Abstract
BackgroundResting metabolic rate (RMR) has been examined as a proxy for low energy availability (EA). Previous studies have been limited to adult athletes, despite the serious health consequences of low EA, particularly during adolescence. This study aimed to explore the relationship between RMR and EA in competitive teenage girl runners.MethodsEighteen girl runners (mean ± standard-deviation; age, 16.8 ± 0.9 years; body mass, 45.6 ± 5.2 kg, %fat, 13.5 ± 4.2 %) in the same competitive high-school team were evaluated. Each runner was asked to report dietary records with photos and training logs for seven days. Energy intake (EI) was assessed by Registered Dietitian Nutritionists. The runners were evaluated on a treadmill with an indirect calorimeter to yield individual prediction equations for oxygen consumption using running velocity and heart rate (HR). Exercise energy expenditure (EEE) was calculated by the equations based on training logs and HR. Daily EA was calculated by subtracting EEE from EI. The daily means of these variables were calculated. RMR was measured early in the morning by whole-room calorimetry after overnight sleep on concluding the final day of the seven-day assessment. The ratio of measured RMR to predicted RMR (RMR ratio) was calculated by race, age, sex-specific formulae, and Cunningham’s equation. Body composition was measured using dual-energy X-ray absorptiometry. Bivariate correlation analyses were used to examine the relationship between variables.ResultsRMR, EI, EEE, and EA were 26.9 ± 2.4, 56.8 ± 15.2, 21.7 ± 5.9, and 35.0 ± 15.0 kcal⋅kg−1 FFM⋅d−1, respectively. RMR reduced linearly with statistical significance, while EA decreased to a threshold level (30 kcal⋅kg−1 FFM⋅d−1) (r= 0.58, p= 0.048). Further reduction in RMR was not observed when EA fell below the threshold. There was no significant correlation between RMR ratios and EA, irrespective of the prediction formulae used.ConclusionsThese results suggest that RMR does not reduce with a decrease in EA among highly competitive and lean teenage girl runners. RMR remains disproportionally higher than expected in low EA states. Free-living teenage girl runners with low EA should be cautiously identified using RMR as a proxy for EA change.
Highlights
Resting metabolic rate (RMR) has been examined as a proxy for low energy availability (EA)
Since RMR is influenced by race, age, sex [22], and the method of measurement [23], we examined a small group of runners from the same team with highly homogeneous backgrounds and measured RMR according to the rigorous testing procedure for basal metabolic rate [23] using a whole room calorimeter
The participants were exclusively thin and lean (Table 1); body mass, Body mass index (BMI), fat mass, fat free mass (FFM), BMDTBLH, and Z-score were significantly higher in participants with low EA than those above the threshold (30 kcal⋅kg−1 FFM⋅d−1)
Summary
Resting metabolic rate (RMR) has been examined as a proxy for low energy availability (EA). Causeeffect relationships between low EA and reproductive hormone disturbances or bone metabolism have been demonstrated [2,3,4]. Since it is difficult to accurately measure EA in practice, reductions in resting metabolic rate (RMR) have been examined as a proxy for low EA [7,8,9]. Only a few studies have directly examined the association between low EA and a reduction in RMR among female athletes [7, 8, 11]. Since EEE is needed to determine EA, further research is required prior to using reduced RMR to identify athletes with low EA
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