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Interaction of Dietary Fat and Thyroid Function with Lipid Metabolism of Fasted Rats

Laboratory of Food Chemistry, Department of Food Science, Faculty of Agriculture, Kagawa University, Kagawa-ken, Japan

The influence of previously feeding a high fat diet followed by fasting on changes in lipid contents of the liver and carcass, and in serum concentrations of free fatty acid (FFA) and free glycerol in rats at various states of thyroid function was compared with the influence of previously feeding a high carbohydrate diet. The higher rate of reduction in carcass lipid content during fasting of rats previously fed the high fat diet was abolished in rats made hypothyroid by feeding propylthiouracil (0.01% of diet) and was not altered in rats made hyperthyroid by intraperitoneal injection of thyroxine (30 µg/rat). Inversely, when rats were previously fed the high carbohydrate diet, the reduction of carcass lipid content during fasting was accelerated in hyperthyroid rats and was not altered in hypothyroid rats. An increase in liver lipid content during fasting of rats previously fed the high carbohydrate diet was abolished in hyperthyroid rats and was accelerated in hypothyroid rats. Rats previously fed the high fat diet did not show any significant change in liver lipid content during fasting regardless of the thyroid status. The responsiveness of serum FFA to thyroid activity was similar to that of liver lipid and slightly different from that of serum free glycerol. The responses of lipid metabolism in the liver and in carcass tissues to thyroid activity might differ. The loss of responsiveness in liver lipids of fasted rats previously fed the high fat diet to thyroid activity might be due to the reduced facilitatory influence of fasting on the accumulation of liver lipids, which might be associated with thyroid activity.

KEY WORDS: • high fat diet • fasting • lipid content • thyroid function

Manuscript received 13 July 1978.