What the Run Fueling Research Actually Shows

Fueling Is a Training-Specific Science

Run fueling research has produced some of the clearest dose-response findings in sports nutrition because endurance performance is objectively measurable and glycogen depletion has a predictable time course. The evidence is strong enough to make specific recommendations — not just "eat enough."

What Replicates Strongly

Carbohydrate ingestion improves performance in runs longer than 75 minutes. Below this threshold, endogenous glycogen stores are sufficient for most athletes at moderate intensity. Above 75 minutes, carbohydrate intake of 30–60g per hour prevents glycogen depletion and significantly improves performance and time trial times in RCTs. This threshold shifts with intensity — higher-intensity running depletes stores faster.

Multiple transportable carbohydrates (glucose + fructose) allow higher absorption rates. The gut can absorb approximately 60g/hour of glucose alone before absorption becomes rate-limiting. Combining glucose and fructose (2:1 ratio) allows absorption up to 90g/hour by using different intestinal transporters simultaneously. Elite marathon research increasingly uses 80–90g/hour protocols during racing. Practising high-carb intake in training is necessary to train gut tolerance.

Caffeine (3–6mg/kg) improves endurance performance by 2–5%. This is one of the most replicated ergogenic findings in sports science. Caffeine reduces perceived exertion, delays fatigue, and enhances fat oxidation at lower intensities. Optimal intake 30–45 minutes before effort; tolerance adaptation requires strategic use (not daily). Mid-race caffeine gels are effective for late-race performance maintenance.

Sodium bicarbonate (0.3g/kg) buffers lactate and improves high-intensity performance. Meta-analyses confirm ~2% improvement in performances lasting 1–7 minutes. GI distress is the primary limiting factor — individual tolerance testing before race use is essential. Some athletes buffer effectively; others experience debilitating nausea.

Pre-run carbohydrate loading increases glycogen stores and improves performance in events over 90 minutes. The classic 3-day glycogen depletion then loading protocol has been simplified to 36–48 hours of high carbohydrate intake (8–10g/kg/day) which achieves maximum muscle glycogen supercompensation. This is well-established for marathon and ultramarathon performance.

What the Research Can't Tell You

Individual gut tolerance, carbohydrate absorption capacity, and fat oxidation rates vary significantly between athletes. Some runners perform well with minimal in-run fueling at moderate intensities; others need aggressive fueling from 45 minutes. Experimenting with fueling protocols in training — not racing — is the safest way to identify your personal optimal strategy.