Race week comes with its own checklist, and by midweek most of it is done. Shoes broken in, bib collected, and a table booked at the pasta place for the night before, because everyone in the running club calls that dinner the way to carb load for a half marathon, and the race is on Sunday. Of everything on that list, the dinner gets questioned least. In the research, carb loading names something no single dinner could ever be.
Two different things hide under that one label. Topping up, which sports scientists call normalising, means eating enough carbohydrate over the 24 hours before a race, on a day of lighter training, to refill the stores your training already built, typically 7 to 10 grams per kilo of body weight across that day. Loading is the bigger protocol, 10 to 12 grams per kilo every day for 36 to 48 hours, built to push those stores past their normal level. So the real decision is which of the two a half marathon calls for.
Should You Carb Load for a Half Marathon, or Just Top Up?
For a half marathon, the research points to topping up rather than loading, typically 7 to 10 grams of carbohydrate per kilo of body weight over the 24 hours before. A 2019 review lists the half among races where that is enough, and the one trial at 20.9 km found loading gave trained runners no performance benefit.
— Burke et al. 2019 · Int J Sport Nutr Exerc Metab · scholarly review; Sherman et al. 1981 · Int J Sports Med · trained runners, 20.9-km run
A 2019 review by four sports-nutrition scientists, going event by event through the distance races, puts the half marathon among races under 90 minutes where topping up is sufficient. Loading, in that review, is kept for the marathon and the 50-kilometre race walk, where glycogen, the carbohydrate your muscles store, can run low enough to limit the race.
Loading's inside story and its outside story split apart in the one trial run at almost exactly this distance. Trained runners who loaded before a 20.9-kilometre performance run stored more glycogen in their calf muscles and burned through more of it with every kilometre, yet their run times did not differ from runners who ate a moderate-carbohydrate diet through the same week of tapering runs. Over 20.9 km, the paper concludes, loading gave trained runners no benefit to performance, and a half marathon is only 200 metres longer.
The smaller of the two strategies is still measured across a whole day, not a single meal. A runner's body weight in kilos, multiplied by 7 to 10, gives the grams of carbohydrate that range describes across the 24 hours before the start.
There is a catch in that 90-minute line, and it is about whose 90 minutes. Elite performers' race times are what that list was built around, and a runner whose half takes longer is not addressed. A finish time that starts with a 2 lands past it.
The 90-minute line measures time on your feet, not the name printed on your race bib.
For efforts past 90 minutes, the general line is careful rather than firm. Such events may benefit from higher glycogen stores, with more field research still needed to define which sporting events actually gain, according to a 2011 review of carbohydrate for training and competition. In running, the loading benefit that review cites is for distance running of 30 kilometres or more.
The closest trial to a two-hour effort is a 30-kilometre treadmill time trial from 1992 that took experienced runners about two and a quarter hours. After a week of extra carbohydrate, the loaded group ran its last 5 kilometres faster than it had the week before, and its six men finished 3.6 minutes faster than their own first attempt. Across the whole group, though, there was no overall difference in finishing time between the loaded runners and a group that ate the same extra calories as fat and protein. Support from Mars Confectionery UK is acknowledged in the paper, and the extra carbohydrate came from confectionery.
Eating more than 7 grams of carbohydrate per kilo the day before went with faster overall race speed and steadier pacing among non-elite London Marathon runners, compared with eating less than 7, in a study built on what runners reported eating, which can show a link but not prove a cause. It is a marathon, not a half, and 7 grams per kilo is where the topping-up range begins, well short of loading territory.
Loading in half marathoners who take longer than 90 minutes remains untested by any trial. The runners in these trials were trained or elite, and part of the dose evidence comes from cyclists. For a runner past the line, the honest answer sits between a guideline that says may benefit and a gap where the trial should be.
Loading also has a price, paid on the scale and in the stomach. A week of extra carbohydrate added 0.83 kilos (almost 2 pounds) to the loaded runners in that 30-kilometre trial, while the energy-matched group's weight held steady. Loading adds the weight of the extra glycogen plus the water stored alongside it, as the 2019 review notes, the same glycogen-and-water arithmetic behind overnight scale jumps. Eating 10 grams per kilo a day brought more stomach fullness and 3,940 calories of food a day, against 2,863 at 6 grams, in a 2026 trial of trained cyclists. No weight change was detected in those cyclists, though the trial's measuring method, by the authors' own warning, may have been too insensitive to pick one up.
For women the research is thinner still. The carbohydrate recommendations rest mostly on male endurance athletes, and women may need to raise their total energy intake substantially on loading days to get a similar effect, according to a 2017 sports-nutrition position stand. Women took part in the 30-kilometre trial, but too few to analyse their times, and none of these trials measured how loading changes women's race times.
Race morning is simpler in the 2019 review. A familiar pre-race meal, then a little carbohydrate after the warm-up. During a half, the suggestion is to try a carbohydrate mouth rinse, going up to an intake of 30 to 60 grams in total from drinks, gels or sweets, rather than the hourly targets used for marathons.
A half finished in under 90 minutes: the 2019 review files races of that length with those where topping up, typically 7 to 10 grams of carbohydrate per kilo over 24 hours, is sufficient.
A half that takes longer than 90 minutes: events of that length may benefit from higher glycogen stores, says a 2011 review that also calls for more field research, and loading in half marathoners at that pace has yet to be tested in any trial.
Two numbers settle the decision, and both are already yours. Your expected finish time tells you which side of the 90-minute line your race falls on, and your body weight turns 7 to 10 grams per kilo into a day of real food. Past race week, whether carb timing matters for your body composition is a separate argument with its own evidence.