Trail Research Weekly #5: Ultra-High Carb Fuelling – Fuelled or Fooled?

Endurance athletes often ask, “How many carbs do I need to consume?” The answer seems to change every year, but right now it is 120 grams of carbohydrate per hour. Scroll through any post-race feed and you’ll see it: the winner ‘took 120g an hour’, the podium ‘never dropped below 100g’, and suddenly the rest of us are staring at our vest pockets wondering if we’ve been missing out. Plews and colleagues (2026) ask the uncomfortable question: are we actually fuelled, or are we being fooled?

What Counts as “Ultra-High”?

Current sports nutrition guidelines place the ceiling for carbohydrate intake at 90 g·h⁻¹. Anything above that is called ultra-high carbohydrate (U-HC) intake. It has become popular through elite athletes, race reports and podcasts, while research has been playing catch-up ever since. High intakes have been recorded in elite sport since at least 2009, but the evidence for moving from 90 to 120 remains extremely thin.

The Three Myths of U-HC Fuelling

1. Ultra‑High Carbohydrate Ingestion Does Not Further Spare Endogenous Glycogen.

It’s logical to assume eating more carbs spares your muscle glycogen from depletion. A recent meta-analysis says otherwise: the sparing effect is small and doesn’t scale with dose. Moving from 90 to 120 g·h⁻¹ doesn’t mean more glycogen sparing. In one study, a higher dose actually drove greater use of your endogenous carbohydrates, the opposite of the sparing theory.

2. There Is a Lack of Evidence for Improved Performance with Ultra‑High Carbohydrate Ingestion Rates

Sports nutrition research has mostly measured substrate oxidation rather than performance itself, and trials that have measured performance directly don’t support a dose-response relationship. Smith et al. (2013) found performance benefits up to roughly 78 g·h⁻¹, with the line essentially flat between 80 and 90 g·h⁻¹, and even projected decrements beyond that range. Fell et al. (2021) showed a positive dose-response up to 90 g·h⁻¹ but did not test higher doses. King et al. (2019) found that carbohydrate beat placebo, but no difference in performance was observed above or below 90 g·h⁻¹. Across these studies, higher intake doesn’t reliably translate into greater oxidation or a measurable performance benefit. Carbohydrate clearly beats no carbohydrate, but there is no clear evidence of a performance dose-response beyond 90 g·h⁻¹.

 3. Ultra‑High Carbohydrate Ingestion Rates and Gut Training

The rationale for ‘gut training’ is that repeatedly loading carbohydrate during training improves gastric emptying and intestinal absorption, reducing GI distress on race day. There is some support for this: subjective tolerance improves and markers of carbohydrate malabsorption decline, particularly in cases where fructose-specific adaptations in gastric emptying have been shown. What gut training does not reliably do is raise your ceiling for exogenous carbohydrate oxidation.

Three Proposed Mechanisms of Action

The authors didn’t stop at ‘there’s no evidence’. They propose three plausible mechanisms for why the world’s best keep pushing carbohydrate intake.

1. Improved Oxidative Efficiency Through Substrate Shift

Carbohydrate yields more energy per litre of oxygen than fat. If U-HC feeding shifts substrate use toward carbohydrate even at lower intensities, you could be running at a marginally lower oxygen cost. For years, we’ve tried to shift the crossover point to the right to burn more fat, which is a much larger energy store. This model argues that once this shift is achieved, improvements may come from increased exercise efficiency. Their own meta-analysis of 16 studies found no clear effect of carb intake on VO2. It’s a hypothesis, not a finding.

2. Carbohydrate‑to‑Lactate Interplay in Trained Metabolism

High fructose intake increases circulating lactate. Lactate is now well established as a preferred oxidative fuel rather than a waste product, and it is shuttled to various tissues for use. The limitation is fructose absorption: GLUT5 fructose transporters saturate around 30–60 g·h⁻¹, so at a 1:0.8 or 1:1 ratio you’re already at the ceiling. Fructose that isn’t absorbed can’t be converted to lactate and may cause gut-related issues.

 3. A Possible Brain‑Derived Effect?

Carb mouth rinsing has been proven to improve performance without any carbohydrate being swallowed. This happens via sweet taste receptors, signalling to reward and motor centres. Learsi et al. (2019) showed lower RPE over 105 minutes and a faster 10 km TT. The hypothesis is that continuous U-HC feeding keeps those oral and gut receptors stimulated, creating a feed-forward loop. This may offer some performance benefit in an ultramarathon, where perception of fatigue is key to performance.

Tim Noakes’s Recent Review

Noakes and colleagues (2026) published a 160-study review that lands on a related conclusion. They argue that the primary benefit of carbohydrate ingestion is preventing exercise-induced hypoglycaemia (low blood glucose). They show that exercise-induced hypoglycaemia is closely correlated with exercise termination. They also show that muscle glycogen depletion alone doesn’t produce whole-body fatigue. Carbohydrate ingestion improves performance even in glycogen-depleted states, and fat-adapted athletes perform equivalently on far lower carbohydrate oxidation. They also demonstrate that carbohydrate ingestion suppresses fat oxidation linearly across the intake range.

Read the two papers together, and you get some tension. Both agree the old glycogen-depletion theory is wrong and that blood glucose is the real threat. Plews et al. (2026) entertain the possibility that maximal carbohydrate reliance is itself the performance lever. Noakes et al. (2026) argue carbohydrate is not an obligatory fuel at all, and that chronically chasing high carbohydrate intake is damaging to metabolic health. Prins et al. found that even moderately elevated chronic carb intake produced prediabetic fasting glucose patterns in 30% of athletes over 31 days.

Why This Matters for Trail Runners

No study to date has examined U-HC intake across the full duration of an ultramarathon. Lab trials run 2 to 3 hours, which eliminates the possibility of observing GI-related issues. That means the carbohydrate tolerance data you see quoted is not contextual to our sport. Ask anyone who has tried to hold 100 g·h⁻¹ into the back half of a mountain ultra how their gut is doing, and you’ll learn the reality.

Furthermore, Hearris et al. (2022) measured oxidation rates ranging from 1.3 to 1.9 g·min⁻¹ across athletes all fed the same 120 g·h⁻¹. The athlete at the top of that range utilises about 114 g·h⁻¹, near-complete use. The athlete at the bottom oxidises about 78 g·h⁻¹ and carries the rest as ballast. Larger athletes tend to oxidise more in absolute terms, which is another reason a single number for everyone is a poor idea.

And on our terrain, the variables the papers can’t control matter enormously. Steep climbing changes gastric emptying. Heat changes it again. Fluid concentrations above 8–10% slow the stomach and bring on fullness, bloating and nausea.

My Takeaway

I’m not anti-carb. Increasing carbohydrate intake remains the most reliable performance intervention in endurance sport. Most trail runners I coach are under-fuelling rather than over-fuelling. But there is a difference between fuelling properly and copying the number off a web article.

Here’s how I’d apply this paper:

Earn 90 before you chase 120. 

The evidence and practical application of 60–90 g·h⁻¹ is robust. 100 g·h⁻¹  is in the realm of anecdote and the metabolically gifted minority. The authors are explicit that recommending U-HC intake to amateur and recreational athletes is unnecessary.

Fix your ratio before you raise your total. 

Across the studies, glucose: fructose ratios of 1:0.8 to 1:1 produced the highest average oxidation rate (88 g·h⁻¹), whereas traditional 2:1 blends averaged only 65 g·h⁻¹. By adding pure fructose sugar to your drink mix, you can very easily improve this.

Treat unoxidised carbs as a cost, not a bonus. 

Anything you swallow but can’t oxidise doesn’t disappear. It sits in the gut and raises your risk of GI distress.

Use your long runs to test your tolerance. 

Individual oxidation testing needs a metabolic cart nobody reading this owns. Your version is a race-simulation long run with race-day products, race-day intensity and race-day heat, tracking grams per hour. Only if 90 g·h⁻¹ is comfortable at hour 5, consider increasing it.

Consider the high numbers a recovery tool rather than a race tactic. 

The strongest case for aggressive fuelling in the paper is day-to-day recovery: multi-day races, back-to-back long runs, and intensified blocks. That is a different physiological goal from race-day performance, and it is where I’d use those extra carbohydrates first, not only within your run, but between runs.

References

Fell, J.M., Hearris, M.A., Ellis, D.G., Moran, J.E.P., Jevons, E.F.P., Owens, D.J. & Morton, J.P. 2021. Carbohydrate improves exercise capacity but does not affect subcellular lipid droplet morphology, AMPK and p53 signalling in human skeletal muscle. The Journal of Physiology, 599, 2823–2849. 

Hearris, M.A., Pugh, J.N., Langan-Evans, C., Mann, S.J., Burke, L., Stellingwerff, T., Gonzalez, J.T. & Morton, J.P. 2022. ¹³C-glucose-fructose labeling reveals comparable exogenous CHO oxidation during exercise when consuming 120 g/h in fluid, gel, jelly chew, or coingestion. Journal of Applied Physiology, 132, 1394–1406. 

King, A.J., O’Hara, J.P., Arjomandkhah, N.C., Rowe, J., Morrison, D.J., Preston, T. & King, R.F.G.J. 2019. Liver and muscle glycogen oxidation and performance with dose variation of glucose–fructose ingestion during prolonged (3 h) exercise. European Journal of Applied Physiology, 119, 1157–1169. 

Learsi, S.K., Ghiarone, T., Silva-Cavalcante, M.D., Andrade-Souza, V.A., Ataide-Silva, T., Bertuzzi, R. & Lima-Silva, A.E. 2019. Cycling time trial performance is improved by carbohydrate ingestion during exercise regardless of a fed or fasted state. Scandinavian Journal of Medicine and Science in Sports, 29, 651–662. 

Noakes, T.D., Prins, P.J., Buga, A., D’Agostino, D.P., Volek, J.S. & Koutnik, A.P. 2026. Carbohydrate ingestion on exercise metabolism and physical performance. Endocrine Reviews, 47(2), 191–243. 

Pfeiffer, B., Stellingwerff, T., Hodgson, A.B., Randell, R., Pöttgen, K., Res, P. & Jeukendrup, A.E. 2012. Nutritional intake and gastrointestinal problems during competitive endurance events. Medicine and Science in Sports and Exercise, 44, 344–351. 

Plews, D.J., Booth, P.D., Krieger, T. & Maunder, E. 2026. Fuelled or fooled? Examining the evidence and mechanisms behind ultra-high carbohydrate intake in endurance athletes. Sports Medicine. 

Smith, J.W., Pascoe, D.D., Passe, D.H., Ruby, B.C., Stewart, L.K., Baker, L.B. & Zachwieja, J.J. 2013. Curvilinear dose–response relationship of carbohydrate (0–120 g·h⁻¹) and performance. Medicine and Science in Sports and Exercise, 45, 336–341. 

FAQs

Q1: Should I be taking 120 g of carbs per hour in my next ultra? 

Probably not, unless you’ve built up to it and proven you tolerate it. The evidence supports 60–90 g·h⁻¹ for most athletes, and the authors state that recommending ultra-high intakes to non-elite athletes is premature.

Q2: Why do the pros seem to get away with ultra-high carb fuelling? 

Two reasons. There are real individual differences in carbohydrate oxidation capacity, with some athletes oxidising up to 1.9 g·min⁻¹ versus 1.3 g·min⁻¹ in others. And larger athletes oxidise more in absolute terms. High intakes may only suit a small minority at the top of that distribution.

Q3: Does eating more carbs during a race save my muscle glycogen? 

Not to the degree we assumed. The sparing effect appears small and isn’t dose-dependent. The most reliable benefit of carbohydrate ingestion is keeping blood glucose stable.

Q4: Does gut training work?

It appears to improve subjective tolerance and reduce malabsorption markers, particularly for fructose. What it does not consistently do is raise your ceiling for exogenous carbohydrate oxidation.

Q5: What about the metabolic health side of eating this many carbs? 

It’s an open question. One study found roughly 30% of athletes developed pre-diabetic fasting glucose patterns on moderately elevated chronic carbohydrate intake over 31 days, an effect that reversed when intake was reduced.