The RMR Ratio Thresholds For Energy Deficiency In Exercising Females Cannot Be Used In Males

Numerous studies have used the resting metabolic rate ratio (RMRratio) as a proxy indicator of energy deficiency in young exercising males, although it has been validated only in females. PURPOSE: To investigate sex differences in the relationship between RMRratio with total triiodothyronine (TT3). METHODS: Young exercising males (M; n = 41) and females (F; n = 42) were submitted to an RMR test via indirect calorimetry, and a blood draw in a 12-h fasted and rested state. Chemiluminescence assessed serum TT3 and dual-energy x-ray absorptiometry (DXA) assessed body composition. RMRratio was calculated as the ratio of the measured RMR to the predicted RMR from four different equations: Harris-Benedict, Cunningham1980, Cunningham1991, and DXA-calculated RMR. The relationship between TT3 and RMRratio was assessed with Pearson correlation coefficients. Independent samples t-tests and Mann-Whitney tests were used for sex comparisons. Data are shown as mean ± standard error of the mean. RESULTS: M were older (22.3 ± 5.5 vs. 20.7 ± 3.5 years, p = 0.023), had lower percent body fat (19.2 ± 4.8 vs. 28.6 ± 7.0 %, p < 0.001), and had higher TT3 (114.2 ± 3.3 vs. 99.1 ± 4.9 ng/dL, p = 0.014) than F, but there were no differences in body mass index (24.0 ± 4.3 vs. 23.1 ± 3.8 kg/m2, p = 0.147) or RMR adjusted for lean body mass (29.2 ± 4.0 vs. 30.0 ± 4.9 kcal/kg LBM/day, p = 0.193). M had higher Harris-Benedict (0.911 ± 0.013 vs. 0.851 ± 0.014, p = 0.002), Cunningham1980 (0.955 ± 0.012 vs. 0.876 ± 0.013, p < 0.001), and Cunningham1991 RMRratios (1.005 ± 0.012 vs. 0.939 ± 0.014, p < 0.001), but similar DXA-calculated RMRratio (0.957 ± 0.013 vs. 0.937 ± 0.013, p = 0.303) compared to F. The DXA-calculated RMRratio was positively correlated with TT3 in M (r = 0.396, p = 0.010) and F (r = 0.514, p < 0.001), but Cunninham1980 and Cunningham1991 were correlated with TT3 only in F (r = 0.435, p = 0.004 and r = 0.454, p = 0.003). Harris-Benedict RMRratio was not correlated with TT3 in neither group (p > 0.05). Men below the cut-off of 0.94 with the DXA-calculated RMRratio, previously validated in females, had clinically healthy mean TT3 (107.5 ± 4.0 ng/dL). CONCLUSION: RMRratio thresholds for energy deficiency in females do not seem suitable for males. Further studies are needed to determine whether the RMRratio is a reliable proxy indicator of energy deficiency in exercising males. This study was funded by the Pennsylvania State University Social Science Research Institute. Ana Carla Chierighini Salamunes is funded by the Fulbright Comission Brasil and CAPES.