Effect of proteins on availability of zinc. II. Bioavailability of zinc from casein and whey protein--retention study in young rats.

The availability of zinc from two semi-synthetic diets with isolated whey protein (Wp D) or with isolated casein (Cas D) as protein component (20% W/W) was compared in a 21-day study with growing male rats (initial weight 40 g; 14 animals/group). Zinc concentration in both diets (18 ppm) was adequate to meet the requirements of the animals fed ad libitum. For radiolabeling approximately 13 micrograms 65Zn (= 4 microCi) was given daily by intragastric intubation to each animal. The investigation was designed primarily as a retention study, but also general parameters like weight development, food and water intake, organ weights etc. were registered and the activity of alkaline phosphatase was determined in serum and femur tissue. A significantly higher percentage of 65Zn was retained in the whole body from the Wp D (36.5%) than from the Cas D (31.6%) during the experimental period. The same is valid for the percentage retention of 65Zn in the femur and for the 65Zn concentration in femur and hair as well as for the total zinc concentration (65Zn and non-labeled zinc) of the femur. The other parameters determined were not unequivocally influenced by the protein component of the diet. The study clearly demonstrated that the availability of zinc by the growing rat was better from a diet with whey protein than from one with casein as the protein component. The reason on this phenomenon has to be elucidated by further investigations.