Diet composition as a factor in the anabolic efficacy of 3,5,3′-triiodo-L-thyronine administered orally to steelhead trout ( Salmo gairdneri)

Abstract

To assess the effect of diet composition on the anabolic efficacy of orally administered, 3,5,3′-triiodo-L-thyronine (T 3), juvenile steelhead trout ( Salmo gairdneri) were fed either a commercial dry diet (Silvercup) or an experimental dry diet with similar protein and gross energy contents. Lipid provided the major part of available non-protein energy in the former diet whereas carbohydrate (dextrin and glucose monohydrate) was the predominant source in the latter. After an initial 42-day period growth rate, appetite and protein efficiency ratios (PER) were similar for groups fed the two basal diets without hormone supplements. Terminal moisture content was significantly reduced and lipid content increased in fish fed the experimental diet. These fish also had a significantly elevated hepatosomatic index (HSI) and histological examination indicated a high degree of hepatic lipid or glycogen deposition. Supplementing the Silvercup diet with 1 or 5 ppm of T 3 did not significantly affect growth rate, appetite, PER, protein conversion efficiency (PCE) or energy conversion efficiency (ECE). However, 25 ppm of T 3 induced a small but statistically significant increase in weight. Fish fed the experimental diet, on the other hand, responded to T 3 supplementation (1, 5 and 25 ppm) with significant increases in weight and length. PER increased in proportion to the T 3 dose. Increases were also noted in PCE and ECE. Whole body protein and moisture content increased but lipid content and HSI decreased in the 25 ppm group. A highly variable enlargement of thyroid follicle epithelial cells was noted in all fish administered T 3. Dietary T 3 affected external morphology to a degree proportional to the T 3 dose. We conclude that diet composition has a pronouced influence on the anabolic efficacy of oral T 3. Also, our data suggest that oral T 3 stimulates growth of steelhead trout through increased carbohydrate utilization.