Dietary phytate reduction improves zinc absorption in Malawian children recovering from tuberculosis but not in well children

High dietary phytate content that compromises zinc nutriture is thought to be a major problem among children of the developing world. Zinc stable isotope techniques permit the quantitative assessment of the effect of phytate reduction on zinc homeostasis. We tested the hypothesis that zinc absorption would be increased in Malawian children fed a reduced-phytate corn-plus-soy diet compared with a standard high phytate diet. Twenty-three children hospitalized in Blantyre, Malawi, were enrolled. Children were selected from those recovering from tuberculosis and from well children (those with minor injuries, those awaiting elective surgery or healthy siblings). Children received a diet of corn-plus-soy porridge (either low phytate or high phytate) for a period of 3-7 d and then participated in a zinc stable isotope study. The study included the administration of oral and intravenous zinc stable isotopes and 7-d collections of urine and stool. The diet was maintained throughout the duration of specimen collection. Zinc isotopic enrichments in urine and stool were measured, and zinc fractional absorption, total zinc absorption, endogenous fecal zinc, net zinc retention and size of the exchangeable zinc pool were calculated. Among the 14 children recovering from tuberculosis, dietary phytate reduction resulted in higher fractional absorption (0.41 +/- 0.14 versus 0.24 +/- 0.09, mean +/- SD, P: < 0.05) and total zinc absorption (169 +/- 55 versus 100 +/- 46 microg/(kg. d), P: < 0.05). No effect of phytate reduction was seen in the well children (n = 9). Phytate reduction did not decrease the absolute endogenous fecal zinc, but it did decrease it relative to total absorbed zinc. These preliminary results indicate that phytate reduction may be beneficial in improving zinc nutriture in groups with increased zinc requirements who consume a cereal-based diet.     

Effect of dietary phytate on zinc homeostasis in young and elderly Korean women

BACKGROUND: Previous studies suggest that consumption of predominantly plant-based diets with high phytate content contribute to zinc deficiency by inhibiting zinc absorption. Age of the individual may also affect the ability to maintain zinc homeostasis. OBJECTIVE: This study was designed to determine the effect of dietary phytate on zinc homeostasis and to evaluate the effect of age on the capacity to maintain the zinc homeostasis with changes in dietary phytate in young and elderly Korean women. DESIGN AND METHODS: Seven healthy young women (22-24 yr) and 10 healthy elderly women (66-75 yr) were studied consecutively for 3 months in 2 metabolic periods (MP) in two different metabolic units. During MP1 the women consumed a high phytate (HP) diet (P:Zn molar ratio = 23) for 9 days. After a 10 d wash-out period at home eating their usual diets, a lower phytate diet (LP) (P:Zn molar ratio = 10) was fed in MP2 for 9 d. Phytase was added to selected foods in the high phytate diet to reduce the phytate content of the meals in the LP period. The zinc content of both diets was about 6.5 mg/d. Stable isotopes of Zn ((70)Zn) were administered intravenously on d 5 of MP 1 and 2 for measuring endogenous fecal zinc excretion. Plasma samples were also collected on d 5 for measuring plasma zinc concentrations by Inductively Coupled Plasma-Atomic Emission Spectrometry (ICP-AES). 24 hr urine samples were collected for 5 d and complete fecal samples were collected for 9 d after isotope administration. Fractional zinc absorption (FZA) was calculated from mass balance corrected for endogenous fecal zinc (EFZ) excretion and EFZ was determined by using an isotopic dilution technique. Isotopic ratios for FZA and EFZ were measured by Inductively Coupled Plasma-Mass Spectrometry (ICP-MS). Statistical analyses were done using ANOVA. RESULTS: Both the young and elderly women were in negative zinc balance during the HP period. This was due to a significant decrease in FZA and total absorbed zinc (TAZ) with a HP diet (43 vs 22% in young women, 34 vs 20% in elderly women, p < 0.001). EFZ excretion did not differ in the young and elderly women during the LP and HP periods. Dietary phytate did not alter plasma zinc concentrations or and urinary zinc excretion in either group. CONCLUSIONS: Adjustments in zinc homeostasis with an increase in dietary phytate did not differ between young and elderly women in this study.     

Fecal phytate excretion varies with dietary phytate and age in women

OBJECTIVE: Information on the excretion of dietary phytate in humans under different conditions is limited. The purpose of this study was to investigate fecal excretion of dietary phytate and phosphorus in a group of young and elderly women consuming high and low phytate diets. METHODS: Fifteen young and fourteen elderly women were fed two experimental diets, high phytate and low phytate, for 10 days each with a washout period of 10 days between the two diet periods. Duplicate diet samples from two different menus and complete fecal samples were collected for 5 days during each diet period and analyzed for phytate and phosphorus contents. Mean daily excretions and percentages of dietary intakes of phytate and phosphorus were calculated. RESULTS: Dietary phytate level does impact phytate excretion, but the effect was observed only in young subjects. Fecal phytate excretion of young subjects during the high phytate diet (313 mg/d) was significantly higher than during the low phytate diet (176 mg/d), however, that of elderly subjects did not vary with dietary phytate levels. Phosphorus excretion, net absorption, and apparent absorption rate were affected by dietary phytate level but not by the age of the subjects. CONCLUSIONS: Results of this study indicate that phytate degradation in the gastrointestinal tract is substantial and more variable in young women than in elderly women. The high capacity of phytate degradation in elderly subjects may be related to long-term phytate intake but needs further clarification. Both beneficial and adverse health effects of phytate need to be studied considering the long-term phytate intake and age of subjects as well as dietary phytate levels.     

A model to produce pure zinc deficiency in rats and its use to demonstrate that dietary phytate increases the excretion of endogenous zinc

An early effect of zinc deficiency in rats is loss of appetite with consequent malnourishment. To obviate the need for pair-feeding, rats were fed liquid semipurified diets by gastric tube feeding. Rats tube fed a zinc-deficient diet at a daily rate of 90-110 g/kg thrived for 6-7 days and then rapidly became seriously ill. In contrast rats fed the deficient diet ad libitum stopped growing after 3 days but remained relatively healthy. Animals tube fed a zinc-replete diet or the deficient diet with subcutaneous injections of zinc grew normally and suffered no ill effects. Rats tube fed the deficient diet and supplemented parenterally with zinc excreted significantly more endogenous zinc into small intestinal luminal washings and into feces than unsupplemented rats. Sodium phytate, added to the tube feed in three groups of rats fed the deficient diet and supplemented daily with 0, 0.33 and 3.3 mg Zn/kg, significantly increased the zinc content of luminal washings in all three groups, increased fecal zinc excretion in two groups and lowered body zinc levels, estimated by femur zinc, in two groups. Tube feeding provides a means to study: 1) pure zinc deficiency without malnourishment of other nutrients; and 2) the excretion of endogenous zinc into the gastrointestinal tract.     

Major variables of zinc homeostasis in Chinese toddlers

BACKGROUND: Measurement of the major variables of zinc homeostasis is an essential prerequisite for estimating human zinc requirements, which currently require a factorial approach. The data required for this approach have not been available for toddlers, whose requirements have been estimated by extrapolation from other age groups. OBJECTIVE: The objective of the study was to measure key variables of zinc homeostasis in rural and small-town Chinese toddlers. DESIGN: Zinc stable-isotope tracers were administered intravenously and orally with all meals for 1 d to 43 toddlers. Subsequent metabolic collections in the homes included duplicate diets, quantitative fecal collections, and spot urine sampling. Fractional absorption of zinc (FAZ) was measured by a dual-isotope tracer ratio technique, and endogenous fecal zinc (EFZ) was measured by an isotope dilution technique. RESULTS: No group or sex differences were found. Therefore, results were combined for 43 toddlers aged 19-25 mo whose major food staple was white rice. Selected results (x+/- SD) were 1.86 +/- 0.55 mg total dietary Zn/d; 0.35 +/- 0.12 FAZ; 0.63 +/- 0.24 mg total absorbed Zn/d; 0.67 +/- 0.23 mg EFZ/d; and 65.0 +/- 8.3 microg plasma Zn/dL. The molar ratio of dietary phytate to zinc was 2.7:1. CONCLUSIONS: The mean intake and absorption of zinc in this population are low in comparison with estimated average dietary and physiologic requirements for zinc, and plasma zinc values are consistent with zinc deficiency. Intestinal losses of endogenous zinc exceed previous estimates for toddlers, and only modest evidence exists of conservation in response to low zinc intake and absorption.     

Adaptation in human zinc absorption as influenced by dietary zinc and bioavailability

BACKGROUND: An understanding of the relations among dietary zinc intake, bioavailability, and absorption is necessary for making dietary intake recommendations. OBJECTIVES: We aimed to assess adaptation in human zinc absorption to controlled differences in zinc and phytate intakes and to apply the results to predictive models. DESIGN: In 3 experiments, radiotracers were used to assess zinc absorption by healthy adults (n = 109) from controlled diets, before and after 4 or 8 wk of dietary equilibration. Subjects consumed 4-29 mg Zn/d from 1 of 10 diets, 5 with molar ratios of phytate to zinc from 2 to 7 and 5 with ratios from 15 to 23. RESULTS: Absorptive efficiency was inversely related to dietary zinc from both low- and high-phytate diets. In response to low zinc intakes (<11 mg/d) for 4-8 wk, zinc absorption was up-regulated to as high as 92%, but only if the diets were low in phytate. The results help validate and refine a published saturable transport model that predicts zinc absorption from dietary zinc and phytate. Possible biomarkers of impaired zinc status, including erythrocyte osmotic fragility, in vitro erythrocyte (65)Zn uptake, and leukocyte expression of the zinc transport proteins Zip1 and ZnT1, were unresponsive to dietary zinc content. CONCLUSIONS: Humans absorbed zinc more efficiently from low-zinc diets and adapted to further increase zinc absorption after consuming low-zinc, low-phytate diets for several weeks. Such adaptation did not occur with higher phytate diets. Zinc absorption can be predicted from dietary zinc and phytate after allowing for dietary equilibration.     

Phytate:zinc and phytate X calcium:zinc millimolar ratios in self-selected diets of Americans, Asian Indians, and Nepalese

The literature indicates that phytate:zinc and phytate X calcium:zinc millimolar ratios of a diet may provide useful indexes of the bioavailability of dietary zinc. However, there is little information on the phytate:zinc and phytate X calcium:zinc millimolar ratios for total human diets. We have therefore determined the phytate:zinc and phytate X calcium:zinc millimolar ratios of self-selected diets of 29 American omnivores, 23 American vegetarians, 30 Asian Indian immigrant vegetarians, and 26 Nepalese lactating vegetarians. Criteria for selection of subjects were: good health, no extreme dietary habits, and no intake of nutrient supplements. According to the limited literature, the suggested critical values for phytate:zinc and phytate X calcium:zinc millimolar ratios in animal diets and retrospective calculations from human diets are greater than 10 and greater than 200, respectively. The mean phytate:zinc and phytate X calcium:zinc millimolar ratios of American omnivorous diets in the present study were less than 10 and 200, respectively. Confirmation of the critical molar ratios as indexes of the bioavailability of zinc in human diets has not been established by experimentation. However, if the data from animal studies are applicable to human diets, the present study suggests that phytate has little influence on zinc bioavailability of most American diets. In contrast, the mean phytate:zinc and phytate X calcium:zinc millimolar ratios of all vegetarian diets were above the proposed critical levels. Those data, therefore, suggest that phytate might increase the risk of impaired zinc bioavailability for vegetarians consuming a relatively high level of calcium.     

[Phytate][calcium]/[zinc] ratios in Asian immigrant lacto-ovo vegetarian diets and their relationship to zinc nutriture

Lacto-ovo vegetarians often have low zinc intakes coupled with concomitant high intakes of dietary fiber, phytate and calcium. The latter two dietary components appear to exert a synergistic inhibitory effect on zinc absorption. In this study 30 1-day diet composites were prepared for analysis from 3-day weighed diet records completed by 112 Asian immigrants consuming predominantly lacto-ovo vegetarian diets and compared with 30 omniverous diet composites. Analysed mean (±S.D.) daily intakes were: energy (kcal) 1892±445, calcium (mg) 1093±467, zinc (mg) 8.5±4.5, phytate (mg) 1487±791, neutral detergent fiber (g) 26.8±13.2. Median phytate: Zn molar ratio for Punjabi diets was 17.7 (range=5-50) and median [phytate][Ca]/[Zn] molar ratio was 475.4 (range=48-1150) compared with median values of 8.3 and 130.9 respectively for omniverous diets. Thirty-seven percent of the Punjabi diets had phytate:Zn ratios>20 and 80% had [phytate][Ca]/[Zn] ratios>200, levels shown to jeopardize zinc status in man. Thirty-two percent of the Punjabi subjects had low serum zinc levels (ie<70 g/dl) which may be associated with the unusually high dietary [phytate][Ca]/[Zn] ratios. The latter are attributed to high intakes of unleavened wholemeal chapatti bread, dairy products, and low intakes of flesh foods.     

Influences of dietary protein levels and phytate contents on zinc requirement in rats

This study investigates whether the effects of increased dietary levels of soy protein on zinc-deficient rats are due to the dietary protein content. Rats were fed two levels of demineralized soy protein (DP) diets and two levels of egg albumin (EA) diets including 7 ppm zinc (Experiment 1). Growth was depressed in rats fed a 20% DP diet (0.43% phytate) but not in those fed a 10% EA diet containing 7 ppm zinc. Zinc concentrations in the serum, femur and kidney were lower in rats fed the 20% DP diet than those fed a 10% DP (0.20% phytate) diet, and they were also lower in rats fed the 10% EA diet than those fed a 5% EA diet. Zinc concentrations in the serum and femur of rats fed the 10% DP diet or the 20% DP diet were decreased compared with those fed the 5% EA diet or the 10% EA diet, respectively. Under zinc-deficient conditions (< 0.4 ppm Zn), the survival time shortened (Experiment 2) and the zinc concentration in the serum and femur decreased faster (Experiment 3) in rats fed the 10% EA diet compared with those fed the 5% EA diet. The survival times and time courses of these parameters show that the zinc requirements of rats increased with the dietary protein level. The increased zinc requirement of rats that accompanied increasing dietary soy protein was due to the dietary protein content as well as the dietary phytate content.     

Influences of dietary protein levels and phytate contents on zinc requirement in rats

This study investigates whether the effects of increased dietary levels of soy protein on zinc-deficient rats are due to the dietary protein content. Rats were fed two levels of demineralized soy protein (DP) diets and two levels of egg albumin (EA) diets including 7 ppm zinc (Experiment 1). Growth was depressed in rats fed a 20% DP diet (0.43% phytate) but not in those fed a 10% EA diet containing 7 ppm zinc. Zinc concentrations in the serum, femur and kidney were lower in rats fed the 20% DP diet than those fed a 10% DP (0.20% phytate) diet, and they were also lower in rats fed the 10% EA diet than those fed a 5% EA diet. Zinc concentrations in the serum and femur of rats fed the 10% DP diet or the 20% DP diet were decreased compared with those fed the 5% EA diet or the 10% EA diet, respectively. Under zinc-deficient conditions (<0.4 ppm Zn), the survival time shortened (Experiment 2) and the zinc concentration in the serum and femur decreased faster (Experiment 3) in rats fed the 10% EA diet compared with those fed the 5% EA diet. The survival times and time courses of these parameters show that the zinc requirements of rats increased with the dietary protein level. The increased zinc requirement of rats that accompanied increasing dietary soy protein was due to the dietary protein content as well as the dietary phytate content.     

Stable isotope studies of zinc absorption and retention in young and elderly men

Zinc absorption was determined with 67Zn and 70Zn, stable isotopes of zinc, in six young men and six elderly men who were confined to a metabolic unit for 12 wk. Their purified formula diets, supplemented with select food items, contained 15 mg of zinc per day. Zinc absorption was determined twice for each subject by combining zinc enriched with either 70Zn or 67Zn with the formula diet. Serum, urinary, and fecal zinc and zinc balance were determined by atomic absorption spectrometry. Zinc absorption averaged 17% in elderly men, significantly less than average zinc absorption of 31% in young men. Serum zinc was also lower in elderly men and increased in both groups during the course of the study. Zinc balance did not differ between groups, and endogenous zinc losses were less in the elderly than in the young men. The results suggest that while zinc absorption is less in elderly men than in young men, the lower absorption may reflect a lower requirement for absorbed zinc by the elderly. Alternatively, less efficient zinc absorption could result in decreased endogenous losses.     

Fermentation of soybean flour with Aspergillus usamii improves availabilities of zinc and iron in rats

Soybean flour was fermented with Aspergillus usamii to improve the availabilities of dietary zinc and iron through the degradation of phytate. Three kinds of experimental diets that differed in protein sources were prepared: one consisting of 40% fermented soybean flour (RS diet), one consisting of 40% fermented soybean flour (FS diet), and one consisting of 20% regular soybean flour and 20% fermented soybean flour (RF diet). Zinc solubilities in the upper and the lower segments of the small intestine were higher in rats fed the FS diet than in rats fed the RS diet. The FS group showed higher solubility of iron in the lower small intestine than the RS group did. Zinc concentrations in the femur and plasma and iron concentrations in the liver and plasma were higher in the FS group than in the RS group. These results suggested that the fermentation of soybean flour improved the availabilities of dietary zinc and iron, which may be induced by increasing the solubilities of these minerals in the small intestine through the reduction of phytate content. Femoral and plasma zinc concentrations in the RF group were higher than in the RS group, but lower than in the FS group. No difference was noted in liver and plasma iron concentrations between the RF group and the FS group. Although phytase activity in FS degrades phytate in the RF diet, higher activity may be needed to degrade phytate completely.     

Endogenous zinc excretion in relation to various levels of dietary zinc intake in the mink (Mustela vison)

Endogenous zinc excretion was studied in adult male mink fed experimental diets for 73 d, including a collection period from d 69 to 73. Dietary zinc levels were 2.8, 26 or 121 mg/kg wet weight. In accordance with the results of a methodological study, also reported here, the animals had an intramuscular injection of 65ZnCl2 12 d before the start of the collection period. Total fecal (endogenous + unabsorbed) zinc excretion for d 69-73 in the three groups was 2.3, 20.4 and 91.0 mg. The endogenous zinc excretion was 1.3, 2.0 and 6.4 mg, corresponding to 80.8, 10.6 and 6.4% of the zinc intake. Thus, the endogenous excretion was mainly important for the zinc homeostasis at low zinc intake, whereas at high intake the homeostasis was regulated via absorption from the digestive tract. The overall conclusion of the experiment was that mink are comparable to other species (including man) in regard to mechanisms controlling zinc homeostasis.     

A high oat-bran intake does not impair zinc absorption in humans when added to a low-fiber animal protein-based diet

Oat bran has a high phytate content and a low or inactivated phytase activity. A high intake of oat bran could therefore result in an impaired absorption of trace elements. The effect of a mean daily intake of 142 g of oat bran (102 g/10 MJ) on absorption of zinc was evaluated by the use of stable isotopes and fecal monitoring in 12 healthy subjects (6 males and 6 females). Each subject participated in two separate diet periods each of 21 d with identical low-fiber diets and with oat bran added in one of the periods. The oat bran was incorporated into bread and served at three daily main meals. The intake of zinc and phytate per 10 MJ was 138 micromol (9.0 mg) and 0.5 mmol, respectively, in the low-fiber period and 225 micromol (14.7 mg) and 4.0 mmol, respectively, in the oat bran period. Stable isotopes of zinc ((70)Zn) were added to the diets at d 7 of each period. The fractional absorptions (means +/- SD) of zinc from the low-fiber and oat bran diets were 0.48 +/- 0.11 and 0.40 +/- 0.15 (P = 0.07), respectively. The higher zinc content in the oat bran period resulted in a greater amount of zinc absorbed (64 +/- 19 micromol and 99 +/- 51 micromol, respectively, P = 0.009). Balance data suggest that the higher absorbed amount of zinc resulted in correspondingly higher intestinal endogenous excretion of zinc. In conclusion, the absorption of zinc was high and not affected by addition of oat bran.     

Studies on the phytate: zinc molar contents in diets as a determinant of Zn availability to young rats

1. Studies were carried out in vitro to examine the effects of phytate on the solubility of the trace elements zinc, copper and manganese. Appropriate volumes of a solution of sodium phytate were added to a mineral solution to achieve phytate: Zn values of from 0 : 1 to 45 : 1. In a second series the same values for phytate: Zn were achieved by varying the amount of added Zn at a fixed phytate concentration. 2. In both experiments greater than 85% of the Zn was rendered insoluble at pH 6.5 even at the lowest value for phytate: Zn (5 : 1). The effect of phytate on Zn solubility was greater than effects on Cu or Mn. 3. In a dietary study, rats were offered a semi-synthetic egg-albumin-based diet with added phytate. Two series of diets were prepared, the first had a constant Zn content (18.5 mg Zn/kg) and the amount of sodium phytate varied so as to achieve values for phytate: Zn of from 0 : 1 to 40 : 1 (series 1). In the second series, the same values for phytate: Zn were achieved by adding a fixed amount of phytate (7.4 g phytic acid/kg) while the amount of Zn was varied (series 2). 4. Dietary phytate caused significant reductions in growth rates, plasma Zn concentrations and hair Zn concentrations and greying of the coat at values for phytate: Zn of 15 : 1, 10 : 1, 15 : 1 and 15 : 1, respectively. 5. While phytate was apparently slightly more effective in reducing Zn status when phytate: Zn values were achieved at the lower absolute levels of phytate and zn (series 1 diets), the differences at equivalent phytate: Zn values were small. It was concluded that phytate: Zn values can be used as an indicator of Zn availability from phytate-rich diets. 6. Rats offered three diets containing soya-bean-based textured-vegetable-protein (TVP) exhibited low rates of weight gain compared with rats offered an egg-albumen-based diet of similar Zn content (14.5 mg Zn/kg). Additional Zn supplied in drinking-water (25 mg Zn//l) was without effect on rats consuming the egg-albumin diet but significantly improved the weight gain of rats on the TVP diets. 7. It was concluded that phytate naturally present in TVP behaves similarly to phytate added to an otherwise phytate-free diet and that the reduced availability of Zn in TVP diets can be accounted for entirely by their phytate contents.     

The effect of varying dietary zinc levels on the concentration and localization of zinc in rat bile-pancreatic fluid

To determine if zinc homeostasis occurs by zinc output from bile-pancreatic secretions, the bile-pancreatic duct and intestine of rats were surgically cannulated, and bile-pancreatic fluid was collected 2 h/d from the day of surgery to 8 d after surgery. The rats were fed diets containing 10, 100 or 1000 micrograms Zn/g diet before and after surgery. The effect of surgery itself was significant; food intake was initially lower than presurgery levels, but returned to presurgery levels by d 5. Protein and zinc concentrations, and carboxypeptidase A (CpA) and carboxypeptidase B (CpB) activities in bile-pancreatic fluid increased after surgery and leveled off at approximately d 6. Among the dietary zinc groups, the concentration of zinc in bile-pancreatic fluid varied significantly, whereas concentrations of calcium and copper did not. Zinc concentration in bile-pancreatic fluid for d 1-8 postsurgery averaged 1.8, 3.2 and 4.4 micrograms Zn/g, in the groups fed 10, 100 and 1000 micrograms Zn/g diet, respectively. Because the percent of zinc ingested that was secreted in bile-pancreatic fluid, estimated to be 57, 9.5 and 1.2% for the groups consuming the diets containing 10, 100 and 1000 micrograms Zn/g diet respectively, was not similar in the three groups, the quantity of zinc in bile-pancreatic fluid is not proportionally related to the amount of zinc ingested. Our results therefore suggest that zinc secretion in bile-pancreatic fluid does not play a major role in zinc homeostasis. Molecular localization of zinc in bile-pancreatic fluid and measurement of activity of CpA and CpB indicated that zinc in bile-pancreatic fluid is associated primarily with these digestive enzymes.     

Zinc homeostasis during lactation in a population with a low zinc intake.

BACKGROUND: There is a major increase in endogenous zinc excretion, specifically via the mammary gland, in early human lactation. Whereas fractional absorption of dietary zinc has been reported to increase in early human lactation, it is not known to what extent adaptive mechanisms may maintain zinc homeostasis, especially when dietary zinc intake is relatively low. OBJECTIVE: The objective of this study was to quantitate major variables of zinc homeostasis during early lactation in subjects from a population whose habitual dietary zinc intake is low. DESIGN: We studied 18 free-living lactating women from a rural community of northeast China whose infants were exclusively breast-fed. The subjects were studied at approximately 2 mo of lactation with use of stable isotopes of zinc and metabolic collection techniques. Milk volume was measured with use of a deuterium enrichment method. RESULTS: The mean (+/-SD) secretion of zinc in milk was 2.01 +/- 0.97 mg/d, the intake of zinc was 7.64 +/- 1.61 mg/d, and the fractional absorption of zinc was 0.53 +/- 0.09, for a total daily zinc absorption of 4.00 +/- 0.71 mg/d. Endogenous zinc excretion in urine and feces was 0.30 +/- 0.10 and 1.66 +/- 0.97 mg/d, respectively. CONCLUSIONS: Zinc balance, including zinc secreted in breast milk, was maintained at approximately 2 mo of lactation in women whose habitual diet was low in zinc. Homeostasis was achieved by high fractional absorption of zinc and intestinal conservation of endogenous fecal zinc.     

Inhibitory effect of soybean protein vs. casein on apparent absorption of magnesium in rats is due to greater excretion of endogenous magnesium.

Apparent magnesium absorption is depressed in rats fed diets containing soybean protein or enriched with sodium phytate or phosphate in comparison with casein, whereas it is raised in rats fed lactose. However, the possibility that changes in apparent absorption are caused by changes in fecal excretion of endogenous magnesium cannot be excluded. We studied the effects of casein, soybean protein, sodium phytate, lactose and phosphate on apparent and true absorption of magnesium. True magnesium absorption was measured with the use of oral and intraperitoneal administration of tracer doses of 28Mg. Fecal excretion of endogenous magnesium was calculated from apparent and true absorption. True magnesium absorption was not affected by either substitution of soybean protein for casein or by the addition of sodium phytate to a diet containing casein. Endogenous magnesium excretion in feces was significantly increased by soybean protein and sodium phytate. Thus the observed impairment of apparent magnesium absorption in rats fed soybean protein or sodium phytate is due to enhanced fecal excretion of endogenous magnesium. With the other dietary treatments, enhanced fecal excretion of endogenous magnesium was not associated with a discrepancy in the effects on apparent and true magnesium absorption. Dietary lactose vs. dextrose and supplemental phosphate both stimulated fecal excretion of endogenous magnesium, but lactose raised both true and apparent magnesium absorption, and phosphate depressed both true and apparent magnesium absorption.     

Metabolic adaptations to low zinc intakes in premenarcheal girls

BACKGROUND: Zinc deficiency is increasingly recognized as an important cause of mortality and morbidity. Children in developing countries are at especially high risk because of relatively low zinc intakes and poor bioavailability. OBJECTIVE: We assessed the effect of 2-wk adaptation to low zinc intake (4 mg/d) on fractional zinc absorption, endogenous fecal zinc excretion, and urinary zinc excretion. DESIGN: Sixteen healthy 9-14-y-old girls were studied twice in random order after 2-wk adaptation to diets providing either 12 mg/d (high) or 4 mg/d (low) zinc. Fractional zinc absorption and endogenous fecal zinc excretion were measured with use of established stable isotope techniques. RESULTS: Plasma zinc was not significantly lower during the low dietary intake period (1.06 +/- 0.18 mg/L) than during the high dietary intake period (1.14 +/- 0.23 mg/L, P = 0.30). Endogenous fecal zinc excretion was significantly lower during the low intake period (1.08 +/- 0.62 mg/d) than during the high intake period (1.82 +/- 0.95 mg/d, P < 0.026), but there was no significant change in fractional zinc absorption (30.6% +/- 12.4% compared with 26.6% +/- 9.0%, P = 0.32) or urinary zinc excretion (0.68 +/- 0.35 mg/d compared with 0.59 +/- 0.24 mg/d, P = 0.30). Approximate zinc balance was significantly lower during the low-intake period than during the high-intake period (P = 0.007) and significantly (P < 0.0001) less than zero. CONCLUSION: Short-term zinc restriction in premenarcheal girls leads to a significant decrease in endogenous fecal zinc excretion, which was inadequate to restore normal zinc balance.     

The addition of milk or yogurt to a plant-based diet increases zinc bioavailability but does not affect iron bioavailability in women

The addition of milk and milk-based products to the diets of individuals subsisting on plant-based diets was reported to have positive effects on nutritional status and functional outcomes such as growth, morbidity, and cognition. We examined the effect of the addition of milk or yogurt on the bioavailability of zinc and iron from a plant-based rural diet. The subjects were 48 Mexican women (30.9 +/- 5.7 y) who habitually consumed a plant-based diet. The women were assigned to 1 of 3 groups: 1) the typical rural Mexican diet, 2) that diet with milk added, or 3) that diet with yogurt for 13 d. Zinc absorption was measured after extrinsically labeling meals with (67)Zn and an i.v. dose of (70)Zn; iron absorption was measured by extrinsically labeling meals with (58)Fe and a reference oral dose of (57)Fe. Including milk and yogurt in the diet increased zinc absorption by 50 and 68%, respectively (P < 0.05). The 3 groups did not differ in the percentage iron absorption. The total amount of zinc absorbed was increased (P < 0.05) by 70% when milk was added to the meal and 78% when yogurt was added. The total amount of iron absorbed did not differ among the groups. The addition of milk and yogurt to a plant-based diet high in phytate increases zinc bioavailability without affecting iron bioavailability.