HIGH CARBOHYDRATE ATHLETIC FUELLING. A FAD METABOLIC DUMPSTER FIRE , PART 1

BY DR. BAYNE FRENCH

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High-Carbohydrate Fuelling: Performance, Health, and Metabolic Reality

As you hang on my every word, I would like to do the following:

• explain the basis for current fashionable recommendations for high carbohydrate intake in endurance fuelling;
• review the biochemical and metabolic pathways we all possess;
• explain how and why high-sugar fuelling may be unhealthy; and
• offer healthier fuelling alternatives that I believe can improve performance and support career longevity.

High-Carbohydrate Fuelling

We have no absolute nutritional requirement for carbohydrate. I appreciate that some readers may disagree with this statement, or regard it as unscientific, but it remains my view based on both clinical experience and the evidence as I interpret it.

This is not true of fat or protein. Without these, we die. The healthiest and leanest patients I see do not consume carbohydrate as a dietary staple. Likewise, the healthiest athletes in my practice tend to use modest amounts of carbohydrate — and not primarily sugar — reserving it for longer, more demanding training sessions and competition. They are able to access fat as a fuel source and generate our essential energy molecule, ATP. More on that later.

Regular participation in intense, prolonged endurance pursuits is relatively new in human history. In one sense, it could be viewed as unnatural. Human beings are of course built for movement — lifting, travelling, fighting and reproducing — but repetitive, strenuous exercise performed in large volumes, particularly in a historical context where conserving energy was essential for survival, is a modern phenomenon. It follows that the nutritional demands of endurance athletes may differ from those of the general population.

There are, of course, outliers: truly high-level, fat-adapted athletes who perform competitively without fuelling with carbohydrate. I will return later to Dr Volek’s work with such athletes. Then there are those in the middle — like me, albeit without the “high-level” label. I have enjoyed some success in obstacle course racing and even won enough prize money to cover my fishing licence for the year. For long training sessions and competition, I did use carbohydrate, but never in particularly large amounts.

Most people, and especially most athletes, are not fat-adapted. They simply do not burn fat efficiently. That probably includes you. The good news is that fat adaptation can be learned, much like a foreign language or a card game. Until that adaptation is achieved, however, the old saying “carb is king” holds some truth. Most people will perform better, most of the time, when using carbohydrate as a fuel source.

The difficulty begins when we ask two important questions: how much carbohydrate is needed, and from what source?

A range of contextual factors must also be considered, including body size, event duration, environmental temperature, and whether the athlete’s everyday diet supports metabolic efficiency and fat adaptation. One must also ask: what is the goal? Is it simply to finish, or to compete for a win?

This is precisely why sweeping statements are so difficult to justify — and why it is irresponsible to suggest that, because some carbohydrate is useful, more must necessarily be better.

The notion that “more is better” has shaped human behaviour for generations. Current fuelling trends are simply an extension of that mindset. But does that logic hold up in relation to athletic performance? Dr John Smith appears to think so.

In 2010, Dr Smith studied 12 high-level cyclists — twelve, not twelve thousand — and found a dose-response relationship between glucose ingestion and performance (Smith et al. J Appl Physiol. 2010).

In 2013, he led a somewhat larger study involving 51 triathletes and cyclists. Twelve different beverages were used, and participants were blinded to what they were consuming. To be clear, they were not literally blinded; they simply did not know which drink they were receiving. Once again, the conclusion was that higher carbohydrate intake was associated with better performance (Smith et al. Med Sci Sports Exerc. 2013).

Autonomic Dysreflexia

At this point, it is worth stepping back and thinking critically.

The performance gains observed in Dr Smith’s studies were modest. The sample sizes were small, making it difficult to assign substantial statistical power or broader significance to the findings.

Nor were these athletes followed longitudinally over extended periods to assess long-term performance outcomes. Measures such as gastrointestinal distress, career longevity, injury burden, body fat percentage, or the incidence of cancer and heart disease were not examined. These omissions matter, and I will return to them later.

Consider the following. When human beings — including highly trained athletes — consume large quantities of sugary products, blood glucose rises. The body recognises this as a physiologically undesirable state and responds by releasing stress hormones such as cortisol and epinephrine (adrenaline). It is possible that this “fight or flight” response contributes, at least in part, to the modest improvement in performance seen in such studies.

A similar stress response has been reported in Paralympic athletes who intentionally induce bodily harm to provoke the neurological reaction known as autonomic dysreflexia, or colloquially, “boosting”. According to a CBC News report from 2012, this illegal practice has included behaviours such as sitting on pins, breaking toes, or twisting the scrotum. The resulting stress response may enhance physical output slightly, but it is clearly incompatible with health.

The blood sugar spikes associated with high-carbohydrate fuelling may provoke a similar physiological response.

High-Sugar Fuelling Recommendations

A number of companies, along with their more vocal advocates, are currently promoting carbohydrate intake recommendations that I consider potentially harmful. These guidelines are not based on robust, long-term studies involving large numbers of participants followed over many years. Nor are they grounded in what I would regard as a genuinely health-promoting model.

We do possess a storage form of carbohydrate in the body: glycogen. For intense exercise lasting approximately one to two hours, this stored carbohydrate is often sufficient.

According to one proponent of high-carbohydrate fuelling, “30–60 g of simple carbohydrates per hour” is required for intense exercise lasting one to two hours. I broadly agree that this range is supported by decades of research and experience.

However, the same author goes on to recommend that, for intense exercise lasting more than two hours, athletes should consume 60 to 90 g of carbohydrate per hour. On what basis?

Author Andy Blow states:
“This is where ‘multiple transportable carbohydrates’ (MTCs) such as glucose/fructose blends can often be preferred in order to help maximise absorption of very high amounts of carb via your gut.”

“MTCs” is essentially a sophisticated way of describing different sources of sugar.

The Gatorade Sports Science Institute (GSSI) recommends that, in the 24 hours before hard training or competition, athletes consume 7–12 grams of carbohydrate per kilogram of body weight. Let us put that into perspective. For a 150-pound athlete, this amounts to 477–816 grams of carbohydrate — roughly equivalent to 50 slices of bread.

Who exactly is this advice for? In my view, the health of many athletes depends on rejecting this sort of recommendation outright.

Gut Training

The gut microbiome responds dramatically to large carbohydrate loads. Fermentation, gas production, and other microbial consequences follow. Unsurprisingly, this can produce highly unpleasant gastrointestinal symptoms.

This has now been formalised into high-carbohydrate fuelling terminology: exercise-induced gastrointestinal syndrome (EIGS) and exercise-associated gastrointestinal symptoms (Ex-GIS).

The question then becomes whether the gut can be “trained” to tolerate large amounts of ingested carbohydrate. It is an attractive idea, especially given that sugary products are often highly palatable.

Martinez et al. (Sports Med. 2023) conducted a systematic literature review on “gut training”. Of 304 studies reviewed, only 8 were deemed suitable for inclusion. The review found that, after a two-week repetitive carbohydrate feeding protocol — in plain English, repeated high-sugar exposure — both gut discomfort and carbohydrate malabsorption were reduced. Significant improvements were seen in both upper gastrointestinal symptoms (such as vomiting) and lower symptoms (such as excessive gas).

Again, however, it is worth pausing to think critically. Competitive eaters also “train” by stretching the stomach and blunting the neurological vomiting response. But neither practice carries any meaningful association with health, disease prevention, or longevity.

Asker Jeukendrup PhD, an influential figure in sports nutrition, published an article entitled Training the Gut for Athletes (Sports Med. 2017). In it, he discusses the transporters involved in moving glucose and fructose from the intestine into the bloodstream. There are indeed specific transporters for each. His review suggests that, through repeated exposure to large quantities of sugary products, the number and efficiency of these transporters may increase. The hoped-for result is less gastrointestinal distress, more sugar absorbed into the bloodstream, and improved performance.

Dr Jeukendrup appears to support current guidelines recommending 60 g of carbohydrate per hour for exercise lasting up to two hours, and up to 90 g per hour for exercise lasting longer than that. It is worth noting that this article was published with support from the Gatorade Sports Science Institute, a division of PepsiCo.

Personally, I place very little confidence in studies funded by organisations that sell the products the studies are effectively promoting. That concern sits alongside a broader issue: the possible long-term consequences of encouraging athletes to consume large amounts of a substance — sugar — that I believe contributes to cancer, cardiovascular disease, obesity, diabetes and Alzheimer’s disease. That is not a legacy I would want attached to my name.

The Alternative to High-Carbohydrate Fuelling

Dr Volek and colleagues conducted the FASTER study — Fat Adapted Substrate Oxidation in Trained Elite Runners — published in Metabolism in 2016. The aim was to examine metabolic adaptations and differences between athletes consuming high-carbohydrate diets and those following low-carbohydrate, high-fat diets.

The study involved 20 elite ultramarathon runners and Ironman-distance triathletes, all in racing condition. More than half had sponsors, one-third held course records, one-quarter had represented Team USA, and several held national or international records. In other words, this was a highly accomplished and serious group of athletes.

Through an extensive screening process, the researchers identified 10 carbohydrate-fuelled athletes (with a dietary ratio of carbohydrate:protein:fat of 59:14:25) and 10 fat-fuelled athletes (10:19:70). The two groups were closely matched in every major category other than diet and fuelling strategy.

The low-carbohydrate group consumed six times less carbohydrate over the course of an average day than the high-carbohydrate group (82 vs 684 g/day). Crucially, the low-carbohydrate athletes had been eating this way for an average of 20 months and were therefore considered well adapted.

Even within two closely matched groups, some interesting differences emerged:

Measure	High-carb (HC)	Low-carb (LC)
Body fat %	9.6	7.8
Lean mass (kg)	57.3	60.9
Fat mass (kg)	6.5	5.5
VO2 max	64.3	64.7

These figures suggest notable differences in body fat percentage and lean mass.

The ability to burn fat was 2.3 times higher in the low-carbohydrate group (1.54 g/minute versus 0.67 g/minute in the high-carbohydrate group). Yet despite these substantial differences in fuel use, there were no significant differences in resting muscle glycogen, exercise-related glycogen depletion, or glycogen restoration during recovery.

Put simply, the low-carbohydrate athletes had a dramatically greater capacity to burn fat while maintaining the same ability to break down and replenish stored carbohydrate as the high-carbohydrate athletes.

Several earlier studies did not show preserved glycogen stores in low-carbohydrate athletes (Phinney et al. Metabolism. 1983; Helge et al. J Physiol. 2001; Zderic et al. Am J Physiol Endocrinol Metab. 2004). In general, however, those studies involved athletes following low-carbohydrate diets for much shorter periods. This suggests that the cellular mechanisms responsible for glycogen preservation and restoration take time to develop. Meaningful adaptation appears to require several months, not merely a few weeks.

Dr Volek compared the glycogen responses of these low-carbohydrate athletes to those of Alaskan sled dogs. These animals have extraordinary endurance capacity and have been shown to maintain glycogen stores despite running 160 km per day for five consecutive days while eating a high-fat diet containing only 15% carbohydrate (McKenzie et al. Med Sci Sports Exerc. 2005).

For the first time, the FASTER study demonstrated that well fat- or keto-adapted endurance athletes can possess a dramatically enhanced ability to oxidise fat while maintaining normal muscle glycogen. Dr Volek concluded:
“Keto-adaptation provides an alternative to the supremacy of the high carbohydrate paradigm for endurance athletes.”

Summary: The Potential Benefits of Fat Adaptation and Ketones

Fat is often considered a more efficient fuel. It is more than twice as energy-dense as carbohydrate and protein. One gram of fat contains 9 calories, whereas carbohydrate and protein each provide 4 calories per gram. In addition, fat is stored in an anhydrous form, meaning it is not stored alongside water.

Human beings possess a vast reserve of stored fat — approximately 50,000 to 100,000 calories. By contrast, glycogen stores are relatively limited, at less than 2,000 calories.

Ketones are produced when fat is metabolised. They may reduce oxidative stress. Intense exertion leads to a sharp rise in mitochondrial free radical production, and ketones appear to upregulate antioxidant pathways, helping to reduce this oxidative burden.

Ketones may also exert anti-inflammatory effects, reducing inflammatory compounds such as IL-6, IL-8 and TNF-alpha. Other markers of inflammation, including white blood cell count and C-reactive protein (CRP), are also reduced in a higher-ketone environment.

Enhanced recovery has been reported, though not directly measured in this study. That remains anecdotal, but the biochemical rationale is plausible, given the reduction in oxidative stress and pro-inflammatory signalling.

There may also be a reduced risk of “bonking” — the abrupt energy crisis that affects both muscles and brain. Ketones readily fuel the brain, while fat and ketones can fuel skeletal muscle efficiently for extended periods. There is a direct linear relationship between blood ketone levels and brain ketone levels.

There is little doubt that ketogenic diets can assist with weight loss. Improved body composition has an obvious effect on power-to-weight ratio.

Finally, there may be implications for career longevity. Animal models (Roberts et al. Cell Metabolism. 2017) have shown increased longevity and improved “healthspan” in animals fed higher-fat diets. Again, there is a plausible rationale here. The gradual accumulation of excess body fat compromises power-to-weight ratio, while oxidative stress and chronic inflammation may contribute over time to soreness, metabolic dysfunction, and impaired performance.