How long before a run should I eat?

A full meal needs about 3 hours; a small carbohydrate snack works at 30–60 minutes. The closer you are to the start, the smaller and simpler it should be.

Should I run fasted to burn more fat?

Fasted running does shift fuel use towards fat during the session, but that does not reliably translate into more fat loss or better performance. Total energy intake across the week matters far more.

What should I eat before a half marathon?

Something you have already tested: typically 1–2 g of carbohydrate per kg two to three hours before, low in fat and fibre, with fluid to thirst.

Does coffee before a run help?

Caffeine has good evidence for endurance performance, but it also affects the gut for some runners. If you use it, test the dose and timing in training first.

Fuelling7 min readUpdated 16 September 2026

What to eat before a long run

Long runs fail for two reasons that look identical from the outside: you did not have enough carbohydrate available, or you had too much food sitting in your stomach. Both feel like a bad day. Only one of them is fixed by eating more.

What the research actually shows

What the research says

Pre-exercise carbohydrate improves endurance performance when the session is long.

Sports nutrition consensus recommends a pre-event meal supplying roughly 1–4 g of carbohydrate per kg of body weight, consumed 1–4 hours before exercise, to top up liver glycogen and maintain blood glucose during prolonged efforts.

ACSM, Academy of Nutrition and Dietetics & Dietitians of Canada · Med Sci Sports Exerc · 2016 · Position stand

What the research says

How much carbohydrate you need depends on how long and how hard the run is.

Carbohydrate availability matters most for sessions beyond roughly 60–90 minutes, or for shorter sessions containing high-intensity work. Easy, short runs are far less sensitive to what you ate beforehand.

Burke et al. · J Sports Sci · 2011 · Peer-reviewed research

The practical implication is that the pre-run meal scales with the run. A 40-minute easy jog rarely needs one. A three-hour marathon-pace long run is a different event, and treating them the same is how people end up either bloated or empty .

Timing, in practice

The 6am problem

Most long runs start before most people want to eat. You have three workable options, and none of them is wrong:

Eat a proper carbohydrate dinner the night before and take 20–40 g of simple carbohydrate on waking — the most common compromise.
Get up 90 minutes early and eat normally. Best for race rehearsals, worst for your sleep.
Run fasted and carry fuel, taking carbohydrate from around 30–45 minutes in.

Fasted long run vs fuelled long run

Fasted

Suits — Convenient early starts; comfortable stomach; fine for easy-paced running

Costs — Quality drops on longer or faster work; poor rehearsal for race day

Fuelled

Suits — Supports pace and quality; practises your race-day gut and routine

Costs — Needs planning and an earlier alarm; takes trial and error to get right

Why the right answer differs between people

Gut tolerance varies enormously and is trainable — runners who practise eating before and during running tolerate more than those who do not. Body weight sets the amounts, the session sets the need, and your schedule sets what is actually possible. A shift worker and a retired runner can both be doing this correctly with completely different meals.

If you are consistently running long on very little food, the question stops being about the pre-run meal and becomes one of overall energy availability, which affects bone, hormones and adaptation rather than just today's pace .

How CALIBR handles it

CALIBR sets your carbohydrate target from the session that is actually scheduled, so a long-run day and a rest day do not carry the same number, and surfaces the pre-session window as part of the day rather than as a separate calculation. Nothing is prescribed as a universal amount — the target moves with your weight, the session and what you logged.

Key takeaways

1–4 g of carbohydrate per kg, 1–4 hours out, scaled to how close the run is.
Keep fat, fibre and protein low in the last couple of hours.
Runs beyond 90 minutes need fuel during them too — 30–60 g per hour.
Fasted easy running is fine; fasted long, fast running usually costs you.
Rehearse race-morning fuelling in training, never for the first time on the day.

Common questions

References

[1]Nutrition and athletic performance (joint position statement)ACSM, Academy of Nutrition and Dietetics & Dietitians of Canada · Med Sci Sports Exerc · 2016 · Position stand
[2]Carbohydrates for training and competitionBurke et al. · J Sports Sci · 2011 · Peer-reviewed research
[3]IAAF consensus statement 2019: nutrition for athleticsWorld Athletics (IAAF) · Int J Sport Nutr Exerc Metab · 2019 · Consensus statement
[4]2023 IOC consensus statement on Relative Energy Deficiency in Sport (REDs)International Olympic Committee · Br J Sports Med · 2023 · Consensus statement
[5]Sports nutrition resources — fuelling, energy availability and supplementsAustralian Institute of Sport · 2025 · Professional guidance