Effects of dietary fish oil on thyroid hormone signaling in the liver

n-3 polyunsaturated fatty acids (PUFAs) present in fish oil (FO) potently decrease serum lipids, which is also an effect of thyroid hormones. Both PUFAs and thyroid hormones affect hepatic lipid metabolism, and here we hypothesized that a long-term diet rich in n-3 PUFAs would enhance thyroid hormone action in the liver. Female rats received isocaloric and normolipid diets containing either soybean oil (SO) or FO during lactation. Male offspring received the same diet as their dams since weaning until sacrifice when they were 11 weeks old. FO group, as compared to SO group, exhibited lower body weight since 5 weeks of age until sacrifice, with no alterations in food ingestion, lower retroperitoneal white fat mass and elevated inguinal fat mass relative to body weight, with unchanged water and lipid but reduced protein percentage in their carcasses. FO diet resulted in lower serum triglycerides and cholesterol. Serum total triiodothyronine, total thyroxine and thyrotropin were similar between groups. However, liver thyroid hormone receptor (TR) β1 protein expression was higher in the FO group and correlated negatively with serum lipids. Liver 5'-deiodinase activity, which converts thyroxine into triiodothyronine, was similar between groups. However, the activity of hepatic mitochondrial glycerophosphate dehydrogenase, the enzyme involved in thermogenesis and a well-characterized target stimulated by T3 via TRβ1, was higher in the FO group, suggesting enhancement of thyroid hormone action. These findings suggest that the increase in thyroid hormone signaling pathways in the liver may be one of the mechanisms by which n-3 PUFAs exert part of their effects on lipid metabolism.