Working Theory 03

Working Theory 03 • Angela Naeth Coaching • May 17, 2026

The carb revolution is real. But the number that matters most isn't grams per hour — it's your blood sugar.

Annotated study notes on endurance fueling research

Angela Naeth is a professional triathlete, certified coach, and the founder of Angela Naeth Coaching. She reviews peer-reviewed research and translates it for athletes. This is not medical advice.

The question

Every gel company, every podcast, every race-nutrition plan in 2026 tells you the same thing: eat more carbs. The number keeps climbing — 60 g/hr became 90, then 120. Pro cyclists are reportedly fueling at 100–140 g/hr during Grand Tour stages. The "carbohydrate revolution" is the dominant narrative in endurance sport.

But three papers published between late 2024 and 2026 complicate that story significantly. One is a massive narrative review of the 100+ g/hr trend in pro cycling. Another is a 160-study review in Endocrine Reviews arguing that blood glucose — not muscle glycogen — is the real performance limiter. The third showed that just 10 g/hr of carbs improved endurance by 22% in trained triathletes, regardless of whether they ate a high-carb or ketogenic diet.

So what does this mean for your race-day fueling? More is better — or is it?

The key insight

The performance benefit of carbs during exercise comes primarily from preventing blood sugar crashes — not from replacing muscle glycogen. The minimum effective dose may be far lower than current guidelines suggest, but more carbs still help for different reasons (recovery, multi-day events, gut comfort). The right amount is individual, and the playbooks give you the framework to find yours.

Study 1: The 100+ g/hr revolution — does it actually work?

Wilson et al. (2025) — Sports Medicine — Narrative review of high-carbohydrate fueling in professional cycling

This review looked at every study comparing ≥100 g/hr vs. 60–90 g/hr of carbs during cycling. The headline finding: there is no clear evidence that 100+ g/hr improves single-session performance over 90 g/hr.

In fact, in two separate studies by King et al., the 90 g/hr dose produced the best time-trial power — not 100 or 112.5 g/hr. The highest doses actually appeared to increase muscle glycogen breakdown rather than spare it.

90 g/hr

Dose that produced highest TT power in controlled studies — not 100 or 120

Where 100+ g/hr might help: multi-day stage races. On-bike carb intake is the strongest predictor of total daily carb intake in pro cyclists (r = 0.89–0.95), meaning eating more on the bike drives overall recovery. In a 3-week Grand Tour, this could mean 3,200 g more carbs over the event. That matters for glycogen resynthesis, energy availability, and avoiding RED-S.

The risk: above ~100 g/hr using a 2:1 glucose-fructose ratio, exogenous carb oxidation does not increase further but GI symptoms do. A 1:0.8 maltodextrin-fructose ratio at 120 g/hr did show higher oxidation, but only ~17% more than 90 g/hr. The practical gain is marginal; the GI gamble is not.

What I take from this: For a single race — half-marathon, marathon, 70.3, Ironman — the 60–90 g/hr range remains the evidence-backed sweet spot. Pushing above 90 g/hr has not been shown to improve acute performance and may increase glycogen breakdown and GI distress. The playbooks reflect this.

Study 2: It's not glycogen — it's blood sugar

Noakes et al. (2025) — Endocrine Reviews — Review of 160+ studies on carbohydrate ingestion, exercise metabolism, and performance

This comprehensive review re-examined the foundational research behind carb-loading and mid-exercise fueling. The central argument: fatigue during prolonged exercise is not caused by muscles running out of glycogen. It's caused by drops in blood glucose — exercise-induced hypoglycemia (EIH) — which triggers the brain to reduce motor output as a protective mechanism.

10–15 g/hr

Amount sufficient to prevent exercise-induced hypoglycemia in most athletes

Key findings from the review: carbs ingested during exercise do not "spare" muscle glycogen — high ingestion rates actually increase glycogen breakdown by suppressing fat oxidation. What carb intake does spare is liver glycogen, which is what maintains blood sugar. The original 1967 study that launched carb-loading actually showed subjects crashing from hypoglycemia, not empty muscles.

What I take from this: The minimum effective dose of carbs is about preventing a blood sugar crash, not about flooding your muscles with fuel. This is a paradigm shift. It doesn't mean "eat fewer carbs." It means the reason carbs work is different than we were told — and that means your fueling strategy should prioritize consistency and timing over sheer volume.

Study 3: 10 g/hr improved performance by 22% — on both diets

Prins et al. (2025) — American Journal of Physiology–Cell Physiology — Randomized crossover trial in trained triathletes

Ten competitive triathletes followed either a high-carb diet (380 g/day) or a very-low-carb ketogenic diet (40 g/day) for 6 weeks each. After each diet period, they performed a time-to-exhaustion test at 70% VO2max — with either a small carb drink (10 g/hr maltodextrin) or a taste-matched placebo.

+22%

Time-to-exhaustion improvement from just 10 g/hr carbs — on BOTH high-carb and keto diets

Results: performance was identical across both diets without carbs. With just 10 g/hr — 6 to 12 times less than current guidelines — performance improved 22% on both diets. The mechanism? The carbs eliminated exercise-induced hypoglycemia. Blood glucose stayed above 3.9 mmol/L in every carb trial. Without carbs, 6 of 10 athletes on the keto diet and 4 of 10 on the high-carb diet became hypoglycemic.

What I take from this: The floor matters more than the ceiling. Before you worry about hitting 90 or 120 g/hr, make sure you're preventing blood sugar crashes. Small, consistent doses early and often. Then layer additional carbs on top for energy, comfort, and recovery — but know that the first 10–15 g/hr is doing the heaviest lifting for your brain and motor output.

Bonus: your gut is unique — and now we can measure it

Wallis & Podlogar (2025) — University of Birmingham / fuelsync — Proof-of-concept for personalized carb oxidation testing

Researchers at Birmingham tested 11 cyclists' individual exogenous carb oxidation rates using carbon-13 isotope tracing. The result: personalized optimal doses ranged from 49 to 80 g/hr, with an average of 65 g/hr. Athletes on their personalized dose achieved the same oxidation as those eating 90 g/hr — with lower stomach fullness and lower perceived effort.

What I take from this: The range in each playbook (e.g., 60–90 g/hr for marathon, 90–120 g/hr for Ironman bike) exists for a reason. Your gut capacity is individual. Train it. Test it. Start at the lower end and build up. The playbooks give you the framework; your training gives you the data.

How this maps to the playbooks

Every ANC playbook includes a fueling section with specific carb, fluid, and sodium targets by race segment. Here's how the research above informs those numbers:

Distance	Carbs/hr	Why this range
Half-Marathon	30–60 g	Short enough that blood sugar maintenance is primary goal; 1–2 gels plus pre-race fuel covers it
Marathon	60–90 g	2–4 hr range where glycogen depletion becomes real; stay in 60–90 g sweet spot per Wilson review
70.3 Bike	60–90 g	Absorb while seated; front-load for the run; gut tolerance is limiting factor
Ironman Bike	90–120 g	4–6 hr bike = within-day energy deficit risk; higher intake protects blood sugar AND total daily energy (Wilson review)
Ironman Run	50–70 g	GI tolerance drops on the run; maintain blood sugar floor above all else

Five practical rules from the research

1. The first 10–15 g/hr prevents hypoglycemia. This is the non-negotiable floor. Start fueling within 30 minutes of starting.

2. 60–90 g/hr is the proven performance range for single-day races. Going higher has not been shown to help acutely.

3. Above 90 g/hr, use a 1:0.8 maltodextrin-fructose ratio — not 2:1 glucose-fructose — to maximize absorption.

4. Your gut capacity is individual (49–80 g/hr in the Birmingham data). Train it. Test it. Don't copy a pro cyclist's fueling plan.

5. For multi-day events or heavy training blocks, higher on-bike carbs protect energy availability and recovery — even if acute performance isn't better.

The bottom line

The carbohydrate revolution is real, but the revolution isn't "eat as much as possible." It's "eat enough, early enough, consistently enough to keep your blood sugar stable — and then find your individual ceiling through training." The playbooks give you the specific targets by distance. The research tells you why they work.

The fueling targets in this article come directly from the ANC playbooks. Each one is free:

Half
Marathon

Marathon

70.3

Ironman

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References

1. Wilson PB. A Narrative Review of the High-Carbohydrate Fueling Revolution: 100 g/h in the Professional Peloton. Sports Medicine. 2025. PMC12982284

2. Noakes TD et al. Carbohydrate Ingestion on Exercise Metabolism and Physical Performance. Endocrine Reviews. 2025;47(2):191. DOI link

3. Prins PJ et al. Carbohydrate ingestion eliminates hypoglycemia and improves endurance exercise performance in triathletes adapted to very low- and high-carbohydrate isocaloric diets. Am J Physiol Cell Physiol. 2025. DOI link

4. Wallis G, Podlogar T. Personalizing carbohydrate intake during exercise. Proof-of-concept study. University of Birmingham / fuelsync. 2025. Birmingham article

5. Hunter B et al. Durability as an index of endurance exercise performance: Methodological considerations. Experimental Physiology. 2025;110(11):1612–1624. DOI link