# Differences in Body Fat in Athletes Categorized by Resting Metabolic Rate
## 1 Introduction  
Resting metabolic rate (RMR) accounts for 40-60% of total daily energy expenditure (TDEE), and therefore plays a pivotal role in energy balance for athletes [1]. Day-to-day changes in energy balance are a key driver for subsequent changes in body weight and body composition [1]. A higher daily TDEE could alter daily energy balance in a way that promotes weight loss, namely fat loss, assuming energy intake remains unchanged or below that of TDEE [2]. Conversely, a lower TDEE may predispose athletes to weight gain and subsequently increase body fat accumulation over time if energy intake is not carefully managed [2, 3]. As such, an athlete's RMR plays a key role in weight management over time. While the assessment of RMR is common across the literature, limited information is available regarding the classification of an athlete's RMR and how it may influence body composition parameters, such as body fat percentage. Expressing RMR relative to body mass may serve as a way to categorize an athlete's RMR and help provide context wheninterpreting values for athletes. Additionally, normalized RMR values allow for more appropriate comparisons between athletes, as body size (i.e., body weight) accounts for a substantial degree of variation in RMR [4]. The expression of RMR relative to body weight may also help when examining relationships between metabolic activity and body composition, as it accounts for differences in body weight. For example, an athlete with a high RMR, when expressed relative to body weight, may exhibit a lower body fat percentage due to a higher TDEE. Additionally, this could predispose an athlete to be a 'hard-gainer' if there is a desire for weight gain or the accrual of fat-free mass (FFM) in training. Furthermore, athletes with a low body fat percentage may require additional increases in energy intake to compensate for a higher RMR, relative to body weight, if attempting to increase body mass. Conversely, previous research has indicated that a low body fat percentage, often a result of a prolonged energy deficit, may result in numerous physiological and metabolic adaptations, which may suppress energy expenditure as a reflection of improved metabolic efficiency and compensate for low energy intake [5; 6]. Therefore, athletes with a lower body fat percentage may exhibit lower, or suppressed, RMR values, even after accounting for differences in body mass and FFM. However, a lower body fat percentage would also indicate a higher percentage of FFM for a given athlete. FFM has been previously shown to be a strong predictor of RMR in athletes, often accounting for 70% of the variance in RMR [7]. Therefore, athletes with a higher percentage of FFM or greater overall FFM will likely have higher RMR values, when expressed relative to body mass, when compared to their counterparts with less FFM [4; 7; 8]. Currently, it is unknown how RMR and various expressions of RMR (e.g., RMR/kg, RMR/kg of FFM) among athletes could be categorized and what level of influence these values may have on body fat percentages. We have previously reported that RMR is higher in male athletes compared to female athletes, largely mediated by greater absolute body mass in male athletes [4; 9]. Further, when expressed relative to body mass and FFM, differences in RMR initially observed between male and female athletes become insignificant [4; 9]. Additionally, strong relationships between body mass and RMR, and FFM and RMR [4; 7; 8; 9; 10; 11; 12; 13; 14], for both male and female athletes have been previously observed. However, more work is needed to categorize RMR values across male and female athletes and examine how this may influence body composition. Additionally, a descriptive summary of RMR values expressed relative to body mass can allow practitioners to better interpret RMR values and classify athletes as having a 'low' or 'high' RMR and allow for the comparison of body composition parameters (e.g., body fat percentage) between athletes.