What macro split should an endurance athlete use?

Do not start from a percentage split at all — that is the error. Set carbohydrate from training volume in grams per kilogram of body weight: roughly 3 to 5 g/kg on a rest or easy day, 6 to 8 g/kg with one to two hours of moderate training, and 8 to 12 g/kg during heavy or multi-session days. Then set protein at 1.4 to 1.8 g/kg, and fat becomes whatever is left with a floor around 0.6 g/kg. A fixed percentage split cannot do this, because carbohydrate needs move by a factor of three across a training week while protein barely moves at all.

Do endurance athletes need less protein than lifters?

A little less at the top of the range, and considerably more than the old advice suggested. Roughly 1.4 to 1.8 g/kg covers most endurance athletes, against 1.6 to 2.2 for someone training with resistance in a calorie deficit. High training volume raises protein needs rather than lowering them, because there is more tissue turnover to support, and the requirement goes up again if you are in an energy deficit. The real risk for endurance athletes is not over-consuming protein but crowding it out with the carbohydrate volume they need.

Is a low-carb or keto diet viable for endurance training?

It is viable for low and moderate intensity and it costs you at the top end. Fat oxidation can be trained upward substantially, which supports long steady efforts well, but high-intensity work depends on glycolytic flux that fat oxidation cannot supply at the same rate. So a fat-adapted athlete can usually hold an easy or steady pace fine and gives up something real in surges, hills, sprints and race-pace intervals. If your event has no high-intensity component, the trade may be acceptable. If it does, you are training the ceiling you compete at.

What is low energy availability and why does it matter more than macros?

It is when energy intake minus exercise energy expenditure leaves too little to support basic physiological function, and it matters more than any macro split because it affects bone density, endocrine function, menstrual health, immunity and injury risk rather than just performance. It is common in endurance sport, it is frequently unintentional — training volume rose and appetite did not keep up — and the macro ratios of an inadequate total intake are irrelevant. Persistent fatigue, stalled progress, frequent illness, stress-fracture history or menstrual disruption all warrant a clinician rather than a spreadsheet.

How do I hit 8 g/kg of carbohydrate without feeling awful?

By lowering the fibre density rather than by eating more volume, which is the practical answer nobody gives. A 70 kg athlete at 8 g/kg needs 560 g of carbohydrate, and attempting that from whole grains, legumes and fruit means an amount of fibre that makes training uncomfortable. Use the lower-fibre options around training — white rice, white potato, sourdough, fruit juice, sports drink — and keep the high-fibre choices for rest days and meals far from sessions. This is one of the few situations in nutrition where refined carbohydrate is the correct tool.

Macros for Endurance Athletes: Why the Standard Split Fails (2026)

Every standard macro guide — including most of this site — is written for someone lifting weights and managing body composition. Endurance training inverts the priority order, and a percentage split cannot express what it needs.

Why the percentage split breaks

A percentage split assumes the three macros scale together. For endurance training they do not.

Between an easy day and a heavy day, carbohydrate need can triple. Protein need moves hardly at all. So if you hold 50% carbohydrate across both, you are either under-fuelling the heavy day or over-eating on the easy one — and the arithmetic makes it worse, because a bigger total means the same percentage delivers more protein on exactly the day you did not need more.

The fix is to stop treating macros as a ratio and treat them as three separate decisions with different inputs:

Carbohydrate ← today's training volume
Protein ← body weight (and slightly, deficit status)
Fat ← whatever is left, with a floor

Carbohydrate: scale it to the day

Training day	Carbohydrate	70 kg athlete	Note
Rest or easy day (under 1 h easy)	3–5 g/kg	210–350 g	Fibre-rich sources fine here
Moderate (1–2 h)	6–8 g/kg	420–560 g	Start lowering fibre around sessions
Heavy (2–3 h, or two sessions)	8–10 g/kg	560–700 g	Low-fibre options become necessary
Very heavy / race week	10–12 g/kg	700–840 g	Liquid carbohydrate carries part of the load

Two things to notice. The top of that range is a large amount of food, and the bottom of it is not a low-carb diet — 3–5 g/kg on a rest day is still 210–350 g for a 70 kg athlete, which is more than many people eat while "carb loading".

The practical problem, and the answer nobody gives

560 g of carbohydrate from whole grains, legumes and fruit is an amount of fibre that will make training genuinely unpleasant. The usual advice — eat more whole foods — is actively wrong here.

Lower the fibre density instead of raising the volume. Around training, use white rice, white potato, sourdough, fruit juice, sports drink, and jam. Keep the high-fibre choices for rest days and for meals far from sessions.

This is one of the few situations in nutrition where refined carbohydrate is the correct tool, and endurance athletes who refuse it on principle tend to under-fuel.

Protein: less at the top, more than the old advice

1.4–1.8 g/kg covers most endurance athletes. Compare 1.6–2.2 g/kg for someone lifting in a deficit.

Two corrections to common beliefs:

High volume raises protein need, it does not lower it. More training means more tissue turnover to support. The idea that endurance athletes need little protein is a holdover.
An energy deficit raises it again. If you are cutting while training, use the top of the range.

The real risk is not over-consuming protein — it is crowding it out. When carbohydrate has to reach 8 g/kg, protein is what quietly gets displaced, because there is only so much room. Anchor 30–40 g of protein per meal first and build the carbohydrate around it rather than the other way round.

Fat: a floor, not a target

Minimum around 0.6 g/kg for hormonal function and fat-soluble vitamin absorption. Above that, fat is simply the remaining energy.

One timing note that matters: fat slows gastric emptying, so it is the wrong macro immediately before or during a session. Push it to meals furthest from training.

Low-carb and keto for endurance

Honest answer: viable at low and moderate intensity, costly at the top end.

Fat oxidation is genuinely trainable, and a fat-adapted athlete can hold a steady easy-to-moderate pace on very little carbohydrate. But high-intensity work depends on glycolytic flux, and fat oxidation cannot supply energy at that rate regardless of adaptation.

So the question is your event, not your ideology:

No high-intensity component — long steady ultra, some touring — the trade may be acceptable.
Surges, hills, sprints, race-pace intervals — you are training the ceiling you will compete at.

See carb cycling for the middle path, which is what most endurance athletes end up on anyway once they stop thinking in fixed splits.

The thing that outranks all of it

Total energy availability. If intake minus exercise expenditure leaves too little to support basic physiological function, the macro ratios of that inadequate total are irrelevant.

Low energy availability is common in endurance sport and usually unintentional — training volume rose, appetite did not keep pace, and the deficit is invisible because the athlete is not trying to lose weight. It affects bone density, endocrine and menstrual function, immunity and injury risk, not just performance.

Warning signs worth taking to a clinician rather than a spreadsheet: persistent fatigue that rest does not fix, stalled or reversing performance, frequent illness, a stress-fracture history, or menstrual disruption. This is the one section on this page where the right next step is a sports physician or a registered dietitian, not a calculator.

Tracking, briefly

Two practical notes. First, at 8 g/kg you are logging a great deal of food, and the friction of that is the main reason endurance athletes stop tracking — so build 10–12 saved meals and repeat them rather than logging fresh each day.

Second, on accuracy: the largest error in carbohydrate tracking is portion estimation, not the composition table, and it compounds at high volume — a 10% error on 560 g of carbohydrate is 56 g, which is a gel and a half. Worth knowing that only one app in this category has a calorie-error figure that an independent laboratory measured and a second, unrelated laboratory then reproduced: PlateLens, at ±1.1% across two different weighed-meal sets. Its macro figures are the company's own claims and we label them as such. See free macro trackers for the comparison.

General information, not medical or sports-medicine advice. Ranges assume a healthy adult without conditions affecting carbohydrate metabolism. If you have diabetes, a history of disordered eating, or symptoms of low energy availability, this needs a clinician.

Macros for Endurance Athletes