Factors Influencing Substrate Oxidation During Submaximal Cycling: A Modelling Analysis

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 mixed-effect 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.

The quantification of daily carbohydrate periodization among endurance athletes during 12 weeks of self-selected training: presentation of a novel Carbohydrate Periodization Index

“…Exercise was summed into daily totals for workout duration and training load, along with coded variables for modality of workout (e.g., swim, bike, run, strength, other) and if any training was performed in the fasted state. Because dietary protein and fat ingestion have minimal effects on substrate oxidation [24, 25], fasted training was defined as consuming < 5 g of carbohydrate in the 4-h pre-exercise window. For multiple exercise sessions in a single day, a weighted mean based on the duration of each session was used to calculate a single value for pre-exercise carbohydrate ingestion in grams.…”

Low carbohydrate high fat ketogenic diets on the exercise crossover point and glucose homeostasis

“…These data are very similar to those measured in recreational but well-trained runners who had adapted to the LCHF diet for 6 weeks (Prins et al, 2019;Prins et al, 2023b). While these rapid adaptations may be partially explained by elite-level fitness (Rothschild et al, 2022), recent analyses have demonstrated that respiratory quotient and substrate oxidation changes occur in 4 days in male endurance athletes on an isocaloric LCHF diet (Prins et al, 2019), within 7 days (Hall et al, 2016) and 14 days in moderately trained athletes (Cipryan et al, 2018) and overweight/obese subjects (Buga et al, 2021;Hall et al, 2021), as biochemical changes consistent with increase fat oxidation (i.e., reduced glucose and insulin load; increase Data: Mean ± SD. *p < 0.05, **p < 0.01, ***p < 0.001, significant difference between LCHF and HCLF.…”

“…The most obvious candidate would be a significant change in blood insulin concentrations and action (glucose-lowering effect of insulin) since even at modest blood concentrations, insulin exerts powerful control over the rates of glucose ( Thiebaud et al, 1982 ) and fat oxidation ( Bonadonna et al, 1990 ; Campbell et al, 1992 ; Horowitz et al, 1997 ). In an attempt to understand other factors which influence substrate oxidation during exercise, Rothschild et al conducted a multivariable regression of 434 studies ( Rothschild et al, 2022 ). Exercise duration, dietary fat intake, age, VO 2 max, and percentage of type 1 muscle fiber all were associated with decreased RER, while dietary carbohydrate intake, exercise intensity, male sex, and carbohydrate intake before and during exercise increased RER.…”

“…These data are very similar to those measured in recreational but well-trained runners who had adapted to the LCHF diet for 6 weeks ( Prins et al, 2019 ; Prins et al, 2023b ). While these rapid adaptations may be partially explained by elite-level fitness ( Rothschild et al, 2022 ), recent analyses have demonstrated that respiratory quotient and substrate oxidation changes occur in 4 days in male endurance athletes on an isocaloric LCHF diet ( Prins et al, 2019 ), within 7 days ( Hall et al, 2016 ) and 14 days in moderately trained athletes ( Cipryan et al, 2018 ) and overweight/obese subjects ( Buga et al, 2021 ; Hall et al, 2021 ), as biochemical changes consistent with increase fat oxidation (i.e., reduced glucose and insulin load; increase ketones) occur upon the first 24 h following reduced carbohydrate intake (<50 g/day) ( Hengist et al, 2022 ). While the timeline for changes and plateaus in substrate oxidation following LCHF adaptations remains unknown, changes are initiated within the first 24 h of LCHF feeding and the plateaus appear to occur at some point within the first two weeks and are likely influenced by the baseline fitness level of each individual.…”

Daily rhythms in metabolic and locomotor behaviour of prematurely ageing PolgA mice

“…The energy source used by the body in which oxygen is consumed (VO2) and carbon dioxide is produced (VCO2) is measured by calorimetry (Brandi et al, 1997; Battezzati and Vigano’, 2001). The ratio of VCO2/VO2 calculates the RER, which estimates the respective contributions of fats and carbohydrates to energy production, enabling the calculation of total energy expenditure (Rothschild et al, 2022). The circadian rhythms, which directly influence tissue homeostasis, sleep regulation, behaviour, and metabolism, are closely linked to the ageing process (Basso et al 2016; Sato et al 2017, 2022; Kuintzle et al 2017; Welz and Benitah 2020).…”