Bioavailability to Rats of Zinc, Magnesium and Calcium in Casein-, Egg- and Soy Protein-Containing Diets12 RICHARD M. FORBES, KARL E. WEINGARTNER, HELEN M. PARKER, ROMA R. BELL 3 AND JOHN W. ERDMAN, JR. Department of Animal Science, and Department of Food Science, University of Illinois, Urbana, Illinois 61801 ABSTRACT The bioavailability of zinc ( Zn ) and magnesium ( Mg ) from several soybean products (SP) as well as the effect of the presence of dietary SP upon the bioavailability of added Zn or calcium (Ca) was investigated. Male, weanling rats were fed all or part of their dietary pro tein as full fat soy flour ( SF ), freeze-dried soy beverage ( SB ) or soy con centrate (SC). In experiments testing the bioavailability of minerals from SP, incremental levels of a SP were substituted on an isonitrogenous basis for egg white (Zn studies) or casein (Mg studies). Graded levels of ZnCOa or MgCOs were added to egg white or casein basal diets. In experiments testing the effect of the presence of SP upon the bioavailability of supple mental minerals, graded levels of CaCOs or ZnCO3 were added to SP-containing diets or casein (Ca studies) or egg white (Zn study) basal diets. Linear regression analyses related total tibia (or femur) Zn, Mg, or Ca to in creased dietary mineral. Growth (Zn studies) and serum Mg were also related to the dietary mineral concentration. The results showed that Zn was poorly available from soy products, especially SC. Mg was highly available from SF and SB; Mg utilization from SC was good but less than from the other SP. Ca added to all SP was highly available, while Zn added to SC was not fully available. Conditions involved in processing SC or its elevated phytate to zinc molar ratio may have resulted in reduced bioavailability of endogenous Zn and Mg and of added Zn. J. Nutr. 109: 1652-1660, 1979. INDEXING KEY WORDS bioavailability •zinc •magnesium • calcium •phytic acid Expanded utilization of soybeans and soy protein-extended foods for human consumption has prompted studies of the nutritional value of these products. Food processing technology has been developed to assure destruction of antinutritional factors and to provide optimal quality of soy protein.
Still
however, from
the
Siderable Components c,
of
Concern
to
nutritionists,
is the bioavailability legume.
Soybeans
quantities
OÕboth
of
dietary , , '
to reduce mineral
fiber .
absorption
In a previous study, Forbes and Parker ( 10) were able to successfully measure zinc bioavailability from full fat soy flour utilizing male weanling rats in a slope-ratio assay procedure patterned after Momcilovic et al. (11). Forbes and Parker reReceived
of minerals contain
phytic
for publication
COn-
' 2 Presented
In part
acid
and
f^Jf"„ft
which are / i r> \
said
' Current address Western Australian
( 1-9 ).
at the
Experiment
18, 1979.
part "y °SRS
Federation
Biology,
: Dept Institute
6102, western Australia. 1652
Downloaded from https://academic.oup.com/jn/article-abstract/109/9/1652/4770873 by University of Glasgow user on 30 July 2018
January
Gnâ„¢oiâ„¢is?i74 under pPwSSe1"
Atlantic
of American City,
New
of Medical Technology, of Technology, Bentley
BIOAVAILABILITY
OF MINERALS
ported that when the log total femur zinc was used as a criterion of response, zinc from the flour was utilized 34% as effi ciently as was zinc from zinc carbonate. However, zinc added as the carbonate to a whole fat soy flour diet was 94% as efficiently utilized as when zinc carbonate was added to an egg white diet. These results suggest that zinc is poorly available from soy flour but the presence of soy flour in a diet has little effect upon the bioavailability of zinc from the rest of the diet. The purpose of the current experiments was to utilize linear regression procedures to investigate the bioavailability of zinc and magnesium from several soybean products as well as to test the effect of their presence upon the bioavailability of added zinc or calcium. MATERIALS
AND METHODS
General procedures. Three soybean prod ucts, full fat soy flour (SF), freeze-dried soy beverage (SB) and a commercial soy concentrate (SC) were utilized for the current studies. The composition of the soy products is shown in table 1. The soy flour and beverage were locally processed in a manner similar to published procedures (12, 13). For full fat soy flour, whole soy beans (Bonus 1975) were dry-cleaned, heated in an air dryer at 93°for 20 minutes and allowed to pass through a spinning drum-plate apparatus in order to split and separate the hulls and cotyledons. The cotyledons were blanched by stirring them in boiling distilled water for 20 minutes. The blanch water was drained, the beans were milled with an equal weight of dis tilled water and the slurry was drum dried. The resulting flake was crumbled to form a flour and stored at 1°until use. The soy beverage was obtained by pro cessing Bonus 1975 beans as described above. Instead of drum drying the ground slurry, it was reheated to 85°in a steam jacketed kettle and immediately homoge nized according to previously described specifications (13). The resulting full fat soybean beverage was freeze-dried and passed through a sieve to form a flour. The commercial soy protein concentrate was obtained directly from the manufac turer. It had been produced according to
Downloaded from https://academic.oup.com/jn/article-abstract/109/9/1652/4770873 by University of Glasgow user on 30 July 2018
1653
IN SOY DIETS
TABLE l Composition of soy products trate95.770.70.28320.220.100.600.471.6752
Solids' (%)Protein' (%)Ether (%)Zn1extract1 (ppm)Ca' (%)Mg» (%)Total (%)Phytate phosphorus« (%)Phyticphosphorus' (%)Phytate/Zn acid« molar ratioFlour90.052.525.6440.170.190.580.361.2428Beverage95.048.026.6440.180.190 ' Analysis performed using methods of AOAC (1975) Twelfth edition. Solids, section 14.076; protein (N X 6.25), section 2.049; ether extract, section 14.080; zinc, section 25.146. 3Atomic absorption spectrophotometry analysis of wet ashed (nitric acid) samples. ' Bartlett, G. R. (1959) J. Biol. i 'hem. 234, 466-468. '• Analysis performed using the method of Earley, E. II. & DeTurk, £. E. (1944) J. Am. Soc. Agron. 36, 803-814, and assuming 28.2% phosphorus in phytic acid.
the following general procedure. To a de fatted soy flour with high protein dispersibility was added water, NaHSOs and enough HC1 to reach the isoelectric point of the protein (pH 4.2). After filtering, the cake was washed with water, filtered, and the second cake was neutralized to pH 6.7 with a NáOH solution. This neutralized product was heated to 50 to 60°and spray dried. Young male Sprague-Dawley derived rats * were individually housed in stainless steel cages in a controlled temperature en vironment. After a minimum 3 day period in the laboratory, animals were fed experi mental diets ad libitum with distilled water freely available. Feed intakes were mea sured and body weights were recorded weekly. At the end of the experimental feeding period rats from the magnesium and cal cium experiments were killed with ether. Cervical dislocation was employed to kill animals in the zinc experiments. Tibias or femurs were removed from all rats, and after drying and fat extraction they were dry ashed and taken up in dilute HC1 prior to analysis for zinc, mag nesium or calcium by atomic absorption ' HarÃ-anIndustry, Inc., Cumberland, Indiana.
1654
FORBES ET AL. TABLE 2 Composition of soy and casein diets1 for calcium studies
The data obtained were statistically analyzed by regression analysis to compare the slopes of the linear portions of the lines relating response per unit of added mineral to provide measure of its relative availa product26.6-39.2%—0.0-9.53.230.01.02.00.4VariableVariableCasein—19.79.0-9.52.830.01.02.00.08VariableVariable bility (11, 14). Soy product2Casein3Corn Experiments with zinc. Isonitrogenous, isoenergetic 20% protein diets were fed to oil1Calcium rats for 21 days. In the first two experi mix6Starch6Vitamin free mineral ments the bioavailability of zinc from SB mix'Fiber8L-methionine*Glucoseand SC was investigated. Control diets con tained 20% egg-white protein,6 8% fat from corn oil ( or corn oil plus the oil from 2O'°Calcium H the soy product), "i>%cellulose, 5% vitamin carbonate"Soy mix,7 3.3% zinc-free salts8 (adjustment of 1g/100 g diet on an as is basis. *Soy flour calcium and phosphorus levels were made (38.1%), soy beverage (39.2%) or soy concentrate where necessary to equalize these between (26.6%) were added to supply 18% protein. 3 Vi diets) and variable amounts of glucose to tamin Free Test, Teklad Test Diets, Madison, Wisconsin, added to provide 18% protein. 4 Corn complete the diets. The control diets were oil was added to make soy and casein diets isosupplemented with 0, 2, 4, 6 or 8 ppm zinc energetic in each experiment. In the first study, as the carbonate. To test for the availability 0.3 and 9.0% corn oil was added to the soy flour of zinc in the soy products, similar levels and casein diets, respectively. In the second study, 0.0, 9.5 and 9.5% corn oil was added to soy beverage, of zinc were obtained by substituting the soy concentrate and casein diets, respectively. 6 Mineral mix modified from Kenney, A. D. & soy product for egg-white on an equivalent protein basis. The third zinc experiment Munson P. L. (1959) Endocrinology 64, 513521. Both diets contained: CuSO4, 14.6 mg/kg; tested the effect of the presence of SC on FeC„H5O7 XH2O (16% Fe), 1.172 g/kg; MnSO, the availability of added zinc carbonate. •H,O,192 mg/kg; KI, 2 mg/kg; NaCl, 6.009 The basal egg diet contained 3 ppm zinc g/kg; MgSO4, 2.131 g/kg diet. In addition, soy and casein diets contains ZnCOs, 160 and 113 from zinc carbonate and the basal soy diet mg/kg; K,HPO4, 18.325 and 13.744 g/kg; NaHPO, contained 5 ppm zinc from soy concentrate. •HjO,3.580 and 2.685 g/kg diet, respectively. Zinc carbonate was added to both diets to «Corn starch. 'Teklad #40060: mg/kg dry provide an additional 2, 4, 6 or 8 ppm zinc. diet: p-Amino-benzoic acid, 110.132; ascorbic acid, Experiments with magnesium. Isoener 1016.604; biotin, 0.441; Bi2, 29.736; calcium pantothenate, 66.709; choline dihydrogen citrate, getic, isonitrogenous 20% protein diets 3496.916; folie acid, 1.982; i-Inositol, 110.132; were fed to rats for 17 (SF study) or 21 menadione, 49.559; nicotinic acid, 99.119; pyridoxine-HCl, 22.026; riboflavin, 22.026; thiamin days ( SB and SC study ). The bioavailabil •HC1,22.026; units/kg dry diet: retinyl acetate, ity of magnesium from SF, SB and SC was (dry) 5947.2 R.E.; ergocaliferol, (dry) 2202.5 IU; investigated. Casein diets contained 20% tocopherol acetate, 121.15 IU. »Solka Floe, Brown Co., Berlin, New Hampshire. 9Ajinoprotein as casein and were supplemented moto Co., Inc., Tokyo. l° Added to bring diets to provide from 50 to 260 to 325 ppm mag up to 100 g. u In the first study calcium as nesium as the carbonate.9 Similar levels of calcium carbonate was added to the soy flour diet magnesium were obtained in the soy diets (containing 0.06% Ca) and to the Casein diet by substituting soy products for casein at (containing 0.0005% Ca) at a rate of 0.0, 0.2, 0.4 or 0.6% dietary calcium. In the second experiment, equivalent protein levels. In the soy flour calcium was added to soy beverage (containing experiment excess magnesium to provide a 0.08% Ca), soy concentrate (containing 0.06% Ca) or casein diets to give 0.08, 0.18, 0.28 or 0.68 dietary calcium.
spectrophotometryv' Blood was obtained from the abdominal aorta of anesthetized animals in the magnesium studies and mag nesium concentration of the serum was de termined directly via atomic absorption analysis.
Downloaded from https://academic.oup.com/jn/article-abstract/109/9/1652/4770873 by University of Glasgow user on 30 July 2018
51'erkin Elmer, Norwalk. Connecticut, Model 306. 6 Seymour Foods. Topeka. Kansas. 'Cerklewskl and Forbes, J. Nutr. 106, 778 (1976). In the zinc experiments biotin was added to provide 4 mg/kg of diet. 8 Contained in g/kg mix : NaCl, 130.5 ; K-CO8, 95.3 ; K..SO,, 54.2 ; K2HPO4, 92.0 ; CaHPO,, 337.5 : CaCO3, 202.2 ; MgCOs, 64.1 ; Ferric citrate, 19.2 ; MnSO.-H2O, 4.5: CuSO4-5H2O. 0.65; KIOS, 0.02; Na2SeO3, 0.06. "Other basal diet constituents for magnesium studies were the same as described for zinc studies except that MgCO3 was deleted and 2nCOs was added to the salt mix to provide 20 ppm zinc In the diet.
BIOAVAILABILITY OF MINERALS IN SOY DIETS
1655
in serum or bone magnesium in response to these diets was attributable to mag nesium and not to some other dietary in fluence. Experiments with calcium. Since the cal cium content of soybean is quite low,10 it was not practical to test for the bioavailability of calcium in soy products. Instead, two experiments were performed to test for the effect of the presence of SF, SB or SC upon the bioavailability of added calcium. Isoenergetic, isonitrogenous 18% protein diets (see table 2) were fed to rats for 4 weeks. The protein source was casein or one of the three soy products. In the first experiment, 0, 0.2, 0.4, or 0.6% dietary calcium as the carbonate, was added to a casein basal diet or to a soy flour basal diet. In the second experiment, enough calcium carbonate was added to casein, SB or SC basal diets to provide 0.08, 0.18, 0.28 or 0.68% total dietary calcium.
©
PPM ZINC ADDED
Fig. 1 Regression of weight gain (fig. 1A, 1C and IE) and log tibia zinc (fig. IB, ID and IF) upon added dietary zinc. Figures 1A to ID demonstrate the effect of zinc added as zinc car bonate to egg-white protein diet (Q) or added as soy beverage ( •) or soy concentrate ( A ) to egg-white protein on an equal protein basis. Figures IE and IF show the effect of zinc added as zinc carbonate to an egg-white protein ( O ) or to a diet containing 63% of its protein as soy concentrate ( A ). Each data point represents the average response of five animals.
total of 650 ppm was added as the car bonate to the casein and to the highest soy flour diet to ensure that any differences
Downloaded from https://academic.oup.com/jn/article-abstract/109/9/1652/4770873 by University of Glasgow user on 30 July 2018
RESULTS Experiments with zinc. In the experi ments on zinc availability, weight gain and log tibia zinc content responded linearly to increased dietary zinc concentration. The regressions of these criteria on dietary zinc for soy beverage are shown in figures 1A and IB. The ratios between the slopes ob tained for zinc added as contained in soy beverage and as ZnCO^ were 0.63 ( t = 4.6, P < 0.001 ) for weight gain and 0.40 (t = 5.4, P < 0.001) for log tibia zinc. Similarly derived data for soy concentrate were 0.41 ( t - 6.4, P < 0.001 ) for weight gain and 0.20 (i = 5.6, P < 0.001) for tibia zinc as shown in figures 1C and ID. Zinc added to the soy concentrate as the car bonate produced slope ratios of 0.77 ( t = 2.63, P < 0.02 ) for weight gain and 0.70 (t = 2.36, P < 0.05) for tibia zinc as shown in figures IE and IF. There were no significant differences be tween intercepts in either of the regression comparisons in zinc studies ( P < 0.05 ). Experiments with magnesium. In the ex periments on magnesium availability, serum magnesium concentrations and tibia magnesium content responded linearly to 10A diet providing 18% protein from soy would only contain 10% of the ratu requirement for cal cium.
1656
FORBES ET AL.
MgCOs were 0.96 (t = 0.8, N.S.) for serum magnesium concentration and 1.06 ( t = 0.6, N.S.) for tibia magnesium content. In the soy beverage experiment (fig. 2C and 2D) the slope-ratios were 1.02 (f = 0.04, N.S.) for serum magnesium and 1.04 (f = 0.13, N.S.) for tibia magnesium. Similarly derived data with respect to soy concen trate yielded slope ratios of 0.77 ( t = 2.05, P < 0.05) for serum magnesium and 0.80 (f = 2.00, P < 0.05) for tibia magnesium as shown in figures 2E and 2F. Weight gain of rats did not respond linearly to increased dietary magnesium concentration ( table 3). TABLE 3 Weight gain of rats in magnesiums studies1 DietSoyCaseinSoy
Mgppmflour
gaing69
study5511022033065060125250370700Wt ±2103±4105 ±1113±2118±282±3108 flourDiet
±4118±4117±8123±3Soy
studyCaseinSoy
beverage
±4121±5125±5118±568±2105±8120±6128±5126±7
beverage441141622032725811518524529270±4105 I5O
200
»0
100
PPM MAGNESIUM ADDED
Fig. 2 Regressions of serum magnesium ( fig. 2A, 2C and 2E ) and tibia magnesium ( fig. 2B, 2D and 2F) upon magnesium added to casein protein diet in the form of magnesium carbonate ( O ) or added as soy flour ( A ), soy beverage ( •) or soy concentrate ( A ) substituted for an isonitrogenous amount of casein protein. Each data point represents the average response of five animals.
increased dietary magnesium concentra tion. The regressions of these criteria on dietary magnesium for soy flour are shown in figures 2A and 2B. The ratios between the slopes obtained for magnesium con tained in soy flour and that added as
Downloaded from https://academic.oup.com/jn/article-abstract/109/9/1652/4770873 by University of Glasgow user on 30 July 2018
studyCaseinSoy concentrate ±4114±6130±6136
±7127±979±1113±6139±5147 concentrate601041562082626011216421826872
"Data points represent means (N = 5)±SEof weight gains of animals fed for 17 days (soy flour study) or 21 days (soy beverage and soy concen trate) study.
1657
BIOAVAILABILITY OF MINERALS IN SOY DIETS
Experiments with calcium. In the experi ments on the effect of protein source upon the availability of added calcium, femur calcium content responded linearly to in creases in dietary calcium concentration. The regression of this criterion on dietary calcium is shown in figures 3A and 3B. The ratios between the slopes obtained for cal cium added to soyflour ( fig. 3A ) and casein was 0.94 (f = 3.4, N.S.). Similarly derived data for soy beverage and soy concentrate (fig. 3B) were 1.06 (i = 0.70, N.S.) and 1.02 (t = 0.22, N.S.), respectively. Weight gain did not respond linearly to increased calcium concentration ( table 4 ). DISCUSSION Forbes and Parker (10) have previously demonstrated that zinc added to egg-white based diets in the form of full fat soy flour was significantly less utilizable for the rat than when zinc was added as zinc car bonate to the egg-white diets. In the cur rent study the bioavailability of SB and SC were tested utilizing identical condi tions. The slope-ratios of weight gain ( 0.63) and bone zinc (0.40) of SB (fig. 1A and IB ) were similar to the previous report for SF (0.55 for weight gain and 0.34 for /j.g femur zinc). The bioavailability of zinc from soy concentrate was poorest of the three soy products. Figures 1C and ID show that zinc added to egg-white based diets as soy concentrate was only utilized 41% as well for weight gain and 20% as well utilized for log /tg tibia zinc as was that added as zinc carbonate. ®
0 % TOTAL CALCIUM IN DIET
006 018 028
0'
XTOTAL CALCIUM IN DIET
Fig. 3 Regressions of femur calcium (fig. 3A and 3B) on percent total dietary calcium added as calcium carbonate to casein ( O ), soy flour ( A ), soy concentrate ( A ) or soy beverage ( • ) based diets. Each data point represents the aver age response of eight animals.
Downloaded from https://academic.oup.com/jn/article-abstract/109/9/1652/4770873 by University of Glasgow user on 30 July 2018
TABLE 4 Weight gain of rats in calcium studies1 Diet
Casein
Total diet
Wt. gain
Soy flour study O
122±4 0.20 235±14 254±8 0.40 0.60 257±7 Soy flour 189±3 0.06 0.26 230±11 238±8 0.46 0.66 239± 9 Soy concentrate and soy beverage study Casein 0.08 191±3 0.18 212±9 0.28 218±4 0.68 226±5 Soy concentrate 168±9 0.08 0.18 199±6 0.28 210±4 220±6 0.68 Soy beverage 0.08 173±5 202±6 0.18 0.28 212±5 218±7 0.68 1Data points represent means (N = 8) ±SEof rats fed for 28 days.
These data, showing a lower slope ratio for response to zinc in terms of bone zinc than in terms of weight gain are in agree ment with the data of Forbes and Parker (10). Zinc response is a more critical mea sure of zinc availability than is weight gain because in the former instance the response measured is the sum of two effects of ab sorbed zinc; one is the promotion of in creased tissue mass and the second is the promotion of increased zinc concentration in the tissue. In addition, the authors be lieve, in agreement with Momcilovic et al. (11), that bone zinc should be the better criterion since the response is linear over a wider span of zinc intake and the data are less susceptible to daily variation than is body weight. Forbes and Parker (10) also tested the effect of a given level of dietary SF ( 13% of diet) on the availability of added zinc carbonate. They found that weight gain and femur zinc responses to increasing levels of zinc carbonate were not influ enced by the presence of soy flour in the
1658
FORBES ET AL.
diet. In the current study, it was found that the presence of soy concentrate (18.7% of diet) in diets did result in a small but significant reduction in growth and bone zinc responses to added Zn (fig. IE and IF). The results from these zinc studies not only reveal that the zinc in these soy prod ucts was poorly available, but that zinc bioavailability varied from product to product and was poorest in the soy con centrate. Rackis and his colleagues (15, 16) have noted that zinc utilization from certain soy isolates was particularly low as compared to that from soybean meals. They speculated that differences in zinc bioavailability were due to the varying food processing conditions used to manu facture the soy protein products. The cur rent zinc studies amplify this concept. Magnesium bioavailability from the three soy products, in contrast to the bio availability of zinc, was very good. Mag nesium utilization from SF and SB was equivalent to the highly available inorganic source, magnesium carbonate. Only the magnesium from soy concentrate was less bioavailable than the inorganic source. Although Roberts and Yudkin (17) re ported that magnesium deficiency signs in rats could be aggravated by addition of sodium phytate to casein-based diets, Forbes ( 18 ) found that magnesium absorp tion, but not balance, was reduced in young rats fed isolated soy protein diets in com parison to egg white protein diets. Guenter and Sell (19) determined that the bio availability of the mineral from soybean meal for chickens was 105% that of MgSO4. Recently, Lo and coworkers " re ported no significant difference in the bio availability of magnesium for rats fed in cremental levels of magnesium carbonate added to isolated soy protein, casein or lyophylized meat-based diet. These latter studies support the view that magnesium is highly available from soy protein products. The current studies clearly demonstrated that the bioavailability of calcium added as calcium carbonate to any of the three soy products was the same as when added to casein diets. These results suggest that calcium fortification of soy protein products
Downloaded from https://academic.oup.com/jn/article-abstract/109/9/1652/4770873 by University of Glasgow user on 30 July 2018
will result in good utilization of the min eral. However, as pointed out in previous publications (1, 2, 6, 7), calcium addition may cause or accentuate poor utilization of zinc, magnesium or copper from soy prod ucts which are high in phytic acid. Soybean products usually contain con siderable quantities of phytic acid. Litera ture values (3, 20) range from 1.4 to 2.2% phytic acid on a dry basis. Soybean prod ucts also can contain appreciable amounts of dietary fiber, especially if the hull is not removed prior to processing. Defatted soymeal produced from whole soybean con tains about 6% fiber; about %coming from the hull (21). Both phytic acid and com ponents of dietary fiber are said to reduce mineral absorption ( 1-9 ). The phytate ion can complex with metal lic ions such as zinc, calcium, and iron and form highly insoluble and poorly absorbed chelate complexes (1-5). The combination of metal ions such as calcium and zinc forms even less soluble complexes ( 1, 2, 6, 7). Some researchers have added pure sodium phytate to food systems and have found reduced mineral uptake. For exam ple, Davies and Nightingale (22) found that the addition of sodium phytate to yield \% dietary phytic acid greatly reduced rat growth and absorption of zinc, iron, cop per and manganese. This type of investiga tion reveals little insight into the chemical binding of naturally occurring phytic acid, especially when excessive levels of sodium phytate are added to food systems. How ever, most studies suggest an inverse re lationship between phytic acid and mineral absorption. In the current study soy concentrate yielded the poorest bioavailability re sponses of the three soybean products. Phytate content and the phytate to zinc molar ratio was highest in this product (see table 1). This suggests that phytate content is responsible for the poor mineral utilization of soy concentrate. At this time one should not directly equate the phytate concentration with poor mineral bioavaila bility in phytate-containing foods since " Lo. G. S., Collins, D. W., Steinke, P. H. & Hop kins. D. T. (1978) Effect of Isolated soybean pro tein on bloavailablllty of mapneslum. Federation Proc. il, 667 (Abstr.).
BIOAVAILABILITY
OF MINERALS
other factors such as the food processing history may be very important. The type of phytate-protein-mineral complexes formed during processing rather than the specific phytate concentrations may be responsible for reduced mineral absorptive capacity in some soy products (3, 15, 16). More re search is needed to identify the processing steps that affect formation of phytate com plexes. Most components of dietary fiber act as monofunctional weak cation exchange resins (7). Studies involving in vitro tech niques (8, 9) demonstrate binding of zinc and iron to wheat bran, lignin, and some hemicellulose fractions of wheat bran and of calcium to non-cellulosic fractions of plant fibers. Although direct extrapolation of in vitro results to in vivo systems cannot be made (3, 7), many components of di etary fiber must be considered as potential reducers of mineral bioavailability. In this current work, it is not possible to evaluate the relative contributions of phy tate, components of dietary fiber or other endogenous chelators to the reduced zinc absorption. However, a recent study from our laboratory (23) demonstrated that in clusion of the soybean hulls (approxi mately 50c/(.of the dietary fiber and 5c/c of the phytic acid of the whole soybean) in soy flour-based diets had no significant effect upon the bioavailability of native zinc or of added calcium. This study sug gests that soy hull fiber plays no role in reducing mineral absorption, at least when fed at levels normally found in whole soy bean products. It can be concluded from this study that magnesium is highly available from soy flour and soy beverage. Magnesium bio availability from soy concentrate is good but less than from the other products. Zinc availability is poor from soy products, especially soy concentrate. The presence of soy concentrate in rat diets also some what reduces the bioavailability of added zinc. A previous study ( 10) showed that zinc added to diets containing soy flour was fully available. Calcium added to all soy products was fully available for utiliza tion. The particular conditions involved in processing soy concentrate or its elevated phytate to zinc molar ratio may result in
Downloaded from https://academic.oup.com/jn/article-abstract/109/9/1652/4770873 by University of Glasgow user on 30 July 2018
IN SOY DIETS
1659
reduced bioavailability of endogenous min erals and added zinc. ACKNOWLEDGMENTS
The authors would like to acknowledge the able technical assistance of Mrs. LuAnn Franzen and Mr. William R. Casebeer. LITERATURE CITED 1. Oberleas, D., Muhrer, M. E. & O'Dell, B. L. (1966) Dietary metal complexing agents and zinc availability in the rat. J. Nutr. 90, 56-62. 2. O'Dell, B. L. (1969) Effect of dietary com ponents upon zinc availability. Am. J. Clin. Nutr. 22, 1315-1322. 3. Erdman, J. W., Jr. (1979) Oilseed phytates: Nutritional Implications. J. Am. Oil Chem. Soc. (in press). 4. Maddaiah, V. T., Kurnick, A. A. & Reid, B. L. ( 1964 ) Phytic acid studies. Proc. Soc. Exp. Biol. Med. 115, 391-393. 5. Vohra, P., Gray, B. A. & Kratzer, P. S. ( 1965) Phytic acid-metal complexes. Proc. Soc. Exp. Biol. Med. 120, 447-449. 6. Forbes, R. M. (1960) Nutritional inter actions of zinc and calcium. Federation Proc. 19, 643-647. 7. Oberleas, D. & Harland, B. F. ( 1977) Nu tritional agents which affect metabolic zinc status. In: Zinc Metabolism: Current aspects in health and disease (Brewer, G. J. & Prasad, A. S., ed.), pp. 11-24, Alan R. Liss, Inc., New York. 8. Ismail-Beigi, F., Faroji, B. & Reinhold, J. G. ( 1977 ) Binding of zinc and iron of wheat bread, wheat bran and their components. Am. J. Clin. Nutr. 30, 1721-1725. 9. James, W. P. T., Branch, W. J. & Southgate, D. A. T. (1978) Calcium binding by ditary fibre. Lancet 25, 638-639. 10. Forbes, R. M. & Parker, H. M. (1977) Bio logical availability of zinc in and as influenced by whole fat soy flour in rat diets. Nutr. Rept. Int. 15, 681-688. 11. Momcilovic, M., Belonje, B., Giroux, A. & Shah, B. G. (1975) Total femur zinc as the parameter of choice for a zinc bioassay in rats. Nutr. Rept. Int. 12, 197-203. 12. Erdman, J. W. Jr., O'Connor, M. P., Weingartner, K. E., Solomon, L. W. & Nelson, A. I. ( 1977 ) Production, nutritional value and baking quality of soy-egg flours. J. Food Sci. 42, 964-967. 13. Nelson, A. I., Steinberg, M. P. & Wei, L. S. ( 1976) Illinois process for preparation of soy milk. J. Food Sci. 41, 57-61. 14. Steel, R. G. D. & Torne, I. H. (1960) Prin ciples and procedures of statistics. McGrawHill Book Comp., Inc., New York. 15. Rackis, J. J., McGhee, J. E., Honig, D. H. & Booth, A. N. ( 1975 ) Processing soybeans into foods: Selected aspects of nutrition and flavor. J. Am. Oil. Chem. Soc. 52, 249A253.
1660
FORBES ET AL.
16. Rackis, J. J. & Anderson, R. L. (1977) Mineral availability in soy protein products. Food Prod. Dev. 11, 38-44. 17. Roberts, A. H. & Yudkin, J. (1960) Dietary phytate as a possible cause of magnesium de ficiency. Nature 185, 823-825. 18. Forbes, R. M. (1964) Mineral Utilization in the Rat. III. Effects of calcium, phosphorus, lactose and source of protein in zinc defi ciency and in zinc adequate diets. J. Nutr. 83, 225-233. 19. method Guenter, forW.determining & Sell, J. L. availability (1974) A "true" of magnesium from food stuffs using chickens. J. Nutr. 104, 1446-1457 20. de Boland, A. R., Garner, G. B. & O'Dell, B. L. ( 1975 ) Identification and properties
Downloaded from https://academic.oup.com/jn/article-abstract/109/9/1652/4770873 by University of Glasgow user on 30 July 2018
of phytate in cereal grains and oilseed prod ucts. J. Agrie. Food Chem. 23, 1186-1189. 21. Smith, A. K. & Circle, S. J. (1972) Chemi cal composition of the seed. In: Soybeans: Chemistry and technology (Smith, A. K. & Circle, S. J., éd.), pp. 61-92, AVI Pubi. Comp., Inc., Westport, Conn. 22. Davies, N. T. & Nightingale, R. (1975) The effects of phytate on intestinal absorption and secretion of zinc, whole-body retention of Zn, Copper, Iron and Manganese in rats. Br. J. Nutr. 34, 243-258. 23. Weingartner, K. E., Erdman, J. W., Jr., Parker, H. M. & Forbes, R. M. (1979) Ef fect of soybean hull upon the bioavailability of zinc and calcium from soy flour-based diets. Nutr. Repts. Int. 19, 223-231.
Still
however, from
the
Siderable Components c,
of
Concern
to
nutritionists,
is the bioavailability legume.
Soybeans
quantities
OÕboth
of
dietary , , '
to reduce mineral
fiber .
absorption
In a previous study, Forbes and Parker ( 10) were able to successfully measure zinc bioavailability from full fat soy flour utilizing male weanling rats in a slope-ratio assay procedure patterned after Momcilovic et al. (11). Forbes and Parker reReceived
of minerals contain
phytic
for publication
COn-
' 2 Presented
In part
acid
and
f^Jf"„ft
which are / i r> \
said
' Current address Western Australian
( 1-9 ).
at the
Experiment
18, 1979.
part "y °SRS
Federation
Biology,
: Dept Institute
6102, western Australia. 1652
Downloaded from https://academic.oup.com/jn/article-abstract/109/9/1652/4770873 by University of Glasgow user on 30 July 2018
January
Gnâ„¢oiâ„¢is?i74 under pPwSSe1"
Atlantic
of American City,
New
of Medical Technology, of Technology, Bentley
BIOAVAILABILITY
OF MINERALS
ported that when the log total femur zinc was used as a criterion of response, zinc from the flour was utilized 34% as effi ciently as was zinc from zinc carbonate. However, zinc added as the carbonate to a whole fat soy flour diet was 94% as efficiently utilized as when zinc carbonate was added to an egg white diet. These results suggest that zinc is poorly available from soy flour but the presence of soy flour in a diet has little effect upon the bioavailability of zinc from the rest of the diet. The purpose of the current experiments was to utilize linear regression procedures to investigate the bioavailability of zinc and magnesium from several soybean products as well as to test the effect of their presence upon the bioavailability of added zinc or calcium. MATERIALS
AND METHODS
General procedures. Three soybean prod ucts, full fat soy flour (SF), freeze-dried soy beverage (SB) and a commercial soy concentrate (SC) were utilized for the current studies. The composition of the soy products is shown in table 1. The soy flour and beverage were locally processed in a manner similar to published procedures (12, 13). For full fat soy flour, whole soy beans (Bonus 1975) were dry-cleaned, heated in an air dryer at 93°for 20 minutes and allowed to pass through a spinning drum-plate apparatus in order to split and separate the hulls and cotyledons. The cotyledons were blanched by stirring them in boiling distilled water for 20 minutes. The blanch water was drained, the beans were milled with an equal weight of dis tilled water and the slurry was drum dried. The resulting flake was crumbled to form a flour and stored at 1°until use. The soy beverage was obtained by pro cessing Bonus 1975 beans as described above. Instead of drum drying the ground slurry, it was reheated to 85°in a steam jacketed kettle and immediately homoge nized according to previously described specifications (13). The resulting full fat soybean beverage was freeze-dried and passed through a sieve to form a flour. The commercial soy protein concentrate was obtained directly from the manufac turer. It had been produced according to
Downloaded from https://academic.oup.com/jn/article-abstract/109/9/1652/4770873 by University of Glasgow user on 30 July 2018
1653
IN SOY DIETS
TABLE l Composition of soy products trate95.770.70.28320.220.100.600.471.6752
Solids' (%)Protein' (%)Ether (%)Zn1extract1 (ppm)Ca' (%)Mg» (%)Total (%)Phytate phosphorus« (%)Phyticphosphorus' (%)Phytate/Zn acid« molar ratioFlour90.052.525.6440.170.190.580.361.2428Beverage95.048.026.6440.180.190 ' Analysis performed using methods of AOAC (1975) Twelfth edition. Solids, section 14.076; protein (N X 6.25), section 2.049; ether extract, section 14.080; zinc, section 25.146. 3Atomic absorption spectrophotometry analysis of wet ashed (nitric acid) samples. ' Bartlett, G. R. (1959) J. Biol. i 'hem. 234, 466-468. '• Analysis performed using the method of Earley, E. II. & DeTurk, £. E. (1944) J. Am. Soc. Agron. 36, 803-814, and assuming 28.2% phosphorus in phytic acid.
the following general procedure. To a de fatted soy flour with high protein dispersibility was added water, NaHSOs and enough HC1 to reach the isoelectric point of the protein (pH 4.2). After filtering, the cake was washed with water, filtered, and the second cake was neutralized to pH 6.7 with a NáOH solution. This neutralized product was heated to 50 to 60°and spray dried. Young male Sprague-Dawley derived rats * were individually housed in stainless steel cages in a controlled temperature en vironment. After a minimum 3 day period in the laboratory, animals were fed experi mental diets ad libitum with distilled water freely available. Feed intakes were mea sured and body weights were recorded weekly. At the end of the experimental feeding period rats from the magnesium and cal cium experiments were killed with ether. Cervical dislocation was employed to kill animals in the zinc experiments. Tibias or femurs were removed from all rats, and after drying and fat extraction they were dry ashed and taken up in dilute HC1 prior to analysis for zinc, mag nesium or calcium by atomic absorption ' HarÃ-anIndustry, Inc., Cumberland, Indiana.
1654
FORBES ET AL. TABLE 2 Composition of soy and casein diets1 for calcium studies
The data obtained were statistically analyzed by regression analysis to compare the slopes of the linear portions of the lines relating response per unit of added mineral to provide measure of its relative availa product26.6-39.2%—0.0-9.53.230.01.02.00.4VariableVariableCasein—19.79.0-9.52.830.01.02.00.08VariableVariable bility (11, 14). Soy product2Casein3Corn Experiments with zinc. Isonitrogenous, isoenergetic 20% protein diets were fed to oil1Calcium rats for 21 days. In the first two experi mix6Starch6Vitamin free mineral ments the bioavailability of zinc from SB mix'Fiber8L-methionine*Glucoseand SC was investigated. Control diets con tained 20% egg-white protein,6 8% fat from corn oil ( or corn oil plus the oil from 2O'°Calcium H the soy product), "i>%cellulose, 5% vitamin carbonate"Soy mix,7 3.3% zinc-free salts8 (adjustment of 1g/100 g diet on an as is basis. *Soy flour calcium and phosphorus levels were made (38.1%), soy beverage (39.2%) or soy concentrate where necessary to equalize these between (26.6%) were added to supply 18% protein. 3 Vi diets) and variable amounts of glucose to tamin Free Test, Teklad Test Diets, Madison, Wisconsin, added to provide 18% protein. 4 Corn complete the diets. The control diets were oil was added to make soy and casein diets isosupplemented with 0, 2, 4, 6 or 8 ppm zinc energetic in each experiment. In the first study, as the carbonate. To test for the availability 0.3 and 9.0% corn oil was added to the soy flour of zinc in the soy products, similar levels and casein diets, respectively. In the second study, 0.0, 9.5 and 9.5% corn oil was added to soy beverage, of zinc were obtained by substituting the soy concentrate and casein diets, respectively. 6 Mineral mix modified from Kenney, A. D. & soy product for egg-white on an equivalent protein basis. The third zinc experiment Munson P. L. (1959) Endocrinology 64, 513521. Both diets contained: CuSO4, 14.6 mg/kg; tested the effect of the presence of SC on FeC„H5O7 XH2O (16% Fe), 1.172 g/kg; MnSO, the availability of added zinc carbonate. •H,O,192 mg/kg; KI, 2 mg/kg; NaCl, 6.009 The basal egg diet contained 3 ppm zinc g/kg; MgSO4, 2.131 g/kg diet. In addition, soy and casein diets contains ZnCOs, 160 and 113 from zinc carbonate and the basal soy diet mg/kg; K,HPO4, 18.325 and 13.744 g/kg; NaHPO, contained 5 ppm zinc from soy concentrate. •HjO,3.580 and 2.685 g/kg diet, respectively. Zinc carbonate was added to both diets to «Corn starch. 'Teklad #40060: mg/kg dry provide an additional 2, 4, 6 or 8 ppm zinc. diet: p-Amino-benzoic acid, 110.132; ascorbic acid, Experiments with magnesium. Isoener 1016.604; biotin, 0.441; Bi2, 29.736; calcium pantothenate, 66.709; choline dihydrogen citrate, getic, isonitrogenous 20% protein diets 3496.916; folie acid, 1.982; i-Inositol, 110.132; were fed to rats for 17 (SF study) or 21 menadione, 49.559; nicotinic acid, 99.119; pyridoxine-HCl, 22.026; riboflavin, 22.026; thiamin days ( SB and SC study ). The bioavailabil •HC1,22.026; units/kg dry diet: retinyl acetate, ity of magnesium from SF, SB and SC was (dry) 5947.2 R.E.; ergocaliferol, (dry) 2202.5 IU; investigated. Casein diets contained 20% tocopherol acetate, 121.15 IU. »Solka Floe, Brown Co., Berlin, New Hampshire. 9Ajinoprotein as casein and were supplemented moto Co., Inc., Tokyo. l° Added to bring diets to provide from 50 to 260 to 325 ppm mag up to 100 g. u In the first study calcium as nesium as the carbonate.9 Similar levels of calcium carbonate was added to the soy flour diet magnesium were obtained in the soy diets (containing 0.06% Ca) and to the Casein diet by substituting soy products for casein at (containing 0.0005% Ca) at a rate of 0.0, 0.2, 0.4 or 0.6% dietary calcium. In the second experiment, equivalent protein levels. In the soy flour calcium was added to soy beverage (containing experiment excess magnesium to provide a 0.08% Ca), soy concentrate (containing 0.06% Ca) or casein diets to give 0.08, 0.18, 0.28 or 0.68 dietary calcium.
spectrophotometryv' Blood was obtained from the abdominal aorta of anesthetized animals in the magnesium studies and mag nesium concentration of the serum was de termined directly via atomic absorption analysis.
Downloaded from https://academic.oup.com/jn/article-abstract/109/9/1652/4770873 by University of Glasgow user on 30 July 2018
51'erkin Elmer, Norwalk. Connecticut, Model 306. 6 Seymour Foods. Topeka. Kansas. 'Cerklewskl and Forbes, J. Nutr. 106, 778 (1976). In the zinc experiments biotin was added to provide 4 mg/kg of diet. 8 Contained in g/kg mix : NaCl, 130.5 ; K-CO8, 95.3 ; K..SO,, 54.2 ; K2HPO4, 92.0 ; CaHPO,, 337.5 : CaCO3, 202.2 ; MgCOs, 64.1 ; Ferric citrate, 19.2 ; MnSO.-H2O, 4.5: CuSO4-5H2O. 0.65; KIOS, 0.02; Na2SeO3, 0.06. "Other basal diet constituents for magnesium studies were the same as described for zinc studies except that MgCO3 was deleted and 2nCOs was added to the salt mix to provide 20 ppm zinc In the diet.
BIOAVAILABILITY OF MINERALS IN SOY DIETS
1655
in serum or bone magnesium in response to these diets was attributable to mag nesium and not to some other dietary in fluence. Experiments with calcium. Since the cal cium content of soybean is quite low,10 it was not practical to test for the bioavailability of calcium in soy products. Instead, two experiments were performed to test for the effect of the presence of SF, SB or SC upon the bioavailability of added calcium. Isoenergetic, isonitrogenous 18% protein diets (see table 2) were fed to rats for 4 weeks. The protein source was casein or one of the three soy products. In the first experiment, 0, 0.2, 0.4, or 0.6% dietary calcium as the carbonate, was added to a casein basal diet or to a soy flour basal diet. In the second experiment, enough calcium carbonate was added to casein, SB or SC basal diets to provide 0.08, 0.18, 0.28 or 0.68% total dietary calcium.
©
PPM ZINC ADDED
Fig. 1 Regression of weight gain (fig. 1A, 1C and IE) and log tibia zinc (fig. IB, ID and IF) upon added dietary zinc. Figures 1A to ID demonstrate the effect of zinc added as zinc car bonate to egg-white protein diet (Q) or added as soy beverage ( •) or soy concentrate ( A ) to egg-white protein on an equal protein basis. Figures IE and IF show the effect of zinc added as zinc carbonate to an egg-white protein ( O ) or to a diet containing 63% of its protein as soy concentrate ( A ). Each data point represents the average response of five animals.
total of 650 ppm was added as the car bonate to the casein and to the highest soy flour diet to ensure that any differences
Downloaded from https://academic.oup.com/jn/article-abstract/109/9/1652/4770873 by University of Glasgow user on 30 July 2018
RESULTS Experiments with zinc. In the experi ments on zinc availability, weight gain and log tibia zinc content responded linearly to increased dietary zinc concentration. The regressions of these criteria on dietary zinc for soy beverage are shown in figures 1A and IB. The ratios between the slopes ob tained for zinc added as contained in soy beverage and as ZnCO^ were 0.63 ( t = 4.6, P < 0.001 ) for weight gain and 0.40 (t = 5.4, P < 0.001) for log tibia zinc. Similarly derived data for soy concentrate were 0.41 ( t - 6.4, P < 0.001 ) for weight gain and 0.20 (i = 5.6, P < 0.001) for tibia zinc as shown in figures 1C and ID. Zinc added to the soy concentrate as the car bonate produced slope ratios of 0.77 ( t = 2.63, P < 0.02 ) for weight gain and 0.70 (t = 2.36, P < 0.05) for tibia zinc as shown in figures IE and IF. There were no significant differences be tween intercepts in either of the regression comparisons in zinc studies ( P < 0.05 ). Experiments with magnesium. In the ex periments on magnesium availability, serum magnesium concentrations and tibia magnesium content responded linearly to 10A diet providing 18% protein from soy would only contain 10% of the ratu requirement for cal cium.
1656
FORBES ET AL.
MgCOs were 0.96 (t = 0.8, N.S.) for serum magnesium concentration and 1.06 ( t = 0.6, N.S.) for tibia magnesium content. In the soy beverage experiment (fig. 2C and 2D) the slope-ratios were 1.02 (f = 0.04, N.S.) for serum magnesium and 1.04 (f = 0.13, N.S.) for tibia magnesium. Similarly derived data with respect to soy concen trate yielded slope ratios of 0.77 ( t = 2.05, P < 0.05) for serum magnesium and 0.80 (f = 2.00, P < 0.05) for tibia magnesium as shown in figures 2E and 2F. Weight gain of rats did not respond linearly to increased dietary magnesium concentration ( table 3). TABLE 3 Weight gain of rats in magnesiums studies1 DietSoyCaseinSoy
Mgppmflour
gaing69
study5511022033065060125250370700Wt ±2103±4105 ±1113±2118±282±3108 flourDiet
±4118±4117±8123±3Soy
studyCaseinSoy
beverage
±4121±5125±5118±568±2105±8120±6128±5126±7
beverage441141622032725811518524529270±4105 I5O
200
»0
100
PPM MAGNESIUM ADDED
Fig. 2 Regressions of serum magnesium ( fig. 2A, 2C and 2E ) and tibia magnesium ( fig. 2B, 2D and 2F) upon magnesium added to casein protein diet in the form of magnesium carbonate ( O ) or added as soy flour ( A ), soy beverage ( •) or soy concentrate ( A ) substituted for an isonitrogenous amount of casein protein. Each data point represents the average response of five animals.
increased dietary magnesium concentra tion. The regressions of these criteria on dietary magnesium for soy flour are shown in figures 2A and 2B. The ratios between the slopes obtained for magnesium con tained in soy flour and that added as
Downloaded from https://academic.oup.com/jn/article-abstract/109/9/1652/4770873 by University of Glasgow user on 30 July 2018
studyCaseinSoy concentrate ±4114±6130±6136
±7127±979±1113±6139±5147 concentrate601041562082626011216421826872
"Data points represent means (N = 5)±SEof weight gains of animals fed for 17 days (soy flour study) or 21 days (soy beverage and soy concen trate) study.
1657
BIOAVAILABILITY OF MINERALS IN SOY DIETS
Experiments with calcium. In the experi ments on the effect of protein source upon the availability of added calcium, femur calcium content responded linearly to in creases in dietary calcium concentration. The regression of this criterion on dietary calcium is shown in figures 3A and 3B. The ratios between the slopes obtained for cal cium added to soyflour ( fig. 3A ) and casein was 0.94 (f = 3.4, N.S.). Similarly derived data for soy beverage and soy concentrate (fig. 3B) were 1.06 (i = 0.70, N.S.) and 1.02 (t = 0.22, N.S.), respectively. Weight gain did not respond linearly to increased calcium concentration ( table 4 ). DISCUSSION Forbes and Parker (10) have previously demonstrated that zinc added to egg-white based diets in the form of full fat soy flour was significantly less utilizable for the rat than when zinc was added as zinc car bonate to the egg-white diets. In the cur rent study the bioavailability of SB and SC were tested utilizing identical condi tions. The slope-ratios of weight gain ( 0.63) and bone zinc (0.40) of SB (fig. 1A and IB ) were similar to the previous report for SF (0.55 for weight gain and 0.34 for /j.g femur zinc). The bioavailability of zinc from soy concentrate was poorest of the three soy products. Figures 1C and ID show that zinc added to egg-white based diets as soy concentrate was only utilized 41% as well for weight gain and 20% as well utilized for log /tg tibia zinc as was that added as zinc carbonate. ®
0 % TOTAL CALCIUM IN DIET
006 018 028
0'
XTOTAL CALCIUM IN DIET
Fig. 3 Regressions of femur calcium (fig. 3A and 3B) on percent total dietary calcium added as calcium carbonate to casein ( O ), soy flour ( A ), soy concentrate ( A ) or soy beverage ( • ) based diets. Each data point represents the aver age response of eight animals.
Downloaded from https://academic.oup.com/jn/article-abstract/109/9/1652/4770873 by University of Glasgow user on 30 July 2018
TABLE 4 Weight gain of rats in calcium studies1 Diet
Casein
Total diet
Wt. gain
Soy flour study O
122±4 0.20 235±14 254±8 0.40 0.60 257±7 Soy flour 189±3 0.06 0.26 230±11 238±8 0.46 0.66 239± 9 Soy concentrate and soy beverage study Casein 0.08 191±3 0.18 212±9 0.28 218±4 0.68 226±5 Soy concentrate 168±9 0.08 0.18 199±6 0.28 210±4 220±6 0.68 Soy beverage 0.08 173±5 202±6 0.18 0.28 212±5 218±7 0.68 1Data points represent means (N = 8) ±SEof rats fed for 28 days.
These data, showing a lower slope ratio for response to zinc in terms of bone zinc than in terms of weight gain are in agree ment with the data of Forbes and Parker (10). Zinc response is a more critical mea sure of zinc availability than is weight gain because in the former instance the response measured is the sum of two effects of ab sorbed zinc; one is the promotion of in creased tissue mass and the second is the promotion of increased zinc concentration in the tissue. In addition, the authors be lieve, in agreement with Momcilovic et al. (11), that bone zinc should be the better criterion since the response is linear over a wider span of zinc intake and the data are less susceptible to daily variation than is body weight. Forbes and Parker (10) also tested the effect of a given level of dietary SF ( 13% of diet) on the availability of added zinc carbonate. They found that weight gain and femur zinc responses to increasing levels of zinc carbonate were not influ enced by the presence of soy flour in the
1658
FORBES ET AL.
diet. In the current study, it was found that the presence of soy concentrate (18.7% of diet) in diets did result in a small but significant reduction in growth and bone zinc responses to added Zn (fig. IE and IF). The results from these zinc studies not only reveal that the zinc in these soy prod ucts was poorly available, but that zinc bioavailability varied from product to product and was poorest in the soy con centrate. Rackis and his colleagues (15, 16) have noted that zinc utilization from certain soy isolates was particularly low as compared to that from soybean meals. They speculated that differences in zinc bioavailability were due to the varying food processing conditions used to manu facture the soy protein products. The cur rent zinc studies amplify this concept. Magnesium bioavailability from the three soy products, in contrast to the bio availability of zinc, was very good. Mag nesium utilization from SF and SB was equivalent to the highly available inorganic source, magnesium carbonate. Only the magnesium from soy concentrate was less bioavailable than the inorganic source. Although Roberts and Yudkin (17) re ported that magnesium deficiency signs in rats could be aggravated by addition of sodium phytate to casein-based diets, Forbes ( 18 ) found that magnesium absorp tion, but not balance, was reduced in young rats fed isolated soy protein diets in com parison to egg white protein diets. Guenter and Sell (19) determined that the bio availability of the mineral from soybean meal for chickens was 105% that of MgSO4. Recently, Lo and coworkers " re ported no significant difference in the bio availability of magnesium for rats fed in cremental levels of magnesium carbonate added to isolated soy protein, casein or lyophylized meat-based diet. These latter studies support the view that magnesium is highly available from soy protein products. The current studies clearly demonstrated that the bioavailability of calcium added as calcium carbonate to any of the three soy products was the same as when added to casein diets. These results suggest that calcium fortification of soy protein products
Downloaded from https://academic.oup.com/jn/article-abstract/109/9/1652/4770873 by University of Glasgow user on 30 July 2018
will result in good utilization of the min eral. However, as pointed out in previous publications (1, 2, 6, 7), calcium addition may cause or accentuate poor utilization of zinc, magnesium or copper from soy prod ucts which are high in phytic acid. Soybean products usually contain con siderable quantities of phytic acid. Litera ture values (3, 20) range from 1.4 to 2.2% phytic acid on a dry basis. Soybean prod ucts also can contain appreciable amounts of dietary fiber, especially if the hull is not removed prior to processing. Defatted soymeal produced from whole soybean con tains about 6% fiber; about %coming from the hull (21). Both phytic acid and com ponents of dietary fiber are said to reduce mineral absorption ( 1-9 ). The phytate ion can complex with metal lic ions such as zinc, calcium, and iron and form highly insoluble and poorly absorbed chelate complexes (1-5). The combination of metal ions such as calcium and zinc forms even less soluble complexes ( 1, 2, 6, 7). Some researchers have added pure sodium phytate to food systems and have found reduced mineral uptake. For exam ple, Davies and Nightingale (22) found that the addition of sodium phytate to yield \% dietary phytic acid greatly reduced rat growth and absorption of zinc, iron, cop per and manganese. This type of investiga tion reveals little insight into the chemical binding of naturally occurring phytic acid, especially when excessive levels of sodium phytate are added to food systems. How ever, most studies suggest an inverse re lationship between phytic acid and mineral absorption. In the current study soy concentrate yielded the poorest bioavailability re sponses of the three soybean products. Phytate content and the phytate to zinc molar ratio was highest in this product (see table 1). This suggests that phytate content is responsible for the poor mineral utilization of soy concentrate. At this time one should not directly equate the phytate concentration with poor mineral bioavaila bility in phytate-containing foods since " Lo. G. S., Collins, D. W., Steinke, P. H. & Hop kins. D. T. (1978) Effect of Isolated soybean pro tein on bloavailablllty of mapneslum. Federation Proc. il, 667 (Abstr.).
BIOAVAILABILITY
OF MINERALS
other factors such as the food processing history may be very important. The type of phytate-protein-mineral complexes formed during processing rather than the specific phytate concentrations may be responsible for reduced mineral absorptive capacity in some soy products (3, 15, 16). More re search is needed to identify the processing steps that affect formation of phytate com plexes. Most components of dietary fiber act as monofunctional weak cation exchange resins (7). Studies involving in vitro tech niques (8, 9) demonstrate binding of zinc and iron to wheat bran, lignin, and some hemicellulose fractions of wheat bran and of calcium to non-cellulosic fractions of plant fibers. Although direct extrapolation of in vitro results to in vivo systems cannot be made (3, 7), many components of di etary fiber must be considered as potential reducers of mineral bioavailability. In this current work, it is not possible to evaluate the relative contributions of phy tate, components of dietary fiber or other endogenous chelators to the reduced zinc absorption. However, a recent study from our laboratory (23) demonstrated that in clusion of the soybean hulls (approxi mately 50c/(.of the dietary fiber and 5c/c of the phytic acid of the whole soybean) in soy flour-based diets had no significant effect upon the bioavailability of native zinc or of added calcium. This study sug gests that soy hull fiber plays no role in reducing mineral absorption, at least when fed at levels normally found in whole soy bean products. It can be concluded from this study that magnesium is highly available from soy flour and soy beverage. Magnesium bio availability from soy concentrate is good but less than from the other products. Zinc availability is poor from soy products, especially soy concentrate. The presence of soy concentrate in rat diets also some what reduces the bioavailability of added zinc. A previous study ( 10) showed that zinc added to diets containing soy flour was fully available. Calcium added to all soy products was fully available for utiliza tion. The particular conditions involved in processing soy concentrate or its elevated phytate to zinc molar ratio may result in
Downloaded from https://academic.oup.com/jn/article-abstract/109/9/1652/4770873 by University of Glasgow user on 30 July 2018
IN SOY DIETS
1659
reduced bioavailability of endogenous min erals and added zinc. ACKNOWLEDGMENTS
The authors would like to acknowledge the able technical assistance of Mrs. LuAnn Franzen and Mr. William R. Casebeer. LITERATURE CITED 1. Oberleas, D., Muhrer, M. E. & O'Dell, B. L. (1966) Dietary metal complexing agents and zinc availability in the rat. J. Nutr. 90, 56-62. 2. O'Dell, B. L. (1969) Effect of dietary com ponents upon zinc availability. Am. J. Clin. Nutr. 22, 1315-1322. 3. Erdman, J. W., Jr. (1979) Oilseed phytates: Nutritional Implications. J. Am. Oil Chem. Soc. (in press). 4. Maddaiah, V. T., Kurnick, A. A. & Reid, B. L. ( 1964 ) Phytic acid studies. Proc. Soc. Exp. Biol. Med. 115, 391-393. 5. Vohra, P., Gray, B. A. & Kratzer, P. S. ( 1965) Phytic acid-metal complexes. Proc. Soc. Exp. Biol. Med. 120, 447-449. 6. Forbes, R. M. (1960) Nutritional inter actions of zinc and calcium. Federation Proc. 19, 643-647. 7. Oberleas, D. & Harland, B. F. ( 1977) Nu tritional agents which affect metabolic zinc status. In: Zinc Metabolism: Current aspects in health and disease (Brewer, G. J. & Prasad, A. S., ed.), pp. 11-24, Alan R. Liss, Inc., New York. 8. Ismail-Beigi, F., Faroji, B. & Reinhold, J. G. ( 1977 ) Binding of zinc and iron of wheat bread, wheat bran and their components. Am. J. Clin. Nutr. 30, 1721-1725. 9. James, W. P. T., Branch, W. J. & Southgate, D. A. T. (1978) Calcium binding by ditary fibre. Lancet 25, 638-639. 10. Forbes, R. M. & Parker, H. M. (1977) Bio logical availability of zinc in and as influenced by whole fat soy flour in rat diets. Nutr. Rept. Int. 15, 681-688. 11. Momcilovic, M., Belonje, B., Giroux, A. & Shah, B. G. (1975) Total femur zinc as the parameter of choice for a zinc bioassay in rats. Nutr. Rept. Int. 12, 197-203. 12. Erdman, J. W. Jr., O'Connor, M. P., Weingartner, K. E., Solomon, L. W. & Nelson, A. I. ( 1977 ) Production, nutritional value and baking quality of soy-egg flours. J. Food Sci. 42, 964-967. 13. Nelson, A. I., Steinberg, M. P. & Wei, L. S. ( 1976) Illinois process for preparation of soy milk. J. Food Sci. 41, 57-61. 14. Steel, R. G. D. & Torne, I. H. (1960) Prin ciples and procedures of statistics. McGrawHill Book Comp., Inc., New York. 15. Rackis, J. J., McGhee, J. E., Honig, D. H. & Booth, A. N. ( 1975 ) Processing soybeans into foods: Selected aspects of nutrition and flavor. J. Am. Oil. Chem. Soc. 52, 249A253.
1660
FORBES ET AL.
16. Rackis, J. J. & Anderson, R. L. (1977) Mineral availability in soy protein products. Food Prod. Dev. 11, 38-44. 17. Roberts, A. H. & Yudkin, J. (1960) Dietary phytate as a possible cause of magnesium de ficiency. Nature 185, 823-825. 18. Forbes, R. M. (1964) Mineral Utilization in the Rat. III. Effects of calcium, phosphorus, lactose and source of protein in zinc defi ciency and in zinc adequate diets. J. Nutr. 83, 225-233. 19. method Guenter, forW.determining & Sell, J. L. availability (1974) A "true" of magnesium from food stuffs using chickens. J. Nutr. 104, 1446-1457 20. de Boland, A. R., Garner, G. B. & O'Dell, B. L. ( 1975 ) Identification and properties
Downloaded from https://academic.oup.com/jn/article-abstract/109/9/1652/4770873 by University of Glasgow user on 30 July 2018
of phytate in cereal grains and oilseed prod ucts. J. Agrie. Food Chem. 23, 1186-1189. 21. Smith, A. K. & Circle, S. J. (1972) Chemi cal composition of the seed. In: Soybeans: Chemistry and technology (Smith, A. K. & Circle, S. J., éd.), pp. 61-92, AVI Pubi. Comp., Inc., Westport, Conn. 22. Davies, N. T. & Nightingale, R. (1975) The effects of phytate on intestinal absorption and secretion of zinc, whole-body retention of Zn, Copper, Iron and Manganese in rats. Br. J. Nutr. 34, 243-258. 23. Weingartner, K. E., Erdman, J. W., Jr., Parker, H. M. & Forbes, R. M. (1979) Ef fect of soybean hull upon the bioavailability of zinc and calcium from soy flour-based diets. Nutr. Repts. Int. 19, 223-231.
