Carbohydrate Loading and Periodisation: An Evidence-Based Guide for Endurance

Why does glycogen matter?

Carbohydrate is stored as glycogen in muscle and liver. During moderate- and high-intensity exercise, glycogen is the muscle's primary fuel. But stores are limited: in an average athlete, total glycogen lasts only about 90–120 minutes of hard effort. When stores run out — what runners call "hitting the wall" — pace drops sharply, perceived effort rises and concentration suffers. This is why endurance performance is largely about maintaining adequate carbohydrate availability throughout the event.

There are two levers: maximising stores before the event (loading) and taking in external carbohydrate during it. This guide focuses on the first — the storage strategy.

Carbohydrate-loading protocols

The idea of carbohydrate loading has been known since the 1960s, but protocols have become more practical and less punishing over time.

The classic (old) protocol

The original method began with a "depletion" phase: glycogen was emptied with a long session ~6 days out, followed by several days of low-carbohydrate eating, then a switch to high carbohydrate in the final 3 days. It works but is exhausting, increases injury risk and is psychologically hard. For most athletes today it is not recommended.

The modern protocol

Current evidence shows the depletion phase is unnecessary. The modern approach is simple: in the 1–3 days before the event, reduce training load (taper) while raising daily carbohydrate to 8–12 g/kg, which fills muscle glycogen as effectively as the classic method. For a 70 kg athlete that is 560–840 g of carbohydrate per day. These amounts are high, so favour low-fibre, easily digestible sources (white rice, pasta, white bread, honey, fruit juice, sports drinks) and spread meals across the day.

One-day rapid loading

When preparation time is very short, fully resting from training and consuming about 10–12 g/kg of carbohydrate over 24 hours can also fill glycogen quickly. This is especially useful between events in multi-day tournaments.

Carbohydrate periodisation

Loading is only a pre-race tactic. In daily training, the key principle is carbohydrate periodisation: matching carbohydrate intake to that day's training intensity and goal. The logic is that on high-intensity, quality training days, carbohydrate is kept high to support performance, while on low-intensity or technical days it can be kept lower.

• High-intensity day: 6–10 g/kg — performance and recovery priority
• Moderate day: 5–7 g/kg
• Light/rest day: 3–5 g/kg

This approach preserves training quality while avoiding an unnecessary energy surplus. Protein and overall energy availability must be maintained in all cases; it is mainly carbohydrate that is periodised.

The "train low" strategy

Popularised in recent years, "train low" (training with low glycogen availability) deliberately performs some sessions with low carbohydrate availability to enhance cellular adaptations (mitochondrial number, fat oxidation). Examples include restricting carbohydrate after an evening session and doing a light fasted session the next morning.

This strategy is an advanced tool to be used carefully. Applied incorrectly, it reduces training quality, suppresses immunity and can cause excessive fatigue. High-intensity and race-specific sessions should always be done with adequate carbohydrate ("train high"). "Train low" should be applied only to selected low-intensity, volume-focused sessions and within a plan.

Putting it into practice: a race-week example

During loading days, weight may rise by a few hundred grams; this is normal, because water is stored alongside each gram of glycogen. This "water weight" is in fact a fuel reserve to be used in the race.

Common mistakes

• Not combining loading with a taper: Loading without reducing training does not fully fill stores.
• High-fibre loading: Lots of high-fibre food on loading days can cause GI problems; reduce fibre on these days.
• Trying new products in the race: Loading and in-race nutrition should be tested in training beforehand.
• High carbohydrate every day: An unnecessary surplus on light days defeats the purpose of periodisation.