Muscle protein breakdown in young rats fed on a energy-depleted diet

The urinary excretion of 3-methylhistidine has been determined in rats fed an energy-depleted diet. Young male rats were fed over a 21-day period on either an adequate control diet (18% lactalbumin), or an energy-depleted diet (containing half of the amount of carbohydrates of the control diet). Urinary urea-N, creatinine, creatine and 3-methylhistidine, as well as body weight changes were monitored throughout the experiment. At the end of the experiment, the levels of insulin and corticosterone, and the weights of livers and gastrocnemius, soleus, tibialis anterior and extensor digitorum longus muscles were determined. A significant (p less than 0.05) reduction in body and liver weight was found in the energy-depleted rats, but no weight differences were found in the four excised muscles. Urinary outputs of urea-N, creatine and creatinine were significantly (p less than 0.05) increased in this latter group. Output of 3-methylhistidine showed an initial rise followed by a significant (p less than 0.05) and progressive decline throughout the experiment in the rats fed the energy deficient diet. Insulin concentration was significantly (p less than 0.01) reduced in those animals, but no differences were found in the serum levels of corticosterone. It is assumed that lack of energy in the diet decreases the rate of myofibrillar protein breakdown in growing rats.