From Metabolism to Medals: Carbohydrate Guidelines for Fueling Endurance Exercise

Written By Samuel Thompson

In a recent research paper by Morton et al. (2026), over 100 years of study on the factors affecting carbohydrate intake on exercise are summarised to provide contemporary recommendations and guidelines.

In this piece, I extract key takeaways for each of the factors that can influence your ability to use carbohydrates (CHO) when exercising in a bite size format.

The Current Guidelines

  • In the most recent sport nutrition guidelines, the upper limit of CHO intake is cited as 90 g/hr (from products providing multiple-transportable CHOs) for scenarios where exercise duration is >2.5 hr.

  • Regardless of dose and format, CHO ingestion rates do not always translate to equivalent rates of exogenous CHO oxidation (how much external CHO your body uses). Therefore, if an athlete intends to oxidise 90 g/hr, they should likely ingest CHO at rates of 90–120 g/hr.

Composition of CHO blends

  • Oxidation rates of glucose or glucose polymer (e.g. maltodextrin) peaks at around 60 g/hr.

  • Fructose and galactose oxidation rates are typically 50%–60% lower than that of glucose.

  • At CHO intakes ≤60 g/hr, single-source CHO (glucose or glucose polymer) is typically recommended.

Ratio of fructose-to-glucose

  • Glucose-fructose blends offer advantages when ingested at high ingestion rates (>60 g/hr) as each uses distinct intestinal transporters, enhancing CHO absorption and availability during exercise.

  • A ratio of fructose to glucose that is between 0.6 and 1.0 provides optimal exogenous CHO availability and oxidation.

Format of CHO ingestion

  • Comparable exogenous CHO oxidation rates are observed when ingesting fluids, gels, or solids prolonged exercise.

  • Solid CHO sources are however associated with increased subjective GI symptoms, often explained by the addition of protein, fat, and fibre within products.

  • A mixed-format approach is advised, using a combination of fluids, gels and solids, with the flexibility to select according to individual preference, overall CHO requirements, and race conditions or logistics.

Sex-specific considerations

  • A limited number of studies have directly compared exogenous CHO oxidation during exercise in males and females.

  • There is no clear evidence to conclude that female athletes require sex specific guidelines in relation to CHO requirements provided energy needs are met.

  • Current recommendations should be used as a basis for adopting an individualised approach that takes into account athlete specific training and competition goals whilst also considering personal symptoms associated with the menstrual cycle.

Environmental considerations

  • Altitude: There is no evidence that current CHO intake recommendations at altitude should differ from those applied at sea level, although acclimation is advisable to optimise performance.

  • Heat: Although exogenous CHO oxidation rates are reduced during exercise in the heat, CHO intake recommendations remain comparable with those used in temperate conditions. However, a modest reduction in intake may be warranted in individuals experiencing GI discomfort.

  • Cold: Exogenous CHO oxidation rates appear to be unaffected by cold exposure, so CHO intake recommendations should remain unchanged when exercising in the cold. This is  especially important as adequate CHO supply supports thermoregulatory heat production and helps prevent reductions in core temperature.

GI considerations

  • “Gut-training”, has been shown to mitigate exercise-associated GI symptoms and improve tolerance to high CHO intakes through the structured and repetitive exposure of the gut to CHO feedings.

  • Increases in exogenous CHO oxidation during exercise have been shown in athletes following higher daily CHO diets. 

  • Practically, progressive exposure to higher-CHO intakes during training and high dietary CHO intakes in the days and weeks before competition improves tolerance and reduces the incidence and severity of GI symptoms.

CHO personalisation

  • There is substantial individual variability in the capacity to oxidise CHO during exercise.

  • Larger athletes oxidise substantially more exogenous glucose and measures of body size correlate strongly with peak exogenous oxidation, although it does not explain all the interindividual variability.

  • Personalisation offers a promising approach to maximise exogenous CHO availability without exceeding an individual’s tolerance, thereby reducing the risk of GI discomfort or unnecessary endogenous glycogen use. Further research is however required to understand individual variability and its application.

Reference:

Morton JP, Fell JM, Gonzalez JT, Hearris MA, Podlogar T, Pugh JN, Wallis GA. From Metabolism to Medals: Contemporary Perspectives and Revisiting Carbohydrate Guidelines for Fueling Endurance Athletes during Exercise. J Nutr. 2026 May;156(5):101442. doi: 10.1016/j.tjnut.2026.101442. Epub 2026 Feb 25. PMID: 41759826; PMCID: PMC13197957.

Samuel Thompson
Next

Is Potato a Sports Food? Part 1