The acute effects of alteration in the dietary concentrations of carbohydrate, protein, and lipid on plasma T4, T3, and glucose levels in rainbow trout, Oncorhynchus mykiss.

The acute (4 hr) postprandial effects of a single isocaloric meal varying in the proportions of either carbohydrate (C)/lipid (L), C/protein (P), or L/P on plasma levels of glucose, T4 (L-thyroxine) and T3 (3,5,3'-triiodo-L-thyronine) were examined in rainbow trout starved for 3 days. Relative to starved controls, plasma T3 was generally uninfluenced by feeding but was increased by diets containing the highest C/L and P/L ratios. Plasma T4 was elevated only in instances where there was sufficient available dietary C to raise plasma glucose to at least 126 mg/100 ml. High dietary P or L levels in combination with low C levels and a postprandial plasma glucose level below 126 mg/100 ml did not elevate plasma T4. For fish fed an acaloric alpha-cellulose diet, plasma T4 was unchanged indicating that gastric filling alone does not contribute significantly to the T4 surge. It is concluded that the previously demonstrated postprandial elevation in plasma T4 is determined mainly by the level of dietary C and the available glucose, and not by P, L, total caloric content, or bulk properties of the ingesta.