DIETARY OMEGA-6 FATTY-ACIDS AND THE EFFECTS OF HYPERTHYROIDISM IN MICE

The influence of the type of dietary fat on the effects of thyroid hor mones was investigated in mice. Hyperthyroidism was achieved by provid ing thyroid hormones (T-3 and T-4) in the drinking water. Both hyperth yroid and euthyroid mice (Mus musculus) were fed isoenergetic diets co ntaining 18% (w/w) total lipid but differing in fatty acid composition . Diets were either low in the polyunsaturated linoleic acid (18:2, om ega 6) and high in saturated fatty acids (SFAs) or low in saturated fa ts and high in the polyunsaturated fatty acid (PUFA), linoleic acid. T reatments were maintained for 21-22 days. Plasma thyroid hormone level s, standard metabolic rate (SMR), changes in body mass, specific activ ities of malic enzyme (ME), Na-K-ATPase and glycerolphosphate dehydrog enase (GPDH) of the liver were measured. Fatty acid composition of the liver phospholipids was also determined. Levels of T-3 (15-17 nM) and T-4 (250-255 nM) were significantly higher in the respective hyperthy roid groups. There was no significant influence of the diet on hormone levels. Hyperthyroidism increased the SMR 37-44% above the euthyroid levels. A significant body weight loss of 14-18% was observed in hyper thyroid mice on the PUFA diet but not in those on the SFA diet. PUFA d iet significantly reduced the activity of ME but had no effect on Na-K -ATPase or GPDH activity. Activities of Na-K-ATPase and GPDH were sign ificantly elevated in all hyperthyroid groups. Mice on T-4 and PUFA di et showed a highly significant 399% increase in GPDH activity above th e euthyroid level. The overall degree of unsaturation of the fatty aci ds in the liver phospholipids was comparable in all groups. Dietary tr eatment substantially changed liver membrane fatty acid composition wh ilst hyperthyroidism resulted in only small changes. The only paramete rs to show an interaction between dietary fat profile and hyperthyroid ism were ME activity, changes in body mass and liver phospholipid fatt y acid composition.