Effect of Carbohydrate and Noncarbohydrate Sources of Calories on Plasma 3,5,3′-Triiodothyronine Concentrations in Man*

Abstract

To evaluate the effect of changes in dietary carbohydrate (CHO) and excessive caloric consumption on circulating thyroid hormone levels, six normal weight subjects were fed five separate diets: three isocaloric diets with 20%, 40%, or 80% CHO and two hypercaloric (+2000 calories) diets with 20% or 40% CHO for 5 days each as outpatients. T4, T3, and rT3 concentrations were measured in plasma samples collected on the morning of the sixth day. At least 1 week of the subjects' usual diets intervened between each experimental diet. Mean T4 and rT3 levels were similar after all diets. Pair-wise comparisons among all five diets revealed significantly (P < 0.005) increased T3 concentrations after both hypercaloric diets compared to the iso-20 and iso-40 diets, and after the iso-80 compared to the iso-20 diet. A multiple regression analysis of the data revealed the highest correlation of T3 levels with total calories (r = 0.68; P < 0.001) rather than with the intake of CHO (r = 0.46; P < 0.025), fat (r = 0.49; P < 0.02), or protein (r = 0.30; P = NS). A repeated measures analysis of variance of the data revealed that over a daily caloric range of 2100-4100 calories, increasing CHO intake between 204-408 g did not influence T:t levels. However, over this same range of CHO intake, increasing calories between 2100-4100 was associated with increased T3 concentrations. Furthermore, this caloric effect on T3 levels was independent of CHO intake. These data suggest that: 1) non-CHO as well as CHO sources are important modulators of plasma T3 concentrations in man; and 2) the influence of non-CHO calories may actually be more pronounced than that of CHO when at least a normal amount (-200 g) of CHO is ingested. (J Clin Endocrinol Metab48: 577, 1979)