Does a low-GI diet improve endurance? Choosing carbohydrate for training and racing

Oats, rice, bread and energy gels all supply carbohydrate. Their digestion, convenience and effects on glucose can differ considerably. For runners, the useful question is where each fits: the everyday meal, the pre-run breakfast or the feed taken halfway through a long race.

What the glycaemic index describes

The glycaemic index, or GI, compares the post-meal glucose response to foods providing an equal amount of available carbohydrate. Food structure, processing and preparation affect digestion and absorption, helping explain why carbohydrate-containing foods produce different responses.⁵

A real meal introduces portion size. Half a bowl of rice and two bowls supply different amounts of carbohydrate. Beans, vegetables, protein and fat also change the meal’s composition and digestion. GI becomes more useful when considered alongside the serving you actually eat and the rest of the meal.⁵

Whole grains, pulses and vegetables can fit into everyday meals, with enough carbohydrate to support the day’s training. Near exercise, comfort becomes especially important. Someone prone to fullness or gastrointestinal distress may tolerate a familiar, lower-fibre breakfast more easily before a run. Higher-fibre meals can be placed further from the session.³

This approach accommodates different needs across the same day. A lunch eaten several hours before training has a different practical job from a gel swallowed during a race. The available time for digestion and the amount of food that can comfortably be eaten are part of that choice.

What happened in the endurance trial?

Hamilton and colleagues studied nine ultra-endurance athletes in a crossover trial published in 2025. Each athlete followed a lower-GI and a higher-GI diet for 28 days, separated by a 14-day washout. Both diets were carbohydrate-rich, supplying approximately 58% of energy from carbohydrate. The recovery drinks also differed: isomaltulose in the lower-GI condition and maltodextrin in the higher-GI condition.¹

The laboratory challenge was demanding. Participants completed a three-hour fasted run, consumed carbohydrate during recovery and then ran on a treadmill to exhaustion. Researchers measured glucose patterns, carbohydrate oxidation and endurance capacity. The lower-GI condition reduced glucose variability, with the coefficient of variation approximately 16% versus 18%, and reduced time below the study’s glucose range. The difference in endurance capacity was not statistically significant.¹

For meal planning, this provides evidence that a lower-GI, carbohydrate-rich pattern can change day-long glucose exposure. The accompanying training outcomes, food tolerance and total carbohydrate supply remain relevant to the athlete’s choice. Details of the small sample and combined food-and-drink intervention are retained in the study notes.

Why race nutrition often uses readily digested carbohydrate

Prolonged exercise creates a continuing demand for carbohydrate. Drinks and gels offer convenient ways to consume it while moving. Their portability and ease of swallowing help athletes meet an hourly target when stopping for a meal would be impractical. Concentration, serving size and fluid intake also affect how tolerable a plan feels.³

In a human cycling experiment, glucose or sucrose ingestion during three hours of exercise maintained liver glycogen, while muscle glycogen continued to fall. The working muscle was using stored carbohydrate alongside the ingested supply.⁴

Sports-nutrition guidance commonly starts at 30–60 g carbohydrate per hour for endurance sessions lasting around 1–2.5 hours. Longer events may justify intakes up to approximately 90 g/hour, using suitable carbohydrate combinations and prior practice. Higher intakes require attention to gastrointestinal tolerance, fluid and the demands of the event.³

A fibre-rich everyday diet and easy-to-digest race feeds can comfortably belong to the same nutrition plan. The timing and purpose of the food help determine its place. Practising the intended feeds during training allows adjustments before the day when the outcome matters most.

How to test your next fuelling plan

Start with a familiar session and record the pre-exercise food, its approximate carbohydrate amount and the interval before starting. A run that feels uncomfortable shortly after breakfast suggests reviewing meal size, composition or timing. Repeated difficulty maintaining the later part of a long session also warrants reviewing total carbohydrate availability.³

Changing one major feature at a time makes the record easier to interpret. You might first compare similarly sized breakfasts, then review feeding frequency during the run. Also record weather, accumulated fatigue and route differences. These notes help you judge whether a change is worth repeating.

The choice should remain practical. A theoretically attractive food that is difficult to carry or repeatedly upsets your stomach will be hard to use consistently. Familiarity and the ability to consume the intended amount deserve a place in the decision.³

Where CGM fits

CGM connects the glucose response across meals, exercise and overnight periods. Alongside food and training records, it can show how the trace changes under different arrangements. A separate 12-person crossover study used glucose trends to time carbohydrate during cycling. Variability was lower with this approach, while heart rate, lactate and perceived effort did not differ significantly.²

When reviewing a session, work through the amount consumed, late-session performance and gastrointestinal comfort, then compare the glucose record. That order makes a proposed adjustment specific: an earlier meal, a different serving size or a product that is easier to tolerate.

For race day, a sensible choice is the food and drink you have practised, in amounts appropriate to the event. Lower-GI meals can remain part of everyday eating, while the exercise plan supplies carbohydrate in a form and schedule suited to the work.³