Energy metabolism and thyroid hormone levels of growing rats in response to different dietary proteins - Soy protein or casein

Energy balances were measured by indirect calorimetry in four experiments o n male growing rats, fed restrictively on isoenergetic and isonitrogenous ( 10% CP) diets based on either casein supplemented with methionine, or soy p rotein isolate (experiments 1, 2 and 3) and soy protein isolate supplemente d with methionine (experiment 0), respectively. At the end of experiments t he rats were killed for body analysis and determination of thyroid hormones and lipids in blood as well as mitochondrial respiration in liver and hear t. Feeding of non-supplemented soy protein resulted in a lower efficiency o f energy utilisation as well as a lower protein utilisation compared to cas ein-fed and supplemented soy protein-fed rats. Chemical body composition wa s not markedly different between the dietary groups. After long-term feedin g of soy protein (experiment 3) mass and mitochondrial protein content of t he interscapular brown adipose tissue were increased compared to casein-fed rats. Serum thyroid hormone levels were not changed (T-4 and free T-4) or were significantly lowered (T-4 and free T-4) following soy protein feeding in comparison with casein feeding (except for experiment 2). Cholesterol a nd triglycerides were decreased on an average in response to soy protein or supplemented soy protein feeding. In two of three experiments a significan t lower efficiency of hepatic mitochondrial respiration with succinate as s ubstrate, expressed by the ratio of added ADP to oxygen consumed, was obser ved in soy protein-fed rats compared to casein-fed rats.