Every runner has heard that carbohydrates are the primary fuel source for running. That's true – but it's only half the picture. Fat is always involved, and understanding when and how your body uses it changes how you think about easy runs, fasted training, and why you don't need to eat before every single workout.
Your Body Uses Both at the Same Time
At any given moment during a run, your body is burning a mixture of carbohydrates and fat. The ratio between them shifts constantly based on how hard you're working.
During easy, conversational-pace running, fat contributes a significant share of the energy. As effort increases toward tempo pace, threshold, and race effort, your body shifts progressively toward glycogen. At very high intensities, glycogen dominates almost entirely. [1]
You track your miles.
We'll handle the meals.
This is why the "fat burning zone" is technically real – you do burn more fat as a percentage of total energy at lower intensities. But the reason most nutrition advice focuses on carbohydrates is that glycogen is the limiting factor. You have enormous fat stores even as a lean runner. You have a finite glycogen supply. Managing that supply is what separates a good race from a bad one.
Why Fat Matters Even If Carbs Dominate
Fat's contribution to running energy matters most in two scenarios.
The first is easy and long slow runs. During your base-building work – those long, slow weekend runs at a pace where you can hold a conversation – fat is doing a lot of the heavy lifting. The longer you can sustain that effort using fat, the more you spare your glycogen for when intensity picks up. This is one of the reasons coaches emphasize truly easy running: it trains your aerobic system to become more efficient at using fat as fuel. [2]
The second is very long events. In a marathon or ultra, glycogen will eventually run low regardless of how well you fuel. Runners who are metabolically efficient at using fat can sustain pace longer before things fall apart. This is the underlying logic behind some long run strategies that intentionally limit carbohydrate intake – not to avoid carbs in general, but to encourage fat oxidation adaptations.
Fat Adaptation: What It Actually Means
"Fat adaptation" gets used loosely in running circles, often as justification for low-carb or keto diets. The reality is more nuanced.
Your body does adapt to use fat more efficiently with consistent aerobic training – regardless of your diet. Regular easy running, especially longer efforts, gradually increases the enzymes and cellular machinery involved in fat oxidation. [2] You don't need to restrict carbohydrates to get this benefit. It comes from putting in the easy miles.
Deliberately following a very low-carb diet to force fat adaptation is a different thing, and the evidence for it as a performance strategy is mixed at best. The problem is that high-intensity running – intervals, tempo work, races – relies on glycogen, and chronically low carbohydrate intake impairs your ability to perform at those intensities. [3] You may become more efficient at easy paces while becoming less effective at hard ones. For most recreational runners, that's not a worthwhile trade.
Fasted Running and Fat Burning
Some runners train fasted – heading out before breakfast – partly to encourage fat oxidation. This is a reasonable practice for easy, shorter efforts. Your liver glycogen is lower after an overnight fast, which means your body draws on fat earlier in the run. [4]
For hard sessions, long runs, or anything that requires sustained intensity, fasted training comes with real costs. You start with less available fuel, which limits how hard you can work and increases the risk of muscle protein breakdown for energy. The adaptation benefit doesn't outweigh the performance and recovery cost for most workouts.
Easy run before breakfast? Often fine. Hard intervals fasted? Not worth it.
Dietary Fat for Runners
Beyond fat as an energy source during running, dietary fat matters for recovery and general health. Omega-3 fatty acids from fatty fish, walnuts, and flaxseed have been consistently shown to reduce inflammation markers following exercise. [5] Fat also supports the absorption of fat-soluble vitamins (A, D, E, K) that play roles in bone health, immune function, and muscle repair.
Salmon is one of the best real-food sources of omega-3s for runners – high in protein, rich in EPA and DHA, and easy to build a recovery meal around:
Salmon Poke Bowl
The practical point is that fat should not be avoided – it's a necessary part of a runner's diet. The timing caveat is the same as fiber: dietary fat slows gastric emptying, which makes it a poor choice in the hours immediately before a run. Save higher-fat meals for recovery and rest periods, not the pre-run window.
For a warm recovery option that uses coconut milk and olive oil as quality fat sources alongside turmeric and ginger:
Coconut Curry with Rice
The Takeaway
Fat and carbohydrates are partners, not competitors. Your body uses both, and training consistently at easy effort improves your ability to use fat efficiently – which spares glycogen for when you actually need it. You don't need to manipulate your diet to get this benefit. You just need to run easy on your easy days, which most runners don't do enough of anyway.
Carbohydrates remain the priority around hard efforts and long runs. But understanding fat's role gives you a more complete picture of what's actually happening when you run, and why the type of run matters as much as the distance.
Fuel.fit tags every recipe by training context. Pre-run meals are deliberately low in fat to keep gastric emptying fast – fat has its place, just not in the hour before you head out.
Sources
[1] Burke LM, Hawley JA, Wong SH, Jeukendrup AE. Carbohydrates for training and competition. J Sports Sci. 2011;29(Suppl 1):S17-27.
[2] Achten J, Jeukendrup AE. Optimizing fat oxidation through exercise and diet. Nutrition. 2004;20(7-8):716-727.
[3] Burke LM. Fueling strategies to optimize performance: training high or training low? Br J Sports Med. 2010;44(1):48-54.
[4] Gonzalez JT, Fuchs CJ, Betts JA, van Loon LJ. Liver glycogen metabolism during and after prolonged endurance-type exercise. Am J Physiol Endocrinol Metab. 2016;311(3):E543-E553.
[5] Rahimi MH, Nasir Y, Nouri Saeidlou S, et al. Effect of omega-3 fatty acids supplementation on inflammatory markers following exercise-induced muscle damage. Sci Sports. 2024;39(5-6):e167-e176.
