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/m2 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; Cunningham1980 : P = .754; Cunningham1991 : P = .739). When TT3 > 73.2 ng/dL, each RMR ratio threshold (Harris-Benedict: P = .021; DXA: P = .019; Cunningham1980 : P = .019; Cunningham1991 : 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).
Keywords: energy deficiency; female athlete triad; metabolism; resting metabolic rate.
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