Determinants of resting metabolic rate and factors contributing to erroneous measurements in the female athlete

Resting metabolic rate (RMR) represents the energy required to maintain the systems of the body and to regulate body temperature at rest. RMR is measured using indirect calorimetry with measurements occurring in an overnight fasted and rested state. Traditionally, an athlete’s RMR has been measured as part of the estimate of total daily energy requirements. A more novel potential use of RMR measurements is as an indicator of low energy availability (LEA) with energy availability representing the energy left over for the body after accounting for the energy expended through exercise. A decreased RMR is a proposed potential indicator of LEA and signals that insufficient energy is being partitioned to one or more systems underpinning body function. However, RMR measurements are not currently supported for use as an indicator of LEA due in part to high measurement variability. There are numerous biological, technical and environmental factors that contribute to variability in RMR measurements as well as factors that may lead to erroneous RMR measurements. This thesis will investigate factors contributing to variability in RMR measurements as well as artifacts that may interfere with a valid and clinically useful interpretation of the data.

Study 1: Effect of menstrual cycle phase and hormonal contraceptives on resting metabolic rate and body composition The purpose of this study was to determine if RMR changes across the menstrual cycle (MC) and differs compared to hormonal contraceptive (HC) users. This was accomplished during a 5-week training camp involving naturally cycling (NC) athletes (n=11) and HC users (n= 7 subdermal progestin implant, n= 4 combined-monophasic oral contraceptive pill, n=1 injection) from the National Rugby League Indigenous Women’s Academy. There was no effect of MC phase on absolute RMR (p=0.875) or relative RMR (p=0.958) nor was there an effect of HC use on absolute RMR (p=0.068) or relative RMR (p=0.309).

Study 2: The temporal effects of altitude and low energy availability manipulation on resting metabolic rate in female race walkers This study investigated the temporal effects of ~1800 m altitude exposure and energy availability (EA) manipulation on RMR. Twenty elite female race walkers underwent a 3-week training camp at an altitude of ~1800 m. During the first two weeks, athletes consumed a high EA (HEA) diet of 45 kcal·kg fat free mass (FFM)-1·day-1. During the final week, half the athletes consumed a low EA (LEA) diet of 15 kcal·kg FFM-1·day-1 while the others continued on a HEA diet. Athletes in the HEA group had RMR measured at baseline (~580 m) prior to altitude exposure (Pre-alt), at 36 hours (36h-alt), 2 weeks (Wk2-alt) and 3 weeks into altitude exposure (Wk3-alt), and at 36 hours post-altitude exposure at ~580 m (36h-post). Athletes in the LEA group underwent RMR measurements at Pre-alt and before (Wk2-alt) and after the 7 days of LEA (Wk3-alt). Compared to Pre-alt, the RMR of HEA athletes was increased at 36h-alt (+5.3±3.1%; p=0.026) and Wk2-alt (+4.9±4.9%; p=0.049) but was no longer elevated at Wk3-alt (+1.7±4.2%; p=0.850). The RMR of HEA athletes at 36h-post was lower than all timepoints at altitude (p<0.05), but was not different from Pre-alt (-3.9±7.2%; p=0.124). The 7-day period of LEA exposure at altitude did not affect RMR (p=0.347).

Study 3: Impact of acute dietary and exercise manipulation on next day RMR measurements and DXA body composition estimates This study investigated the effects of acute diet and exercise manipulation on RMR measurement variability. 10 male and 10 female endurance athletes (12 cyclists, 5 triathletes, 4 runners) of Tier 2 (n=18) to Tier 3 (n=2) calibre underwent five conditions using a Latin square counterbalance design. For 24 hours, athletes consumed a diet providing excessive energy availability (75 kcal‧kg FFM-1) without exercise, high energy availability (45 kcal‧kg FFM-1) without or with exercise, or low-energy availability (15 kcal‧kg FFM-1) without or with exercise. There were no sex differences in relative RMR (p=0.158) nor effects of any of the five conditions on RMR (p=0.358).

Study 4: Barriers and enablers to measuring resting metabolic rate in the high-performance sporting system: A qualitative exploratory study This study investigated barriers and enablers to measuring RMR in real-world, high-performance sport environments. Twelve semi-structured interviews were conducted with RMR technicians (n=6 dietitians, n=6 physiologists) employed across six National Institute Networks, two National Sporting Organisations and one professional sporting code. Barriers included lack of confidence in measuring RMR, burden of measurement on athlete and technician, confusion over measurement responsibility, and scepticism in RMR measurements as an indicator of LEA. Subthemes that contributed to scepticism included: the RMR thresholds used to indicate LEA, unanswered research questions, and measurement errors introduced by athlete presentation, testing equipment and/or environment. Enablers to use of RMR measurements included perceived value of RMR measurements as a “piece of the puzzle” when assessing for LEA and its use as a longitudinal measure.

Summary and future direction: This series of research studies investigated factors contributing to variability and error in RMR measurements. As a result, several gaps in the literature have been addressed and results provide novel insight, including: 1) MC phase does not contribute to variability in RMR measurements in female athletes 2) HC usage does not contribute to variability in RMR measurements in female athletes 3) RMR is acutely increased with low altitude exposure but returns to baseline values with more prolonged exposure in female athletes 4) 7 days of LEA while at altitude does not impact RMR in female athletes 5) Large variations in EI does not introduce error into next day RMR measurements 6) Prolonged continuous cycling at moderate intensity does not introduce error into next day RMR measurements 7) While RMR measurements are seen by practitioners working with the high-performance athlete as a piece of the puzzle when assessing for LEA and valuable when used longitudinally, there are multiple barriers that need to be addressed when measuring RMR in the high-performance sport environment. A uniform approach is needed when measuring RMR both within the high-performance sport environment and research so that measurement variability is minimised. While this thesis addressed some notable gaps in the literature, further research is needed This includes assessing the impact of training status independent of LEA on RMR, and if resistance exercise and high intensity exercise needs to be restricted the day prior to a RMR measurement. A normative database of RMR in athletic cohorts is also needed that considers characteristics that impact the RMR to FFM relationship to better understand what a normal RMR is, and what threshold may indicate LEA. This could also provide insight into novel ways of assessing an athlete’s RMR, such as indexing RMR to regional estimates of FFM.

This work © 2025 by Megan A. Kuikman is licensed under Creative Commons Attribution-NonCommercial-NoDerivatives 4.0 International (https://creativecommons.org/licenses/by-nc-nd/4.0/).