9 1992 by The Humana Press, Inc. All rights of any nature, whatsoever, reserved. 0163-4984/92/3401~0067 $02.20

Absorption, Endogenous Excretion, and Balance of Zinc in Growing Rats on Diets with Various Sugars Replacing Starch E. WEIGAND 1 AND M. KIRCHGESSNER *'2 llnstitut fiJr Tierern~hrung, Justus-Liebig-Universit~t, 6300 Gie/3en, Germany; and 2lnstitut fhr Embhrungsphysiotogie, Technische Universit~it /Vlhnchen, 8050 Freising-Weihenstephan, Germany Received July 15, 1991; Accepted August 1, 1991

ABSTRACT The effect of partially replacing starch for various sugars on the apparent and true absorption, endogenous excretion, and balance of zinc was investigated in a study with growing rats. Six groups of five or six animals with an initial live weight of 39.4 _+ 2.7 g were fed diets that had the same Zn content (22 mg/kg), but differed in the sugar content: 1. 2. 3. 4. 5, 6.

Starch only (56%); Glucose (15%); Fructose (15%); Sucrose (30%); Galactose (15%); and Lactose (30%).

At the start of a 15-d fecal and urinary collection period, each animal was given an intramuscular injection of 380 kBq 65 Zn for estimating e n d o g e n o u s Zn excretion by isotope dilution. The ratio of the specific activity of fecal Zn (after 12 d) to that of urinary Zn (after 9 d) was applied to reflect the ratio of *Author to whom all correspondence and reprint requests should be addressed.

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endogenous to total fecal Zn collected from day 10 to 15. This ratio averaged 0.59, without significant differences among treatments. For this period, apparent and true absorption averaged 87.1 and 94.7% of Zn intake, respectively, and did not significantly differ among diets. Urinary excretion of 65Zn and of stable zinc by the galactose-fed rats was markedly higher than that by the other animals. Their Zn balance was, per unit weight gain, comparable with that of the other groups (30.7 vs 28.2 to 30.2 ~g/g). Index Entries: Absorption, of zinc in rats, true and apparent; balance, of zinc, influence of sugars; carbohydrates, influence on Zn metabolism, sugars; excretion, of endogenous zinc, of zinc in feces, in urine; zinc, absorption, balance, excretion, influence of sugars.

INTRODUCTION Carbohydrates compose major portions of normal diets. A number of studies have shown that the type of dietary carbohydrate, starch or various sugars, may affect absorption, tissue retention, and metabolism of several trace minerals, such as chromium (1), copper (2), iron (3), lead (4), manganese (5), and zinc (6-9). Such effects are most likely to become evident when the dietary supply or availability of the trace mineral is low. It was the aim of the present study to examine the influence of partially replacing starch for various sugars as dietary carbohydrate sources on true absorption, endogenous excretion, and balance of zinc in growing rats fed diets with a marginally adequate Zn content.

MATERIALS AND METHODS Thirty-two weaned male Sprague-Dawley rats with an initial mean live weight of 39.4 _ 2.7 g were allocated to one of six treatment groups with either five or six animals each. They were kept individually in metalfree metabolism cages and were offered the experimental diets in three meals daily, allowing sufficient time to ensure ad libitum consumption. Deionized water was available at all times. All diets were composed of 25.0% casein, 0.2% DL-methionine, 8.8% coconut fat, 4.0% cellulose, 4.0% mineral premix, and 2.0% vitamin premix. Components of the premixes were the same as used previously

(10). Diets differed in the content and type of sugar. The control diet contained 56.0% maize starch without sugar. For three groups, 15.0% of the starch was substituted for D-glucose, D-fructose, or D-galactose, and for another two groups 30% of the starch was substituted for sucrose Biological Trace Element Research

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(equivalent to 15% fructose + 15% glucose) or lactose (equivalent to 15% galactose + 15% glucose). The Zn concentration of all diets, which were fed in the form of dry crumbles (11), was 22 mg/kg dry matter. One-third of this amount had been added as zinc sulfate. The experiment was divided into a 3-d adjustment period and a 15-d balance period without change in dietary regimen. At the start of the balance period (day 0), each rat was given an im injection of 25 I~L saline solution containing 380 kBq 65Zn and about 40 ~g stable zinc. The injection site was the thigh of the left hind leg. Feces and urine were collected separately from each rat in 3-d intervals throughout the 15 d after 65Zn dosing. After 3, 9, and 15 d, the cages were rinsed with deionized water to remove remaining urine. This wash water was analyzed for radiozinc separately from urine samples, and urinary excretion of stable zinc was corrected accordingly. In the groups fed the galactose- and lactose-containing diets, most rats had intermittently soft feces, which tended to adhere to the collecting funnels. Care was taken to collect all adhering feces by wiping with ash-flee filter paper. At the end of the balance period, the rats were anesthetized with ether and bled by decapitation. Digestive tract, liver, pancreas, and kidneys were removed. All preparations were stored at -20~ until analysis. The contents of stable zinc in diets, feces, urine, and body organs were determined by atomic absorption spectrometry after dry-ashing and preparing 0.6M hydrochloride solutions as described by Weigand and Kirchgel3ner (12). The 65Znactivity of these solutions was measured by ~-scintillation spectrometry against blanks and standards to account for decay and counting efficiency. Endogenous fecal Zn and, hence, true Zn absorption were determined by the concept of isotope dilution following the previously described approach (12,13). The proportion of endogenous zinc in feces collected during the last 6 d (day 10 to 15) of the trial was estimated as the ratio of the specific activity of fecal zinc after 12 d to that of urinary zinc after 9 d. The specific activities for these times were obtained, separately for each rat, by regression analysis of the observed values from the four 3-d collection periods covering day 4 to 15. The regression models applied were: for fecal Zn, In Y = a + bX, and for urinary Zn, In Y = a + b In X, where Y denotes the specific activity and X the time elapsed after 65Zn injection in days. This concept assumed that endogenous zinc in feces was excreted with a delay of 3 d, since it was noted that the maximum radiozinc excretion in feces was not reached until about 3 d after injection of the isotope. The results were evaluated by analysis of variance and monovariate regression analysis by standard methods. The Newman-Keuls' test (14) was applied to differentiate among means for variables that showed a significant treatment effect. Biological Trace Element Research

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Biological Trace Element Research

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Zinc Absorption, Excretion, and Balance

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RESOLTS Results summarized in Table 1 show that live weight gain, feed/gain ratio, and digestibility of the organic matter were significantly influenced by treatments, whereas feed intake was not. The rats of the starch group and sucrose group had, on the average, the highest daily live weight gain, feed intake, and organic matter digestibility, and the lowest feed/ gain ratio. These criteria significantly differed from those recorded for the galactose group, except for feed intake. Zn balance data over the 15-d treatment period are shown in Table 2. Overall, Zn intake averaged 148 bLg/d without significant differences among dietary groups. The mean daily fecal excretion of stable zinc ranged from 14.9 ~g in the starch group to 21.0 bLg in the lactose group and was significantly different between these groups. In most groups, urinary Zn loss was higher than fecal Zn excretion and was also significantly influenced by diets. The highest daily Zn excretion in urine was recorded for the galactose-fed group (33.2 ~g), and the lowest for the lactose-fed group (20.8 I~g). The percent of apparent Zn absorption was markedly lower for the rats fed the diets with galactose or lactose than for those fed the other diets, with the exception of those fed the sucrose diet. Zn balance averaged 63.5% of intake in the case of the galactose group vs 70.6-73.1% in the case of the other groups. Based on live weight gain, however, Zn balance of the galactose-fed rats was comparable with that of the others. Data on 65Zn excretion presented in Table 3 show that an overall mean of 8.1% of the 65Zn dose was recovered in feces and 16.2% in urine in the course of the 15-d trial. Fecal 6SZn excretion ranged from 6.2% in the starch group to 9.1% in the lactose group, but failed to be significantly different because of considerable variation within treatments. The animals fed the galactose-containing diet voided about 20% of the 65Zn load via the kidneys. Accordingly, total 65Zn excretion by this group amounted to 29.0% of the dose vs 22.2-24.3% for the other groups. This galactose effect on the renal 65Zn lOSS was evident throughout the entire balance period. The initial 65Zn excretion in feces was recorded in about quarter-day intervals. Peak activity was, on the average, not reached before the third day after 65Zn dosing. This reflects a delay in the appearance of zinc from endogenous pools in feces as compared with its immediate appearance in urine. Overall, feces contained an average of about 3.0% of the dose during the first 3 d and 2.4% during the next 3 d. For urine, the corresponding values were 7.7 and 3.2% of the 65Zn dose. According to results shown in Table 4, there was no marked treatment effect on the specific activity of zinc in the visceral organs analyzed. Zinc in the kidneys and pancreas was labeled to a lower extent than that in the liver and residual carcass. In urine and feces, the specific activity of zinc markedly decreased from one collection period to the next. Table 4 Biological Trace Element Research

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Table 3 Mean 65Zn Excretion in Feces and Urine in % of the Dose (Day 0-15) 65Zn excretion

Dietary group Starch Glucose Fructose Sucrose Galactose Lactose

Feces 6.2 8.2 7.9 8.2 8.8 9.1

Overall mean 8.1 Residual SEt 1.9 Sign. of treatment~ ns ~See footnotes to Table 1.

Urine 16.8"'b'7 16.1''~' 15.8I' 14.0b 20.2" 14.3~' 16.2 2.2 P < 0.01

Total 23.0I' 24.3b 23.7I' 22.2b 29.0" 23.4l' 24.3 2.2 P < 0.01

presents mean values for the specific activity of zinc in urine 9 d and in feces 12 d after 6SZn injection and the mean ratio between these values. The urine values after 9 d were not very different from the specific activities observed for the kidneys and pancreas at the end of the 15-d trial. For the final 6 d of the 15-d balance period, endogenous fecal zinc and, hence, true Zn absorption were estimated (Table 5). The highest amounts of total and endogenous zinc in feces were observed for the groups fed the galactose- and lactose-containing diets, however, without being significantly different from those recorded for the other groups. Also, the percent of apparent and true Zn absorption did not greatly vary among groups. Overall, apparent absorption averaged 87.1% and true absorption 94.7% of Zn intake.

DISCUSSION Zinc balance is subject to homeostatic control attained by varying the absorption efficiency and the fecal excretion of zinc from endogenous pools (10,15,16). Concerning the present study with growing rats, the data shown in Table 5 indicate that neither the percentage of true absorption nor the rate of endogenous fecal excretion was materially modified by the choice of the dietary carbohydrate source. This was evidently also the case during the entire 15-d balance period, since the fecal excretion of the injected 65Zn (Table 3) and the specific activity of tissue zinc at the end of the experiment (Table 4) did not markedly vary among groups. The excretion of parenterally administered radiozinc is generally a sensitive criterion to changes in Zn status (15,17,18). This is also true for the specific activity of tissue zinc, especially in the range of deficient to marginal Zn supply (13). Biological Trace Element Research

Vol. 34, 1992

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Absorption, endogenous excretion, and balance of zinc in growing rats on diets with various sugars replacing starch.

The effect of partially replacing starch for various sugars on the apparent and true absorption, endogenous excretion, and balance of zinc was investi...
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