Bioavailability of lysine in milk-based infant formulas as determined by rat growth response method

Abstract

Bioavailability of lysine in smaples of different forms (powder, liquid concentrates, ready-to-use, premature and/or hypoallergenic) of infant formulas (involving varying degrees of heat processing during preparation, and containing different proportions of casein and whey proteins) obtained from four manufacturers was determined by the rat growth response method using regression analysis (weight gain vs. lysine consumed). A wheat gluten basal diet adequate in all nutrients for rat growth except lysine (0.26%) was supplemented with graded levels of crystalline lysine (0.08, 0.16, 0.24, 0.32, 0.40, 0.48 and 0.64%, standard diets), or infant formulas providing 0.16% supplemental lysine (test diets). These diets were fed to weanling Sprague-Dawley rats for a period of 3 weeks. Bioavailability of lysine was determined after 1 and 3 weeks of test. Values for bioavailability of lysine in the infant formulas after 1 and 3 weeks of test were 78–95 and 84–97%, respectively. Diets containing liquid concentrate form of infant formulas had the lowest lysine bioavailability values. It is suggested that protein quality of liquid concentrates may be lower than that of powder and/or other forms of infant formulas prepared by the same manufacturer.