Fat oxidation and metabolic flexibility
Evidence: moderate
Fat and carbohydrate are burned together at all times, and their relative shares shift with intensity: fat dominates at low intensity, carbohydrate takes over as effort rises. Endurance training raises fat oxidation and spares glycogen. The fuels’ relative use, and the ability to switch between them, shapes how far a given pace can be held.
The aerobic system burns both fat and carbohydrate, and the two are always used together (energy systems). What changes is the ratio. At rest and easy intensities, fat supplies most of the energy; as the pace rises, carbohydrate’s share grows until it dominates. The intensity at which carbohydrate overtakes fat is the crossover point, and pushing harder beyond it shifts the mix further towards carbohydrate while fat oxidation falls (Brooks & Mercier 1994). This is the same shift seen from the glycogen side: the harder the effort, the more it leans on stored carbohydrate.
FATmax
Plotted against intensity, the absolute rate of fat oxidation rises from low to moderate intensity, peaks, then falls away as carbohydrate takes over. The intensity at the peak is called FATmax. In trained men it sits at roughly 63% of VO₂max, and lower in untrained people; above about 65 to 70% of VO₂max, fat oxidation drops sharply (Achten & Jeukendrup 2004). FATmax is therefore a low-to-moderate intensity, in the region of easy-to-steady running rather than threshold work, which is one reason most endurance volume is run easy (the basics).
Why carbohydrate is the higher-power fuel
Fat is the more energy-dense store: a gram of fat yields more than twice the ATP of a gram of carbohydrate, and the body holds far more of it. But oxidising fat costs more oxygen per unit of ATP, so per litre of oxygen consumed, carbohydrate yields more ATP than fat (Brooks & Mercier 1994). Because oxygen delivery is the binding constraint at hard intensities, carbohydrate is the higher-power fuel: it lets the muscle produce ATP faster for the oxygen available. This is the physiological reason a hard pace cannot be sustained on fat alone, and why glycogen depletion forces a slowdown (glycogen).
What raises fat oxidation, and what blunts it
Endurance training is the main lever. It increases mitochondrial content and the enzymes of fat metabolism, which raises the rate of fat oxidation at any given pace and moves the crossover point to a higher intensity (Brooks & Mercier 1994; Granata et al. 2018). The practical payoff is glycogen-sparing: a trained runner burns proportionally more fat at marathon pace, leaving more carbohydrate in reserve.
Two things blunt fat oxidation in the moment: high carbohydrate availability, which raises insulin and suppresses fat release and use, and high intensity, which shifts the mix to carbohydrate regardless of training (Achten & Jeukendrup 2004). Both are expected and not problems to fix in a race, where in-race carbohydrate is what extends pace; they matter mainly when interpreting fat-oxidation tests or planning low-fuel training.
Metabolic flexibility
Metabolic flexibility is the ability to switch fuel selection efficiently as conditions change, between fat and carbohydrate across the fasting-feeding cycle and across the rest-to-exercise transition (Goodpaster & Sparks 2017). A flexible runner draws readily on fat when intensity allows and recruits carbohydrate when it does not. For long-distance racing this matters because both fuels are needed: fat to spare the limited glycogen store, carbohydrate to hold power at the sharp end. Flexibility is part of what underlies durability, the resistance to the late-race drift in economy and fuel use that decides slow-burning events.
The low-carb and train-low link
Two diet strategies aim at fat oxidation directly. Low-carbohydrate and ketogenic diets raise fat-burning capacity substantially, but the gain comes at a cost: the higher oxygen cost of fat impairs economy at race intensities, and the evidence does not support them for performance in events that depend on carbohydrate (low-carbohydrate and keto). Train-low approaches, training with low glycogen to amplify adaptation signals, are real at the molecular level but have not translated into clear performance gains, so they are a targeted tool rather than a default (Hawley et al. 2018; carbohydrate periodisation). Raising fat oxidation is useful for sparing glycogen; sacrificing carbohydrate power to do it is usually a poor trade for racing.