Fuelling endurance sport: The carbohydrate conundrum

Article by Adam Green, freelance journalist.

Endurance athletes, recreational or professional, will be familiar with the experience of ‘hitting the wall’ – feeling suddenly unable to continue – or conversely, being struck by nausea when fueling with sugars. The arithmetic of endurance sports is a challenge. The body needs fuel for performance, but too much can backfire.

Conventional wisdom is that carbohydrate is the gold standard energy source for exercise lasting longer than 90 minutes, spawning a multi-billion pound sports nutrition industry of gels, drinks and chews. However, existing guidelines are generic, and every individual is constitutionally unique. The data are drawn from broad populations which leaves athletes working by trial and error.

Professor Gareth Wallis leads research into exercise metabolism at the University of Birmingham's School of Sport, Exercise and Rehabilitation Sciences. After over two decades exploring nutrition and sports performance, his lab is launching fuelsync®, a testing service that gives endurance athletes a precise and individualised measurement of how much carbohydrate their body can actually use during exercise. This can translate that into a personalised fuelling strategy.

Metabolic meltdown

Current guidelines for carbohydrate intake during exercise recommend that endurance athletes consume between 30 and 90 grams of carbohydrate per hour, depending on the duration and intensity of effort. That is a large range, so titrating the right balance is highly specific to the individual.

The upper limit of what the intestine can absorb from a single carbohydrate source such as glucose is approximately 60 grams per hour. When intake exceeds absorptive capacity the risk of gastrointestinal symptoms such as cramping and bloating increases. These are the symptoms that can derail athletic performance. Studies in endurance triathletes have reported gastrointestinal distress affecting up to 93% of competitors. Across endurance sports more broadly, the incidence ranges from 30% to 90%.

The response from the sports nutrition industry has been to engineer around the problem by combining glucose with fructose, as these two sugars use different intestinal transporters, and blending them can push total oxidation rates higher. This is a significant advance, but it does not answer the more fundamental question of how much carbohydrate any individual can actually use."If you look at what's out there," Wallis explains, "there are loads of guidelines for athletes, but they're very generic. People are essentially using trial and error to work out what's best for them."

Tracking carbon atoms

The technique at the heart of Wallis's work, called exogenous carbohydrate oxidation measurement, tracks how much of the carbohydrate an athlete ingests is actually burned as fuel by working muscles. The word 'exogenous' means that the carbohydrate is derived from outside the body as opposed to the glycogen stores already inside the liver and muscle. In other words, of all carbohydrates consumed, how much ended up being converted to physical power?

The method works by exploiting the natural variation in carbon isotopes. Carbon exists in two stable, non-radioactive forms: the common carbon-12, and the slightly heavier carbon-13. In nature, most plant-derived carbohydrates contain a small but consistent proportion of C-13. When athletes consume a glucose drink containing a slightly elevated proportion of that isotope it acts as a tracer that travels with the glucose into the bloodstream. If the muscle burns that glucose, it produces carbon dioxide (CO₂). If the exhaled CO₂ carries the heavier isotope signature it indicates that the ingested carbohydrate has been metabolised. This is done through capturing breath during exercise and analysing the ratio of C-13 to C-12 in exhaled CO ₂using a specialised mass spectrometer.

The fuelsync® test involves the athlete cycling or running at a sustained, moderate intensity for two and a half hours, consuming a glucose drink every 15 minutes throughout. Wallis's data show that exogenous oxidation rates continue to rise across the first 90 minutes and only plateau between two and two and a half hours, so the test duration must be sufficiently long to capture this peak. Breath samples are processed in Birmingham's on-site mass spectrometry facility. The numbers produced provide data on the individual's maximum capacity to use ingested carbohydrate, and the recommended intake range that would optimise delivery without overloading the gut.

Proof of concept

The possibility to personalise carbohydrate feeding via this kind of direct measurement was discussed in the field for some years but Wallis and his collaborator Dr Tim Podlogar were among the first to argue that individual variability in exogenous oxidation rates justified a personalised approach, The practical question that gives rise to is whether a single laboratory visit could actually produce a measurement reliable enough to serve as the basis for individual recommendations.

This question was answered in a recent proof-of-concept study. The team worked with 11 endurance cyclists and triathletes, measuring each participant's peak exogenous glucose oxidation rate under a high glucose dose of 90g/h. From those individual measurements, a personalised dose was calculated for each athlete. They found that personalised doses ranged from 49-80g/h, averaging around 65 grams per hour. Despite consuming less carbohydrate overall than the 90g/h reference condition, athletes achieved comparable oxidation rates, meaning they were extracting just as much usable energy. Ratings of perceived exertion and stomach fullness were lower on the personalised dose.

This individual variability is the key insight driving the enterprise. On average, people can use around 60 grams of glucose per hour, but the range in the study spanned from roughly 30 to 80 grams per hour. The implications are two-fold. An athlete with low capacity to use ingested carbohydrates who follows a recommendation of 70g per hour is not getting more energy but is accumulating unabsorbed carbohydrates in the gut which may impact comfort and performance. Conversely, an athlete at the high end who stays conservative is leaving gains on the table. "The more that you can get in and utilise, the more it will contribute to your performance," Wallis explains. "But if we identify people who are quite poor at it, the recommendation would be: you shouldn't be trying to push it too much, because you don't have the capacity."

fuelsync® is aimed at endurance athletes, but Wallis believes understanding efficient use of ingested carbohydrate is relevant beyond. Sports matches lasting 90 minutes or more face the same challenge, and it may have increased relevance for endurance events at altitude or in heat, where the body's ability to absorb and burn ingested carbohydrate is significantly reduced. There is also a question of what the test might reveal about the relationship between an individual’s capacity to oxidise ingested carbohydrates and everyday diet. Wallis acknowledges that carbohydrate intake habits may account for a small part of the variability between individuals, so findings could also inform training nutrition and not just race day planning.

Launching fuelsync®

fuelsync® is headquartered at the University of Birmingham, where the service is coordinated within the University’s research infrastructure and supported by its mass spectrometry facility. Delivery will take place through a network of partner laboratories in the UK and Europe, which can offer the cycling or running tests locally. Breath samples can then be analysed through Birmingham’s central facility as part of the wider service, before results and personalised recommendations are returned to the athlete.

The University of Birmingham has been consistently ranked in the global top ten for sports science. Work from Wallis's group and his predecessors has directly shaped industry guidelines and athlete products. The study on which the service is based is a proof of concept and Wallis is careful not to overclaim, but the direction of travel is clear. Sports nutrition has long prided itself on being evidence-based, while simultaneously issuing advice that asks every athlete to find their own version of a population average. What Wallis and his colleagues are building is a way to make the evidence personal. "We don't know why people are different," he says. "So let's measure to be sure."

While their work is geared towards professional and serious amateur athletes, the principle that guides the research – to build the evidence base for individualised approaches to sports and physical activity – are mirrored in a rich portfolio of sports research across the University of Birmingham. Researchers are developing evidence-backed programs in critical areas like illness rehabilitation and healthy ageing, bearing fruit in real world applications ranging from community programs for those at higher risk of disability to home-based rehabilitation protocols following heart failure.