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Fuelling and hydration for training and racing

Consensus ranges for carbohydrate, protein, fluid and caffeine in endurance training. Also where those ranges stop being general and start being about you.

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Carbohydrate during exercise scales with duration

How much carbohydrate is worth taking in mid-session depends mostly on how long the session runs. The ingestion literature converges on a tiered picture: around 30 g per hour for sessions of roughly an hour to two, about 60 g per hour once you are beyond two hours, and up to about 90 g per hour for events past the two-and-a-half to three hour mark ¹.

The top of that range is an absorption limit, not a preference. A single glucose-type carbohydrate saturates its intestinal transporter at roughly 60 g per hour; pairing glucose with fructose recruits a second transporter and lifts total oxidation higher, which is why products aimed at long events are blends rather than one sugar ¹.

Tolerance for intakes that high is trainable. Regularly taking carbohydrate during training raises intestinal absorptive capacity and reduces gastrointestinal symptoms, so what you can handle on race day is partly a product of what you rehearsed in the months before it ¹.

Daily targets follow the training you actually did

Daily carbohydrate reads better as a function of load than as a fixed habit. The joint position stand from the Academy of Nutrition and Dietetics, Dietitians of Canada and ACSM gives ranges of roughly 3–5 g per kg of body mass per day for light activity, 5–7 g/kg for about an hour a day of moderate work, 6–10 g/kg for one to three hours of endurance training, and 8–12 g/kg for very heavy programmes of four hours or more ².

Protein in the same document sits at about 1.2–2.0 g per kg per day for active people, distributed across the day rather than concentrated into one meal ².

These are population ranges from a consensus document written for general guidance. Where any individual sits inside them, and whether these ranges apply at all, is a question for a qualified professional who knows the person. This page is not dietary advice.

Fuel for the work required

Sessions do not all need the same support. The "fuel for the work required" framework periodises carbohydrate availability across the week, so short or easy work can be done with lower glycogen while hard and long sessions are fed properly ³.

Low availability appears to amplify parts of the training signal in endurance-trained muscle, up to a point. Beyond that point it constrains the work itself: sessions performed low come with reduced capacity for high-intensity output, so a key interval set or a long race-specific effort done in that state loses quality ³.

Easy aerobic work tolerates lower availability without much cost to the session.
Hard intensity and long race-pace work are where reduced availability shows up as worse execution.
The question is what this particular workout needs, rather than one number applied to every day ³.

Hydration follows sweat rate, not a rule

Fluid needs vary too widely between people and conditions for a single figure to mean much. Sweat rates reported in the ACSM position stand span roughly 0.3 to 2.4 litres per hour, moving with body size, intensity, heat and how acclimatised you are ⁴.

The workable guardrail is body-mass change across a session. Losses beyond about 2% of body mass start to degrade endurance performance, and the effect is larger in the heat, so drinking is aimed at staying inside that margin ⁴. Going the other way carries its own risk. Taking in more fluid than you are losing can dilute plasma sodium, which is why maximum intake is not the target either ⁴.

Caffeine

Caffeine improves endurance performance on average, and the average is small. A systematic review of sport-specific endurance protocols found mean improvements near 3%, with individual studies scattered from no measurable effect to gains several times that size. Doses in the studied range sit around 3–6 mg per kg of body mass, and going higher has not reliably added anything ⁵.

Spread across individuals, the response is wide enough that the group mean is a poor prediction for any one person, since habitual intake, tolerance and gut comfort all move it ⁵. Whether it helps you is settled in training, not from a table, and anyone with a medical reason to limit stimulants should treat the whole question as a clinical one.

References

Numbered in order of first appearance. Every link goes to the original work.