Carbohydrates are an athlete’s fuel. Scientists have confirmed what has long been standard practice in endurance sport: a significantly higher intake of carbohydrates per hour than was previously recommended. Researchers are therefore calling for a review of the American College of Sports Medicine’s sports nutrition guidelines, which have been in force since 2016. The current recommendations stipulate a maximum of 90 g/h of carbohydrates, but new findings suggest higher levels. “The American College of Sports Medicine’s 2016 guidelines recommend less than 90 g/h for a session lasting 2.5 to 3 hours,” the study states. However, the authors emphasise: “Recent research shows that maximum carbohydrate intake can range between 90 and 120 g/h.”
Metabolism and glycogen stores
The effect of carbohydrate intake is based on several metabolic mechanisms. Studies show that consuming 120 g/h prevents the so-called ‘crossover point’ – the point at which the body switches from carbohydrate to fat metabolism. At lower doses of 45 or 90 g/h, this point is merely delayed. The researchers report that liver stores benefit particularly from a high intake of glucose. Muscle glycogen stores react differently depending on the sport – running consistently shows a protective effect, whilst the effects are more variable in cycling.
Composition: Carbohydrates
The combination of different types of sugar has been shown to be crucial. Glucose and fructose utilise different transport pathways in the gut, which increases absorption. The optimal ratio is between 0.6:1 and 1:1 (fructose to glucose). At very high intake rates of 120–144 g/h, exogenous oxidation rates of 96–105 g/h were measured. Studies using 13C-labelled glucose enabled precise measurements of actual utilisation for the first time.
Forms in which carbohydrates are available
Liquids, gels and chewable tablets show comparable rates of absorption at 108–120 g/h. Solid bars, on the other hand, cause more gastrointestinal discomfort due to delayed gastric emptying. The researchers recommend a mix of different formats. Systematic ‘bowel training’ reduces symptoms and is a prerequisite for consuming large quantities.
Gender-specific differences
The evidence base for female athletes remains insufficient. Fewer than 3 per cent of the studies examined exclusively female participants. Two studies found no differences in absolute exogenous oxidation between men and women, whilst three others showed rates 0.2–0.3 g/min higher in men. The authors therefore call for targeted studies involving female participants.
Environmental factors affecting recycling
Extreme conditions significantly impair carbohydrate utilisation. At high altitudes above 4,000 metres, exogenous oxidation decreases by 20–50 per cent. Heat reduces oxidation rates by 20–30 per cent, even with adequate hydration. Three weeks of altitude acclimatisation partially restores this utilisation capacity. Cold, on the other hand, has no measurable effect on exogenous oxidation.
Cycling and Carbohydrates
In recent years, professional cyclists have consumed 60–90 g/h during Grand Tours. Chris Froome’s 2018 Giro victory was based on a periodised intake of between 52 and 96 g/h, depending on the stage profile. The toughest stage, with an energy expenditure of 6,180 kJ, required the maximum intake. However, carbohydrate intake in professional cycling has since risen significantly: “Recent reports show that a stage winner at the Tour de France consumed 116 g/h of carbohydrates, and some riders are training their digestive systems to tolerate 200–220 g/h,” the authors report. Increased race speeds and earlier attacks are driving up the demand.
Marathons and Carbohydrates
Marathon runners consume significantly less than cyclists – on average just 35 g/h, mainly from drinks. This discrepancy is explained by limited intake capacity and a higher incidence of gastrointestinal complaints whilst running. However, studies of elite marathon runners (all with times under 2:30 h) also showed clear benefits of higher intake rates whilst running. “In one test, a 3 per cent improvement in running economy was observed when consuming 120 g/h instead of 60 g/h”, the study states. However, at 120 g/h, more severe symptoms such as nausea and a feeling of fullness occurred. Eliud Kipchoge used 60–100 g/h during his sub-2-hour attempts.
Updated recommendations on carbohydrates
The authors present a nuanced model based on intensity and duration. For exercise lasting 1–2.5 hours, they recommend 30–60 g/h; for 2.5–6 hours, 60–90 g/h; and for ultra-endurance events lasting over 6 hours, up to 120 g/h. However, this always requires prior training of the digestive system. Body height must also be taken into account. Recent research shows significant individual differences. Taller athletes weighing over 70 kg oxidised an average of 45 g/h of exogenous glucose, whilst shorter athletes weighing under 70 kg oxidised only 33 g/h – a difference of 13 g/h.
Research needs and outlook
The authors identify significant gaps in our knowledge. Over 80 per cent of the studies examined recreational athletes or young athletes, whilst fewer than 5 per cent looked at elite or world-class athletes. Data on female athletes is almost entirely lacking. The mechanisms underlying reduced oxidation in hot and high-altitude conditions remain unclear. They therefore call for further studies.
Kristian Bauer
Editor