Carbs or Fat for Endurance? What the Research Actually Says
Few debates in endurance sport generate as much heat as this one. On one side, decades of sports science built around carbohydrate as the premium race fuel. On the other, a vocal movement arguing that training your body to burn fat unlocks a near-limitless energy supply and frees you from constant feeding. Both camps can point to studies, and both can point to athletes who swear by their approach.
The honest answer is more interesting than either camp admits: your body already uses both fuels all the time, and the ratio shifts with intensity. The real question is not carbs or fat for endurance, but which fuel dominates at the intensity you actually race at — and on that point, the research is a lot clearer than the online arguments suggest.
Two Fuel Tanks of Very Different Sizes
Even a lean 70kg athlete carries roughly 50,000 calories of stored fat — enough energy, on paper, to run several ultramarathons back to back. Stored carbohydrate is tiny by comparison: about 400–500g of glycogen across muscles and liver, worth around 1,600–2,000 calories, or two to three hours of hard exercise. Looked at that way, the appeal of fat-burning is obvious.
The catch is the rate at which each tank can be emptied. Fat oxidation is slow. Even in well-trained athletes it peaks at roughly 0.5–1.0g of fat per minute, reached at a moderate intensity of around 60–65% of VO2 max — researchers call this point FatMax. Push harder and fat's contribution falls away sharply, because the chemistry of burning fat simply cannot keep pace with the energy demand. Carbohydrate can. Above roughly 75–80% of VO2 max — the zone where most people race everything from a 10k to a marathon to a sportive — glycogen becomes the dominant and eventually near-exclusive fuel.
Carbs or Fat for Endurance Performance: The Key Studies
Fat adaptation genuinely works, in one narrow sense. Put athletes on a low-carbohydrate, high-fat diet for a few weeks and their fat-burning rates roughly double — some ultra runners have recorded oxidation above 1.5g per minute. The question the research asked next was the important one: does that make them faster?
The best-known answer came from a series of studies on elite race walkers, where three weeks of ketogenic eating during an intensified training block substantially increased fat oxidation but made the athletes less economical — they needed more oxygen to hold the same race pace — and, unlike the carbohydrate groups, they failed to improve their 10,000m performance despite identical training. Burning fat is oxygen-hungry, and at race intensity oxygen is precisely what you have none to spare. Follow-up work replicated the finding. Meanwhile, the evidence for carbohydrate is unusually consistent: reviews spanning dozens of trials show that taking in carbohydrate during exercise lasting over two hours reliably improves performance, which is why the standard guidance for long events remains 60–90g of carbohydrate per hour from mixed glucose and fructose sources.
Where Fat-First Approaches Do Make Sense
None of this makes fat adaptation useless — it makes it situational. If your event runs at genuinely low intensity for a very long time, think multi-day bikepacking, 100-mile ultras where much of the day is spent hiking climbs, the calculus shifts: intensity sits near FatMax anyway, carrying enough carbohydrate for 24+ hours is impractical, and a bigger fat engine reduces how much you must eat to keep moving.
There is also a middle path used widely in elite endurance sport: periodised carbohydrate, sometimes called "train low, race high". Selected easy sessions are done with low carbohydrate availability to amplify the training signal for fat metabolism and mitochondrial growth, while hard sessions and races are always fully fuelled. You get a better fat engine without sacrificing the ability to burn carbohydrate fast — because that ability, unlike fat burning, deteriorates if you stop using it. Fat-adapted athletes actually downregulate the enzymes that process carbohydrate, which is the hidden cost the low-carb movement rarely mentions.
What About Protein and the "Fat Burns in a Carbohydrate Flame" Problem
One more piece of physiology settles a lot of arguments: fat metabolism itself depends on intermediates that come from carbohydrate breakdown — the old exercise-science line that "fat burns in a carbohydrate flame". Run your glycogen down completely and even your fat-burning slows, which is part of why a full bonk feels less like fatigue and more like the power being cut. Keeping some carbohydrate coming in during long sessions does not just fuel high intensity; it keeps the whole metabolic engine turning over.
Protein, for completeness, is a minor player during exercise — perhaps 5% of energy in a long event, a little more when glycogen runs low — but it matters enormously afterwards. Whatever fuel mix you ride or run on, 20–30g of protein within a couple of hours of finishing supports the repair work that turns training stress into fitness.
The Practical Answer for Most Athletes
If you race anything from a parkrun to an Ironman at an intensity you would describe as "working", carbohydrate is your performance fuel and the research is emphatic about it. Build your everyday diet around quality food in sensible proportions, do some of your easy aerobic work fasted or low-carbed if you enjoy it, but arrive at every hard session and every race with full glycogen and a plan to top it up at 60–90g per hour once you pass the 90-minute mark. Your fat engine will develop plenty through ordinary aerobic training volume — that is what most of your easy miles are already doing.
And remember the gut is trainable too. The athletes who tolerate 90g an hour did not start there; they practised race-day fuelling weekly for six to ten weeks. Whichever side of this debate you lean towards, the worst outcome is being theoretically fat-adapted but practically under-fuelled at kilometre 30.
What We Use at FTP Endurance Fuel
Our approach follows the evidence: easy days on real food, hard days and races fuelled properly with fast carbohydrate. Our pastilles use a blended sugar source so glucose and fructose are absorbed through separate intestinal pathways — the mechanism that makes 90g per hour achievable without gut distress — and because each pastille is a fixed dose, hitting a precise hourly number is a matter of counting rather than guessing.
Most of our athletes keep a Mixed Fruit Pastilles 1kg bulk pack on the shelf for training blocks — the per-gram cost makes weekly gut training affordable in a way boxed gels never are. If you are new to the format, the Taster Pack is the low-commitment way to find out how your stomach gets on with them before a big block.
The carbs-or-fat question has a boring but liberating answer: both, in their proper places — fat for the long easy hours, carbs when it counts. Sort your everyday diet, fuel your key sessions, and spend the energy you save arguing online on your next long ride instead. When you are ready to practise race fuelling properly, our pastilles are built for exactly that job.
