Influence of physical activity and dietary restraint on resting energy expenditure in young nonobese females

An understanding of the physiological and behavioral determinants of resting energy requirements is important to nutritional considerations in females. We examined the influence of endurance training and self-reported dietary restraint on resting metabolic rate and fasting plasma hormones in 44 nonobese females characterized for body composition, maximal aerobic power [Formula: see text], and daily energy intake. To examine the association of metabolic rate and dietary restraint with hormonal status, fasting plasma levels of insulin, glucose, and thyroid hormones (total and free fractions of thyroxine and triiodothyronine) were determined. In univariate analysis, resting metabolic rate (kcal∙min⁻¹) was positively related to [Formula: see text] (L∙min⁻¹) (r = 0.54; p < 0.01). This relationship, however, was partially dependent on body size, since fat-free mass was also related to resting metabolic rate (r = 0.42; p < 0.01) and [Formula: see text] (L∙min⁻¹) (r = 0.75; p < 0.01). After controlling for fat-free weight using partial correlation analysis, the relation between RMR and [Formula: see text] was weaker but still significant (partial r = 0.38; p < 0.05). On the other hand, high levels of dietary restraint were associated with higher levels of body fat (r = 0.31; p < 0.05) and a lower resting metabolic rate (r = −0.29; p = 0.07). These associations persisted after control for differences in fat-free mass. Total energy intake as well as total and free levels of triiodothyronine were not related to resting metabolic rate or level of dietary restraint. Our results suggest that the level of endurance training (i.e., [Formula: see text]) and dietary restraint, independent of differences in fat-free mass, contribute to individual variation in resting metabolic rate of nonobese females. These findings appear to be unrelated to fasting plasma concentrations of thyroid hormones. Whereas high levels of endurance training are associated with increased energy requirements at rest, higher levels of dietary restraint are associated with a lower resting metabolic rate and possibly a propensity to gain body fat.Key words: endurance training, dietary restraint, resting metabolic rate, females, energy intake.