Published on June 14, 2026 03:33 am
These days everyone is hyper-focused on how many carbohydrates they can eat. during A race. But those efforts go in vain if you start the race with a partially empty fuel tank. Carbohydrate-loading protocols have been around for more than 50 years, but there is still debate about the best way to ensure that your pre-race fuel stores are fully maximized. A new study provides some useful data.
The study comes from a group at Liverpool John Moores University led by Robin Jones and Julian Lewis, and is published in Scandinavian Journal of Medicine and Science in Sports. It tests three different two-day carb loading protocols under realistic conditions – that is, with trained endurance athletes eating and training as they normally would in the days before a big race – and finds that, within the range of carb intake tested, more is better, with no significant downsides.
Carb Loading Backstory
The idea of carb loading emerged in the 1960s, when scientists in Scandinavia figured out how to take muscle biopsies to measure how much carbohydrate (in the form of glycogen) was stored in leg muscles. They observed that endurance improved when athletes started out with more glycogen in their legs, and also that the amount of glycogen can be increased – supercompensated – if you first deplete your glycogen stores by exercising to exhaustion.
These insights led to the creation of the first carbohydrate loading protocols, which were complex and demanding. The classic week-long protocol included a long bout of exercise until exhaustion, three days of a low-carb diet, and another long bout of exercise until exhaustion and then three days of a high-carb diet. It may have boosted glycogen stores, but it wasn’t a very pleasant way to spend your final week before a big race.
Later studies showed that elaborate depletion protocols were not actually necessary, as trained athletes were always exercising, even in the week before the race. By the early 2000s, the pendulum had swung so far in the opposite direction that some scientists suggest You needed to eat more carbs a day than normal (10 grams per kilogram of body weight, or g/kg, a quantity we’ll return to) to maximize your glycogen stores.
These days the American College of Sports Medicine official stance It is suggested to spend two to three days eating 10 to 12 g/kg to maximize muscle glycogen. This is standard practice among elite athletes, but when Jones reviewed the literature as part of her doctoral study, she found evidence about how much and when was not as clear as she expected — which is why a new study was needed.
new study
Jones’ study tested a two-day carb-loading protocol with three different amounts of carbohydrate: 6, 8, and 10 g/kg, compared to a baseline diet of 4 g/kg. They recruited 11 trained endurance athletes (8 men, 3 women), each of whom completed three trials of a five-day protocol designed to mimic real-life racing:
- Day 1: 60 minutes moderate cycling; Eat 4 g/kg carbs
- Day 2: 30 min easy, then hard intervals (8 x 5:00) with muscle biopsy before and after; eat 4 g/kg
- Day 3: 60 minutes moderate cycling; Eat 6, 8, or 10 g/kg carbs
- Day 4: 30 minutes moderate cycling; Eat 6, 8, or 10 g/kg carbs
- Day 5: Muscle Biopsy
The main result is that eating more carbs stored more glycogen in their leg muscles. Here are the data for four different conditions (including pre-load measurements from 4 g/kg carbs):
Jones and his colleagues hypothesized that there would be a “ceiling effect”, so going from 8 to 10 g/kg would not provide as much of a boost as going from 6 to 8. But there is no evidence of any ceiling effect: 10 g/kg is much better than the alternatives, which leaves open the question whether 12 g/kg might be even better.
There are some potential caveats to carb loading. One is that 10 g/kg is a Very Of carbohydrates. A one-cup serving of cooked pasta is believed to contain 42 grams of carbohydrates; If you weigh 150 pounds, 10 g/kg is equal to 16 servings. Feeling absolutely full and bloated is hardly what you’re aiming for the night before a marathon.
The study measured a range of GI outcomes, but contrary to expectations, they found no higher prevalence of symptoms such as nausea, cramps or flatulence with the highest carb load. Subjects reported feeling more full with 10 g/kg, but not to problematic levels. They even ate two blue-tinged muffins on the fourth day, and saw no significant difference in how long it took for the blue color to appear in the toilet (20.2, 19.0, and 18.7 hours in the low-, medium-, and high-carb conditions, if you’re curious).
The second concern is that carb loading can lead to weight gain and bloating, as it is believed that every gram of glycogen is stored along with four grams of water. But Jones and his colleagues saw no difference in total body weight. Here is the data:
carb loading in exercise
What I learn from this is that two days at 10g/kg is a good carb-loading protocol – if you can handle it. It seems clear that less is bad, and volunteers in this study had no problems at 10 g/kg. On the other hand, it’s possible that more is better: ACSM guidelines, after all, go for three days at 12 g/kg. But this is getting into some seriously challenging food.
In this study, researchers provided the subjects with all of their food. There aren’t many details about meals, but for the two higher carb levels, they included things like “jelly sweets,” orange juice and sports drinks to reduce the risk of GI symptoms: good sources of carbohydrates that contain minimal fiber. In general, this is probably a good principle to keep in mind for the food you eat during those days, too. Cyclist Mike Woods once told me that his Taper diet follows “the diet of a five-year-old,” avoiding things like whole grains and high-fiber vegetables.
In morten’s account In Sebastian Savay’s sub-two hour marathon fuel plan, he mentions that he was adding an additional sports drink with 80 grams of carbs two days before the race. The day before the race, he drank 160 grams of carbohydrates in a sports drink, and added another 40 grams in an energy bar. If Sava is aiming for about 600 grams of carbohydrates per day (based on an estimate that he weighs about 130 pounds), he’s already getting 200 grams from his easily digestible sports nutrition products.
There is one final problem to consider. Even after all that carb loading, the glycogen in your liver – the second most important storage depot after your leg muscles – gets depleted overnight because you’re still fueling your brain. For this reason, you still need to top-up your store on the morning of the race. Savay reportedly had a light breakfast, drank a sports drink on the bus to the start line, and took a gel with 25 grams of carbohydrates five minutes before the start. Yomif Kjellcha, who broke two hours to finish second behind Savay in London Allegedly Drank a sports drink the morning of the race and took a gel containing 45 grams of carbohydrate shortly before the start.
Of course, it’s worth remembering that no medals are awarded for the highest glycogen level. South African sports scientist Tim Knox, once a booster of carb loading, now believes that glycogen levels are irrelevant to race performance and you only need enough carbohydrates to keep your blood sugar high. Most sports scientists believe otherwise—and apparently Soave and Kjellcha do too.
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