Fractional Zinc Absorption Research Articles

Zinc absorption in premature infants: comparison of two isotopic methods

The fractional absorption of an oral dose of zinc can be measured in adults when given simultaneously with an intravenous dose and subsequently measuring the ratio of the double isotopic enrichment of urine. To test this method in very-low-birth-weight (VLBW) premature infants [n=5 females and 7 males, 1160 +/- 290 g (chi +/- SD) birth weight, 29 +/- 4 wk gestational age], an oral dose of either 300 or 1200 micrograms 68Zn.kg(-1).d(-1) was equilibrated with formula or human milk and administered simultaneously with either 50 or 100 micrograms 70Zn.kg(-1).d(-1) given intravenously 35 +/- 3 wk postconception. Urine and fecal samples were collected for 3-6 d and analyzed by inductively coupled plasma mass spectrometry. Endogenous fecal zinc (EFZ) was determined from isotopic enrichment, whereas net absorption and retention were calculated by traditional methods. The mean fractional absorption calculated from urine was 0.22 +/- 0.09 and from feces it was 0.25 +/- 0.07. Zinc intake averaged 1821 +/- 330, fecal excretion 1637 +/- 419, and urinary excretion 67 +/- 30 micrograms.kg(-1).d(-1). EFZ averaged 390 +/- 270 micrograms.kg(-1).d(-1) and ranged from 48 to 889 micrograms.kg(-1).d(-1). Net absorption was 220 +/- 316 micrograms.kg(-1).d(-1) and net retention was 131 +/- 334 micrograms.kg(-1).d(-1). True absorption was 373 +/- 161 micrograms.kg(-1).d(-1). Fecal collection is difficult, tedious, and often incomplete, and may be replaced by urine collection for the fractional absorption of zinc in groups of premature infants.

Zinc, Copper and Magnesium Absorption from a Fibre-rich Diet

Zinc, copper and magnesium absorption and retention from a high-fibre and high-phytate diet of conventional foods were investigated in 8 healthy subjects. Fractional absorption of the three minerals was estimated from fecal monitoring of zinc, copper and magnesium stable isotopes added to one day's diet. In addition, apparent absorption, retention and endogenous losses were estimated for the respective minerals based on fecal and urinary excretion over the last 16 days of 21 days constant intake. The diet contained 1 mmol phytic acid, 29 g dietary fiber, 140 mumol zinc, 13 mumol copper, and 9 mmol magnesium per 10 MJ. The absorption of zinc, copper and magnesium was 29 +/- 12% (x +/- SD) (48 +/- 20 mumol), 44 +/- 7% (8 +/- 1 mumol) and 46 +/- 6% (5 +/- 1 mmol), respectively. The retained amounts of minerals were -7 +/- 19 mumol, -5 +/- 4 mumol, and -3 +/- 1 mmol, respectively and the estimated endogenous intestinal losses were 40 +/- 20 mumol, 12 +/- 4 mumol and 3 +/- 1 mmol, respectively. In conclusion, the fractional absorption of zinc, copper and magnesium from the fibre-rich diet was not sufficient to cover intestinal and urinary losses of these elements, resulting in negative balances.

Measurement of zinc bioavailability from beef and a ready-to-eat high-fiber breakfast cereal in humans: application of a whole-gut lavage technique

We measured zinc absorption in humans from two different foods, using a whole-gut lavage technique. Healthy adults (n = 8) were given test foods containing approximately 4 mg (61.2 mumol) Zn as either beef or a high-fiber, ready-to-eat breakfast cereal. Basal zinc loss in the gut was measured after giving only 200 mL water alone after an overnight fast. Mean polyethylene glycol (PEG) recovery, a nonabsorbable fecal marker, was 95 +/- 2% (mean +/- SE). There was no significant difference of PEG recoveries with the various treatments. True absorption of zinc was 11.0 +/- 5.4 mumol (716.3 +/- 355.9 micrograms) for breakfast cereal and 34.8 +/- 3.5 mumol (2274.1 +/- 230.2 micrograms) for beef. Fractional zinc absorption was significantly lower for breakfast cereal compared with beef, 14.7 +/- 6.7% vs 55.1 +/- 4.6%, respectively. Our findings demonstrate that zinc absorption can be measured in humans from single foods containing a physiological dose of zinc by using the whole-gut lavage absorption technique and that zinc bioavailability from beef is about fourfold greater than from a high-fiber breakfast cereal.

The analysis of stable isotopes in urine to determine the fractional absorption of zinc

We measured isotopic enrichment in urine after oral and intravenous administration of stable isotopes of zinc to determine fractional absorption (FA). 68Zn and 70Zn were administered orally and intravenously to four normal adults. Subsequently, urine and fecal samples were collected for 7 and 14 d, respectively, ashed, and passed through ion-exchange columns to separate zinc from other elements. Samples were analyzed by fast-atom-bombardment mass spectrometry. From 32 h onwards the enrichment of 68Zn and 70Zn in urine declined proportionately so that FA could be determined as follows: FA = enrichment (oral/iv) x dose (iv/oral). FA determinations from urine and feces (cumulative excretion) were, respectively, for subject ZK1, urine 0.79 +/- 0.03 and feces 0.70 +/- 0.01; ZK2, 0.79 +/- 0.05 and 0.69 +/- 0.02; ZK3, 0.26 +/- 0.01 and 0.25 +/- 0.01; and ZK4, 0.41 +/- 0.02 and 0.37 +/- 0.02. ZK1 and ZK2 received the oral isotope while fasting whereas ZK3 and ZK4 received the oral isotope with meals. FA of zinc can be determined by measurement of isotope enrichment in urine.

Milk consumption and zinc retention in postmenopausal women : J. Nutr. 1990 120/4 (398–403)

We measured intestinal 65Zn absorption and whole-body retention in postmenopausal women following a meal extrinsically labeled with 65Zn. The meal was consumed with 200 mL of milk or with a calcium phosphate supplement. Fifteen elderly (mean age 69) subjects were studied during three 12-d experimental periods in which they were fed either a basal diet containing 16 mg Zn or the basal diet supplemented either with 400 mL of milk or with dibasic calcium phosphate supplements containing an equivalent amount of calcium (468 mg) and phosphorus (360 mg). Measurements of fractional zinc absorption were made in 9 of the 15 subjects by a dual-isotope technique using 65Zn and 51Cr (as nonabsorbable fecal markers). Fractional zinc absorption was 0.22 +/- 0.06 (mean +/- SEM) after the standard meal, 0.23 +/- 0.06 with added milk and 0.22 +/- 0.03% with the added calcium phosphate supplement. The corresponding values for 65Zn retention in 15 subjects 12 d after dosing were 0.10 +/- 0.01% for all treatments. No significant effect of milk or calcium phosphate supplementation was observed on either 65Zn absorption or retention. We conclude that increased milk consumption with meals has no detrimental consequences on zinc nutriture in elderly women.

Zinc absorption in women: comparison of intrinsic and extrinsic stable-isotope labels

Fractional absorption of extrinsic and intrinsic zinc from milk-based formulated diets was measured in 10 young women by using a simultaneous triple-isotope method based on fecal monitoring of unabsorbed enriched stable-isotope labels. Zinc intake was held nearly constant (mean zinc intake = 1.48 mumol.kg body wt-1.d-1) throughout the 18-d controlled feeding phase of the study by addition of extrinsic 64Zn to all formula feedings, with substitution of the extrinsic 67Zn label for some of the 64Zn when formula intrinsically labeled with 70Zn was given. Fractional absorption values of intrinsic and extrinsic zinc labels, 0.267 +/- 0.092 (mean +/- SD) and 0.282 +/- 0.086, respectively, were highly correlated (r = 0.83, P less than 0.05) and did not differ significantly. The ratio of fractional absorption of the extrinsic to that of the intrinsic label, 1.08 +/- 0.20 (mean +/- SD), was not significantly different from unity. The results show that extrinsic labeling by use of enriched stable isotopes is a valid means of determining zinc absorption from milk-based diets in adult humans.

Milk Consumption and Zinc Retention in Postmenopausal Women

We measured intestinal 65Zn absorption and whole-body retention in postmenopausal women following a meal extrinsically labeled with 65Zn. The meal was consumed with 200 mL of milk or with a calcium phosphate supplement. Fifteen elderly (mean age 69) subjects were studied during three 12-d experimental periods in which they were fed either a basal diet containing 16 mg Zn or the basal diet supplemented either with 400 mL of milk or with dibasic calcium phosphate supplements containing an equivalent amount of calcium (468 mg) and phosphorus (360 mg). Measurements of fractional zinc absorption were made in 9 of the 15 subjects by a dual-isotope technique using 65Zn and 51Cr (as nonabsorbable fecal markers). Fractional zinc absorption was 0.22 +/- 0.06 (mean +/- SEM) after the standard meal, 0.23 +/- 0.06 with added milk and 0.22 +/- 0.03% with the added calcium phosphate supplement. The corresponding values for 65Zn retention in 15 subjects 12 d after dosing were 0.10 +/- 0.01% for all treatments. No significant effect of milk or calcium phosphate supplementation was observed on either 65Zn absorption or retention. We conclude that increased milk consumption with meals has no detrimental consequences on zinc nutriture in elderly women.

Intrinsic and Extrinsic Stable Isotopic Zinc Absorption by Infants from Formulas

The kinetics of fecal elimination and the extent of fractional absorption of intrinsic and extrinsic zinc from infant formula were studied in 11 human infants by using a direct, simultaneous dual-label method based on fecal monitoring of unabsorbed stable isotope labels. The relative positions of the two stable isotopic tags (67Zn, 70Zn) as extrinsic and intrinsic labels were reversed in two separate determinations of fractional absorption. Administration of doubly labeled formula for 24 h or less permitted elimination of unabsorbed labels before the end of a 72-h metabolic interval. The extent of enrichment and the kinetics of fecal elimination of unabsorbed extrinsic stable isotopic zinc tag were observed to agree closely with the corresponding values for the intrinsic tag. Fractional absorption of intrinsic and extrinsic tags was highly correlated and did not differ significantly. The ratio of fractional absorption of extrinsic tag to that of intrinsic tag was 1.05 +/- 0.19 (mean +/- SD). The results constitute strong support for validation of the use of enriched stable isotopes at low levels (less than or equal to 10%) of extrinsic addition to milk-based diets for studies of zinc absorption by humans.

Effect of Protein Level and Protein Source on Zinc Absorption in Humans

The effect of increasing levels of various protein sources on zinc absorption from a legume-based meal was studied in humans with the use of a radionuclide technique. The meals were extrinsically labelled with 65Zn and absorption was determined from measurements of the whole-body retention of the isotope. The mean fractional zinc absorption for the 13 meals was 24.7 +/- 6.9% and was only influenced by the protein content of the meal to a limited extent (r = 0.45). However, the amount of zinc absorbed from the meals was strongly correlated with both the protein (r = 0.85) and zinc content (r = 0.86): 5.9 +/- 1.7 mumol of zinc was absorbed from the basal bean meal which had the lowest protein content; the addition of low zinc chicken doubled the protein content and increased zinc absorption to 10.3 +/- 2.0 mumol; the addition of zinc-rich beef also doubled the protein content, however, zinc absorption was increased to 15.9 +/- 4.7 mumol. It is concluded that the zinc content of the main protein source of the diet determines the amount of zinc absorbed to a large extent. However, relatively small amounts of animal protein can significantly improve the value of a legume-based meal as a source of zinc.

Effect of Milk and Lactose on Zinc Absorption in Lactose-Intolerant Postmenopausal Women

We evaluated the effect of lactose intolerance on intestinal zinc absorption under various test conditions in a group of postmenopausal women. Fractional zinc absorption was studied with a dual-isotope technique using 65Zn and 51Cr (as nonabsorbable fecal markers). We performed four absorption tests on sixteen subjects with water (control), whole cow milk, lactose-free cow milk and lactose alone. Mean (+/- SEM) fractional zinc absorption was 0.558 +/- 0.032 with water, 0.276 +/- 0.031 with milk, 0.246 +/- 0.028 with lactose-free milk and 0.564 +/- 0.051 with lactose. No significant differences in zinc absorption were found under any of the test conditions between lactose-tolerant and lactose-intolerant subjects. Milk and lactose-free milk significantly reduced zinc absorption in both groups.

A potential role for prolactin in zinc homeostasis

Zinc is an important trace metal co-factor for many metallo-enzymes and may have a critical role in the stabilization and function of biomembranes. Zinc-depleted animals increase their fractional absorption of zinc to restore zinc homeostasis. This increase in fractional absorption is not currently known to be hormonally-mediated. Recently, zinc has been shown to suppress the output of prolactin from dispersed pituitary cells in vitro at physiologic concentrations. Clinical states associated with a tendency to zinc deficiency are also associated with a tendency to hyperprolactinemia. These observations are consistent with the hypothesis that prolactin regulates the uptake and distribution of zinc and that zinc suppresses prolactin in closure of a negative feedback regulatory loop.

Absorption of stable70 Zn in healthy young men in relation to zinc intake

Zinc absorption was measured in two groups of four healthy young men using the method of stable tracer isotope neutron activation analysis. All absorption measurements were made using a test dose of ZnCl2 in subjects who participated in an overnight fast and also in subjects from whom food was withheld for 5 h after ingestion of tracer. In one group of experimental subjects, dietary zinc intake was held constant (approximately 15 mg/day), and three different test doses of the tracer were administered at weekly intervals. Fractional absorption was 81% from a 4.52-mg dose, 67% from a 6.47 mg dose and 61% for a 24.52-mg dose. In the other group of subjects, dietary zinc was reduced from approximately 15 mg to less than 2 mg/day. This was accompanied by a significant increase in fractional zinc absorption of a fixed tracer zinc dose (1.19 mg), from 81 to 92%. This increase, which occurred within 6 days, was also measurable when the tracer zinc dose was increased to 4.76 mg. These data indicate that there is a significant effect of the dose level of stable zinc isotope on its fractional absorption. They also demonstrate that a restriction in dietary zinc intake results in a prompt increase in the absorption of a fixed pulse dose of zinc in healthy young men.

Absorption of dietary zinc in man: comparison of intrinsic and extrinsic labels using a triple stable isotope method

Determination of true absorption of dietary minerals in human subjects using "extrinsic-tag" approach with stable isotopes requires establishment of its validity. The present study was conducted to test with quantitative aspects absorption of an extrinsic-tag of zinc labeled with 70Zn as compared with an "intrinsic-tag" of the mineral as 68Zn-labeled chicken meat given simultaneously to health male subjects. Three diet periods were used in which diet modulation with respect to zinc dialy intake and nature of protein source (chicken/soy protein isolate) was also examined. Absorption was measured via quantitative fecal isotope balance of 64Zn, 68Zn, 70Zn. For the three diet period 1 (protein: chicken; Zn intake: ZN intake: 10 to 11 mg/day), 2 (protein: chicken/soy protein isolate, 50/50; Zn intake: 10 to 11 mg/day), and 3 (protein: chicken; Zn intake: 7 mg/day), fractional absorption of the extrinsic tag (mean +/- 1 SEM) was 0.46 +/- 0.06, 0.46 +/- 0.06, and 0.66 +/- 0.04 respectively. The comparable values for intrinsic 68Zn were 0.57 +/- 0.06, 0.57 +/- 0.06, and 0.72 +/- 0.04. There was a highly significant correlation (r=0.91), between zinc absorption from the two labels. However, absorption of intrinsic 68Zn was significantly higher (p less than 0.02) during all periods than that for the extrinsic 70Zn. A 50% replacement of protein from chicken meat with the soy protein isolate did not later fractional absorption of zinc from either tag. The ratio of fractional absorption of the extrinsic/intrinsic tag (mean +/- 1 SEM) was 0.79 +/- 0.06, 0.79 +/- 0.04, and 0.92 +/- 0.03 for periods 1, 2, 3, respectively.