Characterizing the resting metabolic rate ratio in ovulatory exercising women over 12-months.

Purpose A reduced resting metabolic rate (RMR) ratio and suppressed total triiodothyronine (TT3) have been demonstrated to reflect metabolic compensation to chronic energy deficiency. However, it is unknown whether the relationship between RMR ratio and TT3 remains constant over time. Objective To examine the relationship between RMR ratio and TT3 in free-living exercising, ovulatory, weight-stable women (n = 14) for a 12-month observational period. Methods Dual-energy X-ray absorptiometry (DXA) and indirect calorimetry provided data on anthropometrics and energy expenditure. Harris-Benedict(,) DXA, and Cunningham (1980 and 1991) equations estimated RMR and RMR ratio (measured RMR/predicted RMR). Repeated measures analysis assessed changes over time (ANOVA and Friedman). Generalized linear modeling tested whether RMR ratio threshold predicted TT3 > 73.2 ng/dL or TT3 > 80 ng/dL over 12-months. Results Women were 25.9 +/- 5.4 years, weighed 59.6 +/- 5.2 kg with BMI 22.3 +/- 1.4 kg/m(2) at baseline, which remained constant throughout the study (weight: P = .523; BMI: P = .511). There was no significant effect of time for RMR (P = .886), TT3 (P = .890), energy availability (P = .212), and RMR ratio (Harris-Benedict: P = .852; DXA: P = .607; Cunningham(1980): P = .754; Cunningham(1991): P = .739). When TT3 > 73.2 ng/dL, each RMR ratio threshold (Harris-Benedict: P = .021; DXA: P = .019; Cunningham(1980): P = .019; Cunningham(1991): P = .016) significantly predicted participants as energy replete; however, when using a more lenient clinical TT3 threshold of >80 ng/dL, only the DXA ratio threshold yielded a significant prediction of TT3 (P < .001). Conclusions The relationship between RMR ratio and TT3 remains significant and consistent over time in free-living exercising women, validating the use of RMR ratio for the longitudinal characterization of energetic status in this population (ie, prospective serial monitoring).