Availability of zinc: loading tests with human milk, cow's milk, and infant formulas

Uptake of zinc with human milk, cow's milk, and four infant formulas investigated using a standard zinc loading test. Female subjects consumed 25 mg of zinc with the milk or formula, the amount of which was calculated to provide 5 gm of protein, after an eight-hour fast. Blood samples were taken prior to (base line) and at 30-minute intervals for three hours after consumption of zinc. Plasma zinc response was calculated as the area under the curve change in plasma zinc from the base line value, plotted against time over three hours. The plasma response with human milk was significantly greater (P less than .005) than with cow's milk and all the formulas. The response with cow's milk and a cow's milk-based formula was one third that with human milk; responses with a soy-based and two casein hydrolysate-based formulas were lower. These results are compatible with the concept, strongly suggested by indirect evidence, that the bioavailability of zinc with human milk is better than with cow's milk or infant formulas.