Effect of long-term calorie restriction with adequate protein and micronutrients on thyroid hormones

Caloric restriction (CR) retards aging in mammals. It has been hypothesized that a reduction in T(3) hormone may increase life span by conserving energy and reducing free-radical production.

The objective of the study was to assess the relationship between long-term CR with adequate protein and micronutrient intake on thyroid function in healthy lean weight-stable adult men and women.

In this study, serum thyroid hormones were evaluated in 28 men and women (mean age, 52 +/- 12 yr) consuming a CR diet for 3-15 yr (6 +/- 3 yr), 28 age- and sex-matched sedentary (WD), and 28 body fat-matched exercising (EX) subjects who were eating Western diets.

Serum total and free T(4), total and free T(3), reverse T(3), and TSH concentrations were the main outcome measures.

Energy intake was lower in the CR group (1779 +/- 355 kcal/d) than the WD (2433 +/- 502 kcal/d) and EX (2811 +/- 711 kcal/d) groups (P < 0.001). Serum T(3) concentration was lower in the CR group than the WD and EX groups (73.6 +/- 22 vs. 91.0 +/- 13 vs. 94.3 +/- 17 ng/dl, respectively) (P < or = 0.001), whereas serum total and free T(4), reverse T(3), and TSH concentrations were similar among groups.

Long-term CR with adequate protein and micronutrient intake in lean and weight-stable healthy humans is associated with a sustained reduction in serum T(3) concentration, similar to that found in CR rodents and monkeys. This effect is likely due to CR itself, rather than to a decrease in body fat mass, and could be involved in slowing the rate of aging.