Somatomedin-C and zinc status in rats as affected by Zn, protein and food intake.

1. The objective of the present experiment was to study the level of plasma somatomedin-C (SM-C) and the status of zinc in rats as affected by three levels of Zn given in combinations with two levels of protein. 2. Six groups of rats were fed, for 21 d, on six different diets based on combinations of two levels of dietary protein (low protein, 75 g/kg; high protein, 200 g/kg) and three levels of zinc (low Zn, 0.9 microgram/kg; moderate Zn, 55 micrograms/kg; high Zn, 110 micrograms/kg). All groups were pair-fed with the group receiving the low-Zn-low protein diet. An additional group of six rats served as an ad lib.-fed control group and was fed on a diet that contained 55 micrograms Zn/kg and 200 g protein/kg ad lib. 3. Body-weight gain and food intake were recorded daily. Rats were killed at the end of the experimental period (21 d). Zn was assayed in plasma, tibia and liver by atomic absorption technique. Plasma SM-C was assayed by radioimmunoassay. 4. In rats given the low-Zn-low-protein diet, the level of plasma SM-C increased in response to the increase in the amount of Zn or Zn and protein in the diet. However, no change was observed when the level of protein alone was increased. 5. Among all groups tested, ad lib.-fed rats showed the highest level of plasma SM-C. Thus it may be concluded that a balanced diet combined with adequate food intake is necessary to maintain an optimal level of plasma SM-C.