Abstract
Using a short-duration step protocol and continuous indirect calorimetry, whole-body rates of fat and carbohydrate oxidation can be estimated across a range of exercise workloads, along with the individual maximal rate of fat oxidation (MFO) and the exercise intensity at which MFO occurs (Fatmax). These variables appear to have implications both in sport and health contexts. After discussion of the key determinants of MFO and Fatmax that must be considered during laboratory measurement, the present review sought to synthesize existing data in order to contextualize individually measured fat oxidation values. Data collected in homogenous cohorts on cycle ergometers after an overnight fast was synthesized to produce normative values in given subject populations. These normative values might be used to contextualize individual measurements and define research cohorts according their capacity for fat oxidation during exercise. Pertinent directions for future research were identified.
Highlights
During prolonged exercise, carbohydrate and fat are the primary substrates oxidized to fuel energy metabolism (Romijn et al, 1993; van Loon et al, 2001)
Humans predominantly store carbohydrates as glycogen in skeletal muscle (Bergström and Hultman, 1967; Bergström et al, 1967) and the liver (Nilsson, 1973; Nilsson et al, 1973), with modest quantities found in the brain, kidneys, and adipose tissue (Biava et al, 1966; Rigden et al, 1990; Meyer et al, 2002; Oz et al, 2003), and ∼4 g circulating in plasma as glucose (Wasserman, 2009)
The purpose of the present review is to extend previous summaries (Jeukendrup and Wallis, 2005; Purdom et al, 2018) by systematically exploring key determinants of maximal rate of fat oxidation (MFO) and Fatmax for consideration during laboratory assessment, and to for the first time contextualize individually measured values in given subject populations with normative values
Summary
Carbohydrate and fat are the primary substrates oxidized to fuel energy metabolism (Romijn et al, 1993; van Loon et al, 2001). MFO has been significantly positively correlated with insulin sensitivity in a large cohort (N = 57) of young, healthy males (Robinson et al, 2015), and absolute Fatmax (Watts) has been positively correlated with insulin sensitivity in non-insulin-resistant obese males (Lambert et al, 2017) This link might be explained by mitochondrial function, given βoxidation of fatty acids to acetyl CoA, oxidation of fatty acid or non-fatty acid-derived acetyl CoA in the citric acid cycle, and oxidative phosphorylation along the electron transport chain all occur in the mitochondria (McBride et al, 2006; Holloszy, 2011; Wu et al, 2014), and that increases in mitochondrial volume density (Hoppeler et al, 1985; Montero et al, 2015), mitochondrial oxidative capacity (Granata et al, 2016a,b), and mitochondrial enzyme content and activity (Spina et al, 1996; Scalzo et al, 2014; Granata et al, 2016a) occur in response to exercise training. This search approach yielded 53 studies for inclusion in the review
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