Nutrition, thyroid function and basal metabolism of the Evenki of central Siberia

Considerable research has shown that indigenous circumpolar populations have elevated basal metabolic rates (BMR). Such elevations are likely promoted by changes in thyroid hormone levels; however, to date, this link between thyroid function and variation in BMR has not been explicitly demonstrated in an indigenous northern group. Thus, the purpose of this paper is to examine 1) the influence of dietary and nutritional factors on variation in thyroid hormone levels, and 2) the influence of thyroid levels on BMR in a sample of indigenous Siberian herders (the Evenki). Measures of body size and composition, dietary intake, thyroid hormones (total and free T3 and T4), thyroid stimulating hormone (TSH) and BMR were obtained for 60 adult subjects (19 males; 41 females) ranging in age from 18-56 years. Thyroid hormone levels in the Evenki were significantly correlated with measures of dietary intake and body composition. Among men, total T3 levels were positively correlated with body weight and fat-free mass (FFM), and negatively correlated with percent body fat; free T4 levels were positively correlated with dietary energy, protein and fat consumption. Among wo-men, there were no clear associations between body composition and thyroid levels; however, dietary energy and protein intakes were po-sitively correlated with total T4 and free T3 levels, respectively. Variation in thyroid hormone levels was associated with differences in BMR. In both sexes, deviations from predicted BMR were positively correlated with free T4 levels. These results demonstrate a link between thyroid hormones and elevated BMR in an indigenous circumpolar population. Further, they show the importance of dietary and nutritional factors for influencing thyroid function and, in turn, metabolic rates among such northern groups.