The three numbers that govern everything
Underneath every fuelling plan there are three figures, and none of them is a matter of taste.
Grams of carbohydrate per hour. This is the number that decides whether you finish or fade. It depends on the duration and the intensity of the session, and it has a physiological ceiling set by the kind of sugar in your product:
| Type of carbohydrate | Absorption ceiling | What it is for |
|---|---|---|
| Glucose or maltodextrin only | 60 g/h | Sessions up to 2 h |
| Glucose + fructose 2:1 | 90 g/h | Olympic, 70.3, marathon |
| High fructose 1:0.8 | 120 g/h | 140.6 and ultras, with a trained gut |
Millilitres per hour. Set by what you sweat, not by what you fancy: 400 to 500 ml/h in the cold, around 700 in mild conditions and above 900 in real heat. Drinking far more than you sweat improves nothing and in long events it is dangerous.
Milligrams of sodium per hour. Sweat carries between 200 and over 2000 mg of sodium per litre depending on the person. At an average of 500 mg per litre, an hour of mild sweating asks for about 350 mg. The late cramps and the bloated feeling despite drinking plenty almost always live here.
Why the figure comes from the session and not from a table
The duration based recommendations — 30 g/h from one to two hours, 60 up to two and a half, 90 above that — are a good starting point, but they ignore intensity, which is exactly what decides how much carbohydrate you burn. A four hour easy ride and a four hour 70.3 do not ask for anything like the same thing.
If the session is planned with an IF, a TSS, a kilojoule figure or a zone breakdown — and if you use TrainingPeaks, Garmin or a written plan, it is — that figure can be calculated instead of estimated: intensity gives energy, intensity and energy give the share supplied by carbohydrate, and that gives the grams you oxidise per hour. Subtract what you keep in the tank and you have what you need to eat. That is what the session plan calculator does.
And why knowing the figure is not enough
Knowing that your race asks for 90 g/h does not mean you can take them. Carbohydrate absorption is trainable, and it trains slowly: about 5 to 10 grams per hour more each week, in the long sessions, with the product you will carry on race day. That is the work planned by the gut training calculator, including the final two weeks where nothing is added and everything is rehearsed.
The most expensive mistake in sports nutrition is not getting the figure wrong: it is using the right figure for the first time on race day.