Dietary \g=w\6fatty acids and the effects of hyperthyroidism in mice

(Requests for offprints should be addressed to A j Hulbert) The influence of the type of dietary fat on the effects of thyroid hormones was investigated in mice. Hyperthyroid-ism was achieved by providing thyroid hormones (T3 and T4) in the drinking water. Both hyperthyroid and euthy-roid mice (Mus musculus) were fed isoenergetic diets containing 18 % (w/w) total lipid but differing in fatty acid composition. Diets were either low in the polyunsaturated linoleic acid (18:2, \g=w\6)and high in saturated fatty acids(SFAs) or low in saturated fats and high in the polyunsatu-rated fatty acid (PUFA), linoleic acid. Treatments were maintained for 21\p=n-\22days. Plasma thyroid hormone levels, standard metabolic rate (SMR), changes in body mass, specific activities of malic enzyme (ME), Na-K\x=req-\ ATPase and glycerolphosphate dehydrogenase (GPDH) of the liver were measured. Fatty acid composition of the liver phospholipids was also determined. Levels of T3(15-17 nm) and T4 (250\p=n-\255nm) were significantly higher in the respective hyperthyroid groups. There was no significant influence of the diet on hormone levels. Hyperthyroidism increased the SMR 37\p=n-\44%above the euthyroid levels. A significant body weight loss of 14\p=n-\18% was observed in hyperthyroid mice on the PUFA diet but not in those on the SFA diet. PUFA diet significantly reduced the activity of ME but had no effect on Na-K\x=req-\ ATPase or GPDH activity. Activities of Na-K-ATPase and GPDH were significantly elevated in all hyperthyroid groups. Mice on T4 and PUFA diet showed a highly significant 399 % increase in GPDH activity above the euthyroid level. The overall degree of unsaturation of the fatty acids in the liver phospholipids was comparable in all groups. Dietary treatment substantially changed liver membrane fatty acid composition whilst hyperthyroidism resulted in only small changes. The only parameters to show an interaction between dietary fat profile and hyperthyroidism were ME activity, changes in body mass and liver phospholipid fatty acid composition