EFFECTS OF DIETARY-COMPOSITION ON SOMATOMEDIN ACTIVITY IN GROWING-RATS

Young growing rats were used to evaluate the effect of dietary composition on growth rate, body and organ weights, biochemical composition of brain and liver, plasma growth hormone concentration, and plasma somatomedin (SM) activity. Young male rats (10/group) were fed four levels of dietary protein (casein: 2.5, 5, 10 and 20%) and three levels of dietary fat (corn oil: 1.2, 10, and 20%) over a 30-day period. Changes in total body and organ weights paralleled increasing dietary protein and were affected to a lesser extent by dietary fat. Liver protein paralleled increasing dietary protein. Glycogen, protein, DNA, and RNA determinations on the brain were not significantly influenced by dietary manipulation. Plasma growth hormone concentrations were not significantly different due to dietary protein or fat. Plasma SM activity was positively correlated with dietary protein levels (0.80 units/2.5% protein for all fat levels to 1.35 units/20% protein for all fat levels). Plasma SM was suppressed approximately 25% with increasing dietary fat in rats fed the 20% protein diets. Furthermore, SM appears to be directly related to food efficiency (g gain/100 kcal) and body weight change. These data suggest that dietary composition has a direct effect on plasma SM activity and that the severe growth retardation associated with protein malnutrition may be related to its additional effect on serum SM.