The training induced increase in whole-body peak fat oxidation rate may be attenuated with aging

Factors Influencing Substrate Oxidation During Submaximal Cycling: A Modelling Analysis

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, VO_2max, 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.

Maximal Fat Oxidation Rate Is Higher in Fit Women and Unfit Women With Obesity, Compared to Normal-weight Unfit Women

CONTEXT

The maximal fat oxidation rate (MFO) is higher in aerobically fit vs unfit young men, but this training-related increase in MFO is attenuated in middle-aged men. Further, it has also been found that unfit men with obesity may have an elevated MFO compared to unfit normal-weight men.

OBJECTIVE

Based hereupon, we aimed to investigate whether a fitness-related higher MFO were attenuated in middle-aged women compared to young women. Also, we aimed to investigate if unfit women with obesity have a higher MFO compared to unfit normal-weight women. We hypothesized that the training-related elevated MFO was attenuated in middle-aged women, but that unfit women with obesity would have an elevated MFO compared to unfit normal-weight women.

METHODS

We recruited 70 women stratified into 6 groups: young fit (n = 12), young unfit (n = 12) middle-aged fit (n = 12), middle-aged unfit (n = 12), unfit young women with obesity (n = 12), and unfit middle-aged women with obesity (n = 10). Body composition and resting blood samples were obtained and MFO was measured by a graded exercise test on a cycle ergometer via indirect calorimetry. Subsequently, a maximal exercise test was performed to establish peak oxygen uptake (V̇O2peak).

RESULTS

Young and middle-aged fit women had a higher MFO compared to age-matched unfit women, and young fit women had a higher MFO compared to fit middle-aged women. Unfit women with obesity, independent of age, had a higher MFO compared to their normal-weight and unfit counterparts.

CONCLUSION

The training-related increase in MFO seems maintained in middle-aged women, and we find that unfit women with obesity, independent of age, have a higher MFO compared to unfit normal-weight women.

The influence of age, sex and cardiorespiratory fitness on maximal fat oxidation rate

Fat oxidation decreases with age, yet no studies have previously investigated if aging affects the maximal fat oxidation rate (MFO) during exercise in men and women differently. We hypothesized that increased age would be associated with a decline in MFO and this would be more pronounced in women due to menopause, compared with men. In this cross-sectional study design, 435 (247/188, male/female) subjects of varying ages performed a DXA scan, a submaximal graded exercise test and a maximal oxygen uptake test, to measure MFO and cardiorespiratory fitness (CRF) by indirect calorimetry. Subjects were stratified into 12 groups according to sex (male/female), age (<45, 45-55 and >55 years), CRF (below average and above average). Women aged <45 years had a higher MFO relative to fat free mass (FFM) (mg/min/kg) compared with men, regardless of CRF. However, there were no differences in MFO (mg/min/kg FFM) between men and women, in the groups aged between 45-55 and >55 years. In summary, we found that women aged <45 years display a higher MFO (mg/min/kg FFM) compared with men and that this sexual divergence is abolished after the age of 45 years. Novelty: Maximal fat oxidation rate is higher in young women compared with men. This sex-related difference is attenuated after the age of 45 years. Cardiorespiratory fitness does not influence this sex-related difference.