A new metabolic study of eight elite male marathoners, published this month in the Journal of Applied Physiology and widely circulated in the sports-nutrition community in the fortnight after Boston, suggests that the ceiling for in-race carbohydrate intake has moved again. The runners — whose average age was 29, whose VO2 max sat at 70.6 ml/kg/min and whose average marathon personal best was 2:22:54 — each completed three 120-minute treadmill runs at marathon effort while ingesting 60, 90 or 120 grams of carbohydrate per hour in randomised order. The headline finding: oxidation of ingested carbohydrate scaled with dose. Runners fuelling at 120 g/h burned a materially higher fraction of what they swallowed than those on the traditionally accepted 90 g/h cap, and they defended muscle glycogen better in the final 30 minutes of the run.
What this means in practical terms is a gentle but real loosening of one of endurance running's longest-held rules. For two decades, the consensus figure for peak exogenous-carbohydrate oxidation has hovered around 90 g/h, derived from work by Asker Jeukendrup and others in the early 2000s. That ceiling has been crumbling for some time — the current generation of hydrogel and dual-source (glucose plus fructose) products was designed to push past it — but the new data is notable for showing that the upper bound isn't 100 g/h or 110 g/h but, at least for this sample, 120 g/h and possibly higher. The authors are careful to note that they tested only one end of the pyramid: well-trained male runners whose guts had been deliberately conditioned through weeks of progressive fuelling in training.
Elite marathon practice has moved quickly in the same direction. At Boston on Monday, nearly every athlete on the podium was fuelling in the 90-to-110 g/h range according to shoe-sponsor nutrition-team briefings shared with World Athletics, with at least two runners experimenting with 115 g/h. John Korir's own build-up included a four-week progression from 90 g/h up to 110 g/h in his long runs, according to his Athletics Kenya coaching staff. The practical logistics — roughly one Maurten Gel 100 every 20 minutes, or one 45g drinks-bottle sip every six minutes — are demanding enough that teams have now begun to practise the fuel-handover choreography during pre-race shakeout runs.
The fly in the ointment, as every amateur who has ever tried to follow an elite fuelling plan knows, is the gut. The new study reports moderate to severe gastrointestinal distress in half of the runners at 120 g/h. That includes bloating, nausea and, for two of the eight, an aborted run. The authors caution that the efficiency gains of 120 g/h will only matter if the runner can actually hold the fuel down, and that gut training — a slow, months-long progression of in-training fuelling — is almost certainly a prerequisite. The same caution applies to recreational runners, who do not run at the sustained metabolic intensity of the test subjects and therefore have both less need for and less tolerance of the higher rates.
For readers who race, the sensible take-home has not actually changed. Build up fuelling gradually across a long-run block; rehearse race-day intake at race-day pace; and treat 90 g/h as the number that has an excellent evidence base rather than the maximum possible. What the study does change is the shape of the conversation at the elite end. With the men's world record down to 2:00:35, sub-2:00 speculation is no longer restricted to theoretical physiology papers, and every marginal improvement — shoes, pacing lights, wind-shield formations and, now, ingested fuel — is being exploited in parallel. The next time a major championship marathon is won, it will probably be won by an athlete whose stomach has been quietly training as hard as their legs.