EFFECT OF CALORIC RESTRICTION AND DIETARY COMPOSITION ON SERUM T₃ AND REVERSE T₃ IN MAN

Abstract

To evaluate the effect of caloric restriction and dietary composition on circulating T₃ and rT₃, obese subjects were studied after 7–18 days of total fasting and while on randomized hypocaloric diets (800 kcal) in which carbohydrate content was varied to provide from 0 to 100% calories. As anticipated, total fasting resulted in a 53% reduction in serum T₃ in association with a reciprocal 58% increase in rT₃. Subjects receiving the no-carbohydrate hypocaloric diets for two weeks demonstrated a similar 47% decline in serum T₃ but there was no significant change in rT₃ with time. In contrast, the same subjects receiving isocaloric diets containing at least 50 g of carbohydrate showed no significant changes in either T₃ or rT₃ concentration. The decline in serum T₃ during the no-carbohydrate diet correlated significantly with blood glucose and ketones but there was no correlation with insulin or glucagon. We conclude that dietary carbohydrate is an important regulatory factor in T₃ production in man. In contrast, rT₃, concentration is not significantly affected by changes in dietary carbohydrate. Our data suggest that the rise in serum rT₃ during starvation may be related to more severe caloric restriction than that caused by the 800 kcal diet.