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

MAYER B. DAVIDSON and INDER J. CHOPRA

Department of Medicine, UCLA School of Medicine, Center for Health Sciences, Los Angeles, California 90024

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 Metab 48: 577, 1979)

^* This work was supported in part by USPHS Grants AM-14702 and AM-16155; NIH Grant RCDA-KO-70225; a grant from the American Diabetes Association, Southern California Affiliate; and Clinical Research Center Grant RR-865. It was presented in part at the Western Meeting of the American Federation for Clinical Research, Carmel, CA, February 4-6, 1976.

To whom requests for reprints should be addressed.

Received July 21, 1978.

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