Thyroid and growth responses of young Zucker obese and lean rats to a low protein-high carbohydrate diet

Experiments were conducted to study the effects of a low protein-high carbohydrate diet on growth and thyroid function in obese and lean male and female Zucker rats. The nine feeding regimens included animals ad libitum fed either a 22% casein and 59% carbohydrate diet (control) or an 8% casein and 73% carbohydrate diet (low protein) and appropriate pair-fed groups to control for the lean rats eating less than the obese rats and the rats fed the low-protein diet eating less than those fed the control diet. The rats were 4 weeks old at the start of the experiment which lasted 7 weeks. Final body size, tibia length and nonfat dry mass of the lean rats were dependent primarily on the amount of protein consumed, whereas growth of the obese rats was related to total energy intake rather than to protein intake. The relative hyperphagia, decreased efficiency of energy utilization and increased oxygen consumption and serum T3 concentrations in the lean rats fed the low-protein diet were consistent with the development of an adaptive thermogenesis, allowing the excess non-protein energy to be dissipated through excess heat production. There was no evidence for such an adaptive thermogenesis in the obese rats. The suggestion that the obese rats were already overeating for protein and storing the excess energy as fat and that the decreased thyroid response might be part of a protective mechanism against overheating was discussed.