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.

Abnormalities in zinc homeostasis in young infants with cystic fibrosis.

Low plasma zinc concentrations have been reported in approximately 30% of young infants with cystic fibrosis identified by newborn screening. The objective of this study was to examine zinc homeostasis in this population by application of stable isotope methodology. Fifteen infants with cystic fibrosis (9 male, 6 female; 7 breast-fed, 8 formula-fed) were studied at a mean (+/-SD) age of 1.8 +/- 0.7 mo. On d 1, 70Zn was administered intravenously, and 67Zn was quantitatively administered with all human milk/formula feeds during the day. Three days later, a 3-d metabolic period was initiated, during which time intake was measured and complete urine and fecal collections were obtained. Fractional zinc absorption, total absorbed zinc, endogenous fecal zinc, and net absorbed zinc were measured; fecal fat excretion was also determined. Fractional absorption was significantly higher for the breast-fed infants (0.40 +/- 0.21) compared with the formula-fed group (0.13 +/- 0.06) (p = 0.01), but with the significantly higher dietary zinc intake of the formula-fed group, total absorbed zinc was higher for those receiving formula (p = 0.01). In 1 infants with complete zinc metabolic data, excretion of endogenous zinc was twofold greater for the formula-fed infants (p < 0.05); net absorption (mg zinc/d) was negative for both feeding groups: -0.04 +/- 0.52 for breast-fed; -0.28 +/- 0.57 for formula-fed. Endogenous fecal zinc losses correlated with fecal fat excretion (r = 0.89, n = 9, p = 0.001), suggesting interference with normal conservation of endogenously secreted zinc. These findings indicate impaired zinc homeostasis in this population and suggest an explanation for the observations of suboptimal zinc status in many young infants with cystic fibrosis prior to diagnosis and treatment.

Microwave method for preparing erythrocytes for measurement of zinc concentration and zinc stable isotope enrichment.

A microwave digestion method to prepare human erythrocytes for measurement of Zn concentration by atomic absorption spectrophotometry and stable isotope enrichment by mass spectrometry is described. Also described is a process for purifying digested erythrocyte samples enriched with Zn stable isotope for analysis by fast atom bombardment mass spectrometry. Microwave digestion was investigated as a way to increase sample throughput by replacing a more time-consuming conventional oven ashing/hot plate wet digestion method. Pooled red blood cells and NIST bovine liver standard reference material were digested by the two different methods and zinc recoveries compared by atomic absorption spectrophotometry. Microwave and conventional methods yielded 11.7 +/- 0.1 and 11.7 +/- 0.2 micrograms/g (wet wt), respectively, for the pooled erythrocytes, and Zn recovery from NIST bovine liver standard (certified 123 +/- 8 micrograms/g) was 128.2 +/- 1.2 and 127.4 +/- 1.3 micrograms/g, p > or = 0.282, respectively. Microwave digestion improved the processing of erythrocytes for atomic absorption spectrophotometry and mass spectrometry by reducing digestion time from 1 week to 2 h. In addition, a procedure for purifying digested erythrocyte samples by either extraction and ion-exchange chromatography in preparation for mass spectrometry analysis of Zn stable isotope enrichment is outlined.

Meal-induced changes in plasma, erythrocyte, and urinary zinc concentrations in adult women.

We attempted to determine whether there is a limit to the transient, meal-induced decline in plasma zinc and whether there is a concomitant increase in erythrocyte and erythrocyte membrane concentrations. Premenopausal women participated in a 17-h fasting trial, a one-meal trial with breakfast at 0700, and a three-meal trial with meals at 0700, 0900, and 1100. During fasting, plasma zinc increased 9%; it decreased 11% and 19% in the one- and three-meal trials, respectively (P < 0.001). A limit to the decline in plasma zinc was reached after the second meal in the three-meal trial. Erythrocyte, erythrocyte membrane zinc, and serum calcium and phosphorus concentrations did not change significantly during the three trials. Serum glucose concentrations were weakly related to plasma zinc concentrations, suggesting that the postprandial decline in plasma zinc is associated with the metabolic changes caused by food intake.