An experimental model for studies of zinc bioavailability from milk and infant formulas using extrinsic labeling

The ability to assess zinc bioavailability from various diets is essential as the zinc content of many foods can be low or marginal. We have investigated the absorption of zinc from human milk, cows' milk, cows' milk formula (whey-adjusted) and soy protein formula as these fluids can comprise the majority of an infant's diet. Radiozinc was added to the diets in tracer amounts. The extrinsic ⁶⁵Zn was shown by ultracentrifugation, ultrafiltration, and gel filtration to add to milk fractions and individual binding compounds in a manner analogous to the distribution of native zinc, validating the use of extrinsically labeled milk diets. Labeled diets were fed by intubation to 16-day-old suckling rats. Animals were killed after 4 h and tissues removed and counted. Zinc bioavailability was 28% from human milk, 24% from whey-adjusted cows' milk formula, 15% from cows' milk, and 10% from soy formula. Intubation studies using adult rats showed that zinc absorption was lower from all the diets; however, it was still highest from human milk and cows' milk formula. These results show that the rat pup model may provide a rapid, inexpensive, and sensitive method to assay bioavailability of zinc from infant foods.