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

Abstract

Energy balances were measured by indirect calorimetry in four experiments on male growing rats, fed restrictively on isoenergetic and isonitrogenous (10% CP) diets based on either casein supplemented with methionine, or soy protein isolate (experiments 1, 2 and 3) and soy protein isolate supplemented with methionine (experiment 0), respectively. At the end of experiments the rats were killed for body analysis and determination of thyroid hormones and lipids in blood as well as mitochondrial respiration in liver and heart. Feeding of non-supplemented soy protein resulted in a lower efficiency of energy utilisation as well as a lower protein utilisation compared to casein-fed and supplemented soy protein-fed rats. Chemical body composition was not markedly different between the dietary groups. After long-term feeding of soy protein (experiment 3) mass and mitochondrial protein content of the interscapular brown adipose tissue were increased compared to casein-fed rats. Serum thyroid hormone levels were not changed (T3 and free T3) or were significantly lowered (T4 and free T4) following soy protein feeding in comparison with casein feeding (except for experiment 2). Cholesterol and triglycerides were decreased on an average in response to soy protein or supplemented soy protein feeding. In two of three experiments a significant lower efficiency of hepatic mitochondrial respiration with succinate as substrate, expressed by the ratio of added ADP to oxygen consumed, was observed in soy protein-fed rats compared to casein-fed rats.