Sixty years of carb-loading research, 63 experiments, and the whole evidence base is 641 people. Only 74 of them were women, and nobody's group averaged over 37

The pasta dinner the night before a marathon is one of the best-known rituals in sport, and the idea behind it is sound. Muscles keep their own store of carbohydrate, called glycogen, and it is the main fuel for hard endurance exercise. The store is small. The paper puts it at about 400 g in the muscles and another 100 to 120 g in the liver on a normal day. Run it down and the pace drops. So for sixty years athletes have eaten more carbohydrate and trained less in the days before a race, to start with the tank as full as it will go.

How much carbohydrate, and for how many days, has been argued over for just as long. A group at Liverpool John Moores University, with a colleague at Flinders University in Australia, set out to settle it from the published record. Their review is in the October 2026 issue of the Scandinavian Journal of Medicine & Science in Sports. They kept only studies that reported what people ate and how they exercised for at least 24 hours and then measured glycogen directly, from a muscle biopsy. From 13,503 search results they ended up with 63 experiments.

The broad answer is the one coaches already give. In the 18 randomised trials strong enough to pool, moving from a low or moderate intake (under 6.5 g of carbohydrate per kilogram of body weight a day) to a high one raised muscle glycogen by about 192 units in the crossover trials, where the same people tried both diets. For scale, a rested muscle on an ordinary diet sits around 400 of those units. When the team plotted every study group, glycogen rose with intake in close to a straight line for recreational and endurance-trained people, with no clear ceiling up to about 10 g per kilogram.

Now the small print, most of which the authors supply themselves. The trials disagreed with each other far more than chance allows, and sorting them by dose, by days or by fitness did not explain it. The five trials that compared separate groups of people, not the same people twice, gave a smaller gain of about 108 units that did not reach statistical significance. On timing, 24 to 48 hours of loading looked at least as good as 72 or 96. But the 24-hour figure comes from two comparisons with four participants between them, and the trials using different durations also fed different amounts. The paper calls these estimates 'untrustworthy' and says an optimal loading period cannot be worked out from them.

The line about fitter athletes is easy to misread. Among well-trained people, intake explained only about 9% of the differences in glycogen, and the link was not statistically significant. That could mean highly trained muscle fills up on less. The authors think a plainer explanation is at least as likely: there were fewer studies in that group, and none fed less than 4.7 or more than 12 g per kilogram, so there was little spread to detect a slope with.

The part we found most striking is who was studied. Across six decades, 641 people. Seventy-nine per cent of the experiments used men only, and 74 women took part in total. Four in five experiments used volunteers aged 18 to 30, and no study group had an average age above 37. Look at the start line of any big-city marathon and compare. On the limited evidence there is, women stored as much glycogen as men once they ate the same amount of carbohydrate for their weight. For runners in their forties, fifties and sixties, there is simply nothing.

There is a second gap between the lab and the race. In 65% of the experiments people rested completely during the loading days, where real athletes keep jogging through a taper. And 72% of the protocols were on a bike. The measured outcome throughout is glycogen in a thigh or calf muscle, not finishing time. This review does not test whether a fuller muscle wins races, and it says so.

Our reading: the practical advice survives, with softer edges than a training plan usually admits. The authors land on more than 8 g of carbohydrate per kilogram a day for 36 to 48 hours before an endurance event, timed after the last hard session, with easy training allowed. Current sports guidelines say 10 to 12 g. For a 70 kg runner (our arithmetic, not the paper's) that is upwards of 560 g of carbohydrate a day against 700 to 840 g, which is a lot of food either way, and the paper notes that very high intakes can upset the gut and suggests low-fibre choices. It also adds a line worth passing on to anyone loading up for a short race or a weekend football match: for team sports and shorter events, maximising glycogen is unnecessary, and 6 to 10 g per kilogram is enough to top up to normal.