Factors Influencing Substrate Oxidation During Submaximal Cycling: A Modelling Analysis

Sports Medicine https://doi.org/10.1007/s40279-022-01727-7 ORIGINAL RESEARCH ARTICLE Factors Influencing Substrate Oxidation During Submaximal Cycling: A Modelling Analysis Jeffrey A. Rothschild1 · Andrew E. Kilding1 · Tom Stewart1 · Daniel J. Plews1 Accepted: 20 June 2022 © The Author(s) 2022 Abstract Background Multiple factors influence substrate oxidation during exercise including exercise duration and intensity, sex, and dietary intake before and during exercise. However, the relative influence and interaction between these factors is unclear. Objectives Our aim was to investigate factors influencing the respiratory exchange ratio (RER) during continuous exercise and formulate multivariable regression models to determine which factors best explain RER during exercise, as well as their relative influence. Methods Data were extracted from 434 studies reporting RER during continuous cycling exercise. General linear mixedeffect models were used to determine relationships between RER and factors purported to influence RER (e.g., exercise duration and intensity, muscle glycogen, dietary intake, age, and sex), and to examine which factors influenced RER, with standardized coefficients used to assess their relative influence. Results The RER decreases with exercise duration, dietary fat intake, age, VO2max, and percentage of type I muscle fibers, and increases with dietary carbohydrate intake, exercise intensity, male sex, and carbohydrate intake before and during exercise. The modelling could explain up to 59% of the variation in RER, and a model using exclusively easily modified factors (exercise duration and intensity, and dietary intake before and during exercise) could only explain 36% of the variation in RER. Variables with the largest effect on RER were sex, dietary intake, and exercise duration. Among the diet-related factors, daily fat and carbohydrate intake have a larger influence than carbohydrate ingestion during exercise. Conclusion Variability in RER during exercise cannot be fully accounted for by models incorporating a range of participant, diet, exercise, and physiological characteristics. To better understand what influences substrate oxidation during exercise further research is required on older subjects and females, and on other factors that could explain additional variability in RER. 1 Introduction Energy production during continuous, submaximal exercise comes primarily from the oxidation of fat and carbohydrate. The respiratory exchange ratio (RER) represents an indirect measure of the skeletal muscle respiratory quotient (RQ)— the quantity of CO2 produced in relation to O2 consumed [1]. The RER can be used to estimate the relative contributions of fat and carbohydrate to energy production with higher values equating to increased carbohydrate reliance and lower values representing increased fat reliance [2]. Several factors are known to influence the RER during exercise * Jeffrey A. Rothschild 1 Sports Performance Research Institute New Zealand (SPRINZ), Auckland University of Technology, Auckland, New Zealand including exercise duration [3], exercise intensity [4], training status [5], sex [6], dietary intake [7–9], the pre-exercise meal [10, 11], and carbohydrate ingestion during exercise [3, 12]. However, the relative influence and interaction between these factors is unclear. For example, RER decreases with exercise duration (i.e., increased reliance on fat oxidation), but increases with exercise intensity and carbohydrate intake [13], leaving the net effect on RER unclear when multiple factors are being manipulated. Therefore, a better understanding of the factors influencing RER during exercise is needed. The ability to effectively oxidize fat for fuel, represented by a lower RER, is important for metabolic health [14] and long-duration exercise performance [15, 16], and many athletes attempt to manipulate substrate oxidation during exercise as part of a periodized nutrition and training plan [17, 18]. However, managing substrate oxidation during exercise is challenged by the influence of both modifiable Vol.:(0123456789) J. A. Rothschild et al. Key Points Several factors are known to influence substrate oxidation during exercise, but the effect of simultaneously modulating multiple factors on the respiratory exchange ratio (RER) is unclear. Factors known to influence substrate oxidation during exercise, such as exercise duration and intensity, age, sex, fitness level, muscle glycogen, and daily dietary intake, together explain ~ 59% of the variation in RER during exercise. The easily measured and easily modifiable factors related to exercise such as exercise duration and intensity, daily macronutrient intake, and pre- and peri-exercise carbohydrate intake, can only explain roughly one-third of the variation in RER during exercise. This suggests most of what dictates RER during exercise cannot be easily controlled on a daily basis. and non-modifiable factors, which may or may not be easily measured (Table 1). Previous studies have investigated factors influencing substrate oxidation, but none have considered variables often manipulated by athletes such as the duration or intensity of exercise, the pre-exercise meal, or carbohydrate ingestion during exercise. Goedecke et al. [19] Table 1  Factors influencing respiratory exchange ratio (RER) during exercise and ease of day-to-day modification and measurement Easily modified Not easily modified found the most important factors influencing RER during endurance exercise were mitochondrial enzyme activity, muscle glycogen and triglyceride concentrations, dietary fat intake, training volume, and free fatty acid concentrations, which collectively explained 42–56% of the variation in RER during exercise. Distinct from RER, others have studied the determinants of maximal fat oxidation rates and found 34–79% of the variance was related to factors such as maximal oxygen consumption (VO2max), sex, body composition, physical activity level, dietary macronutrient intake, resting fat oxidation, and fasting duration [20–24]. To our knowledge, the relative influence of the modifiable, easily measured factors influencing RER during exercise (e.g., dietary intake before and during exercise, exercise duration, and exercise intensity) has yet to be established. Using multivariable regression models, it would be possible to account for multiple factors influencing RER during exercise and predict the response under various circumstances. Therefore, the purpose of this analysis was to investigate factors influencing the RER during cycling exercise and formulate regression models to determine which factors best explain RER during exercise, their relative influence, and the result of multiple variables being modulated simultaneously. To this end, we performed the largest pooled analyses to date (~ 3400 RER observations) of studies examining substrate oxidation during exercise and provide novel insig (...truncated)