# Differences in Body Fat in Athletes Categorized by Resting Metabolic Rate
## 5 Discussion  
The primary aim of the current study was to examine differences in BF% between athletes grouped by relative RMR. The main findings from the current study indicate that athletes in the high relative RMR group had significantly lower BF% values than those in the low RMR group. Athletes in the high RMR group were also smaller in stature, as seen with a lower body weight and BMI compared to those in the low RMR group. This is the first study to categorize RMR values among athletes and examine differences in BF% across groups of athletes categorized by RMR. Classifying an athlete's RMR when using absolute values is challenging, as absolute measures of RMR are largely influenced by an athlete's body size [4]. For example, we previously [4] demonstrated that height, body mass, body mass index, fat-free mass, and fat mass were positively associated with absolute measures of RMR in male and female athletes (r = 0.4-0.8) and that body mass was the strongest predictor of RMR, accounting for 78% and 83% of the variation in RMR for male and female athletes, respectively. Moreover, we found that every 1 kg increase in body mass was associated with a 19 kcal and 20 kcal increase in RMR for male and female athletes, respectively [4]. Therefore, expressing RMR relative to body weight, or adjusting for independent factors that influence RMR, is a way to account for differences in body size and allow for more appropriate comparisons between athletes. This may be particularly important when comparing RMR values between men and women. For example, in a study by [PERSON] et al., [18] premenopausal women were found to have a 24% lower RMR compared with younger men (1377 \(\pm\) 115 vs. 1811 \(\pm\) 198 kcal/day; \(p<0.01\)). However, after adjusting for fat-free mass and weight, only a 4% lower RMR in the premenopausal women was observed when compared with the men (1618 \(\pm\) 143 vs. \begin{table}
\begin{tabular}{l c c c c c c c} \hline \hline  & **Mean \(\pm\) SD** & **95\% CI** & **R\({}^{2}\)** & \(\beta\) & **Slope** & **SEE** & \(p\) Value** \\ \hline Body fat (\%) & 18.1 \(\pm\) 8.3 & 16.8, 19.4 & & & & & \\ \hline Weight (kg) & 81.5 \(\pm\) 21.1 & 78.2, 84.8 & 0.026 & 0.162 & 13.7 & 8.2 & 0.026 \\ \hline Fat-free mass (kg) & 66.1 \(\pm\) 16.2 & 63.5, 68.6 & 0.066 & \(-0.257\) & 27.6 & 8.1 & \(<\)0.001 \\ \hline RMR (kcals/day) & 2228 \(\pm\) 582 & 2136, 2319 & 0.004 & \(-0.062\) & 20.9 & 8.3 & 0.398 \\ \hline RMR/kg (kcal/kg) & 27.5 \(\pm\) 3.1 & 26.9, 27.9 & 0.260 & \(-1.017\) & 53.0 & 6.8 & \(<\)0.001 \\ \hline RMR/kg of FFM (kcal/kg) & 33.8 \(\pm\) 4.0 & 33.2, 34.5 & 0.134 & 0.934 & \(-6.6\) & 7.4 & \(<\)0.001 \\ \hline \hline \end{tabular} Data presented as mean \(\pm\) standard deviation. BMI = body mass index; RMR = resting metabolic rate; kg = kilograms; FFM = fat-free mass; SEE = standard error of the estimate.  
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Table 4: Linear regression summary for all athletes. Figure 2: (**A**) Association between body fat percentage and measures of resting metabolic rate per kilogram of fat-free mass; (**B**) Association between body fat percentage and relative resting metabolic rate. 1681 \(\pm\) 125 kcal/day; \(p\) < 0.01). We previously reported that male athletes have higher RMR values compared to female athletes (2595 \(\pm\) 433 vs. 1709 \(\pm\) 308 kcals; \(p\) < 0.001). However, after adjusting for differences in body mass (_p_ = 0.064) and fat-free mass (_p_ = 0.084), the observed differences were no longer significant. Results from the current study found that the average RMR for male athletes was 27.9 kcals/kg when expressed relative to body weight, with 95% of the athletes having an RMR above 22.6 kcals/kg. For female athletes, the average RMR was 25.9 kcals/kg when expressed relative to body weight, with 95% of the athletes having an RMR above 21.9 kcals/kg. These 5 th percentiles may therefore represent a low metabolic rate for athletes when expressed relative to body weight. Moreover, relative RMR values at the 95 th percentile may represent individuals with a high metabolic rate.