone Miner Res 1990: 11 : 1139-42. 3 0 Heaney RP. Weaver CM. Fitzsimmons ML. The influence of
calcium load on absorption fraction. ] Bone Miner Res 1 9 9 0 : 11: 1135-8. 31 Heaney RP. Smith KT. Recker RR. Hinders SM. Meal effects on calcium absorption. Am ] Clin Nutr 1 9 8 9 : 49: 372-6. 32 Horowitz M. Need AG, Philcox JC, Nordin BEC. Biochemical effects of a calcium supplement in osteoporotic postmenopausal women with normal absorption and malabsorption of calcium. Miner Electrolyte Metab 1 9 8 7 : 13: 112-6. 3 3 Heaney RP. Fecal calcium density: a measure of calcium compliance. ] Bone Miner Res 1 9 9 1 : 6 (5): 469-71. Received 1 9 July 1991. accepted 2 October 7991. Correspondence ;Peder Charles. MD PhD, Medicinsk endokrinologisk afdeling. Aarhus Amts Universitetshospital. Tage Hansengade. DK-8000 Aarhus C, Denmark.
