The truth about carbs and athletic performance is that carbohydrate is the main fuel your muscles burn during hard exercise, and running low on it is one of the clearest limits on how long you can keep up a high intensity. How much you need depends on your training load, from about 3-5 grams per kilogram of body weight a day on light days to 8-12 g/kg on very heavy ones. Timing matters most when sessions are long or close together.
Key Takeaways
- Above roughly 60% of maximal effort, blood glucose and muscle glycogen become the main fuels your muscles use.
- Sports nutrition guidance sets carbohydrate needs in grams per kilogram of body weight, scaled to training: about 3-5 g/kg a day for light days, 5-7 for moderate, 6-10 for hard and 8-12 for very hard or very long training.
- During exercise, about 30-60 g of carbohydrate an hour suits longer sessions, and events over about 2.5 hours may benefit from up to 90 g an hour.
- The "eat within 30 minutes" rule matters mainly when you train again within hours; otherwise total daily intake matters more.
- In controlled studies of elite race walkers, a ketogenic low-carb diet raised fat burning but made athletes less economical and slower at race pace.
Evidence grade: Strong. Carbohydrate guidance for athletes rests on decades of controlled human trials summarized in joint position statements from the American College of Sports Medicine, the International Society of Sports Nutrition and IOC consensus authors, with repeated randomized studies on low-carbohydrate diets.
Why Carbohydrates Matter for Athletic Performance
Your muscles run on ATP, a molecule they constantly use up and remake. They can make ATP from fat or from carbohydrate. During exercise above about 60% of your maximal oxygen uptake, blood glucose and muscle glycogen (the stored form of carbohydrate) become the main fuels, partly because harder efforts recruit more fast-twitch muscle fibers that rely on carbohydrate.
Your body stores only a limited amount. The International Society of Sports Nutrition (ISSN) puts it at roughly 80-100 g of glycogen in the liver and 300-400 g in skeletal muscle. Liver glycogen keeps blood glucose steady, which matters because the brain normally depends on glucose for fuel. Muscle glycogen fuels the working muscle directly. As glycogen runs down, the ability to hold a given pace or power output falls.
That is why the 2016 joint position statement from the American College of Sports Medicine, the Academy of Nutrition and Dietetics and Dietitians of Canada says that well-chosen nutrition strategies, including the type, amount and timing of carbohydrate, improve both performance and recovery.
Simple and Complex Carbohydrates
Carbohydrates range from simple sugars, such as glucose and the sugars in fruit and milk, to complex starches in grains, potatoes and legumes. Both end up as glucose that your muscles and liver can store or burn. The practical difference for athletes is speed and comfort: low-fiber, quickly digested carbohydrates are easier to use close to or during exercise, while whole grains, beans, fruit and vegetables bring fiber and micronutrients that suit everyday meals.
A 2018 review on glycogen for coaches and athletes describes high-quality carbohydrate foods as minimally processed grains, vegetables and fruit, which also supply nutrients many people under-eat, such as fiber, potassium and magnesium. Close to a hard session or race, though, high-fiber foods are one of the factors linked with stomach and gut discomfort during exercise.
How Much Carbohydrate Do Athletes Need?
Older advice expressed carbohydrate as a percentage of calories, such as 60-70%. The ISSN notes that this has fallen out of favor because percentages give the wrong answer for athletes who eat very large amounts of food or who are restricting energy. Current guidance uses grams per kilogram of body weight per day, matched to the training you are actually doing.
| Training load | Example | Daily carbohydrate | For a 70 kg (154 lb) athlete |
|---|---|---|---|
| Low | Easy activity such as yoga, walking or light effort where you can talk or sing | 3-5 g/kg | About 210-350 g |
| Moderate | About an hour of jogging, swimming or cycling at a conversational pace | 5-7 g/kg | About 350-490 g |
| High | An hour or more of hard exercise such as interval training or hard running | 6-10 g/kg | About 420-700 g |
| Very high | Very hard or very prolonged training, such as long interval sessions or team sports at full effort | 8-12 g/kg | About 560-840 g |
The ranges come from a 2018 review in Nutrition Reviews, adapted from the 2016 ACSM joint position statement. The same review stresses that 10 g/kg a day is needed only when glycogen deficits are large or before an event where you want to load glycogen. Intake should rise and fall with training, lower on light days and higher on heavy ones.
Fueling Before, During and After Exercise
Before training or competition
A carbohydrate-rich meal in the hours before exercise can top up liver and muscle glycogen. The ISSN position stand on nutrient timing notes that carbohydrate meals or snacks for several hours before harder sessions longer than about 90 minutes are commonly recommended, and that they matter more if you ate little carbohydrate in the preceding days. For shorter, high-intensity sessions, see our guides to nutrition for high-intensity interval training and pre-workout meals for HIIT.
During exercise
A 2011 consensus paper prepared for the IOC scales carbohydrate during exercise to the event:
- About 1 hour of sustained hard effort: small amounts of carbohydrate, or even rinsing the mouth with a carbohydrate drink, can help.
- Longer sessions: about 30-60 g an hour.
- Events over about 2.5 hours: up to 90 g an hour, ideally from products that blend different sugars, such as glucose and fructose, so more can be absorbed.
A 2014 review by Asker Jeukendrup adds that one type of carbohydrate can be burned at up to about 60 g an hour, and that these amounts do not need to be scaled to body weight. For sodium and fluid in long events, see our article on electrolyte management during long-distance races.
After exercise
The popular advice to eat carbohydrate within 30 minutes of finishing is most relevant when you need to train or compete again soon. When recovery time is short, the ISSN recommends about 1.2 g/kg of carbohydrate an hour for 4-6 hours. Adding protein (about 0.2-0.5 g/kg an hour) speeds glycogen refilling when carbohydrate intake is below that level, and protein supports muscle repair either way.
When you have a full day before the next hard session, timing matters much less. In elite athletes doing heavy training, a diet providing at least 8 g/kg a day restored glycogen within 24 hours when muscle damage was only moderate.
Carbohydrate Loading for Endurance Events
Carbohydrate loading means eating more carbohydrate in the days before a long event so that muscles start with extra glycogen. According to the ISSN, early protocols combined a depletion phase with a high-carbohydrate diet, but simpler approaches work: reduce training for three to four days and eat a very high-carbohydrate diet (about 8-10 g/kg a day) for one to three days. In one study, a single day of 10 g/kg of high-glycemic carbohydrate after a short all-out cycling test nearly doubled baseline muscle glycogen.
Loading is most useful for prolonged events (longer than about 60-90 minutes) that will substantially drain glycogen, such as marathons, long triathlons or long cycling races. The ISSN also notes that women may need to increase total calories on loading days to get effects similar to men. Practice your loading plan before an important race.
What the Research Shows on Low-Carb and Keto Diets for Athletes
Some endurance athletes follow low-carbohydrate, high-fat (LCHF) or ketogenic diets to train the body to burn more fat. The research shows that these diets do change fuel use, but the performance results are mixed at best:
- Fat burning rises sharply. In a 2016 study of 20 elite ultra-endurance athletes, those who had followed a low-carb diet for about 20 months burned about 2.3 times more fat at peak than athletes on a high-carbohydrate diet. Their muscle glycogen use and refilling during and after a 3-hour run were similar.
- Economy and race-pace performance fell in elite race walkers. In a 2017 controlled study, 3 weeks of a ketogenic LCHF diet raised fat burning but increased the oxygen cost of walking at race speeds. Athletes on high or periodized carbohydrate improved their 10 km race times by about 5-7%; the LCHF group did not improve.
- The finding was repeated. A 2020 follow-up study found the same impairment, and a 2021 study found that even 5-6 days on the diet was enough to raise the oxygen cost of race-pace walking, with no rebound benefit once carbohydrate was restored.
These studies were in highly trained athletes performing sustained, high-intensity efforts, so they do not settle every question about lower-carbohydrate eating. The IOC consensus authors also note that it is still unclear whether deliberately training with low carbohydrate on selected days ("train low") improves performance in well-trained people. For hard training and racing, the evidence favors having carbohydrate available.
Hydration and Electrolytes Alongside Carbohydrate
Carbohydrate is only part of fueling. The ACSM position stand on fluid replacement advises starting exercise well hydrated, drinking during exercise to avoid losing more than about 2% of body weight, and building a personal plan because sweat rates vary widely. It notes that drinks with electrolytes and carbohydrate can offer benefits over water alone in some situations. Our guide to hydration strategies for sports performance covers this in more depth.
Related product
OPA ION+ Select Electrolyte is a scoop-and-mix powder with magnesium, potassium, sodium from Himalayan pink salt, coconut water powder and vitamin B12, made without added sugars or stimulants. Because it has no added sugar, it is a hydration option rather than a carbohydrate source: during long sessions, pair it with carbohydrate from food, drinks or gels to meet the hourly targets above.
Safety and Interactions
- Stomach and gut comfort: a review in Sports Medicine reports that 30-50% of athletes experience gastrointestinal complaints during exercise. Fiber, fat, protein and fructose have all been linked with a higher risk, as has dehydration. Choose familiar, lower-fiber carbohydrate before hard sessions and practice your race-day plan in training.
- Medicines that lower blood sugar: if you take insulin or another glucose-lowering medicine, talk to your doctor or pharmacist before making large changes to carbohydrate intake or exercise fueling.
- Electrolyte drinks and potassium: the NIH Office of Dietary Supplements notes that people with chronic kidney disease, and people who take ACE inhibitors or potassium-sparing diuretics, can develop high blood potassium even from typical intakes. Talk to your doctor or pharmacist before using potassium-containing drink mixes if that applies to you.
- Restrictive diets: very low-carbohydrate or low-energy eating during heavy training is a big change. Young athletes, and anyone with a history of disordered eating, should work with a registered dietitian or sports dietitian.
Frequently Asked Questions
Are carbohydrates necessary for high-intensity exercise?
For most athletes, yes. At intensities above about 60% of maximal effort, muscle glycogen and blood glucose are the main fuels, and low glycogen limits how long you can hold a hard pace.
Can a low-carb or ketogenic diet improve endurance performance?
Such diets greatly increase fat burning, but in repeated controlled studies of elite race walkers they also raised the oxygen cost of race-pace exercise, and athletes did not improve their race times the way high-carbohydrate groups did.
How many grams of carbs per day does an athlete need?
It depends on training: about 3-5 g/kg of body weight on light days, 5-7 g/kg for about an hour of moderate exercise, 6-10 g/kg for hard training and 8-12 g/kg for very hard or very long training.
Is carbohydrate loading beneficial for all types of endurance events?
No. It mainly helps prolonged events, longer than about 60-90 minutes, that substantially drain muscle glycogen. Shorter events rarely need it.
How does carbohydrate intake during prolonged exercise impact performance?
It keeps blood glucose up and spares glycogen. Consensus guidance suggests about 30-60 g an hour for longer sessions and up to 90 g an hour, from mixed sugars, for events over about 2.5 hours.
Do I need to eat carbs within 30 minutes after a workout?
Only if you train or compete again within a few hours. Then about 1.2 g/kg an hour for 4-6 hours speeds glycogen recovery. With a full day between sessions, total daily intake matters more than the exact timing.
Sources
- Thomas DT, Erdman KA, Burke LM. American College of Sports Medicine Joint Position Statement. Nutrition and Athletic Performance. Med Sci Sports Exerc. 2016.
- Murray B, Rosenbloom C. Fundamentals of glycogen metabolism for coaches and athletes. Nutr Rev. 2018.
- Kerksick CM, Arent S, Schoenfeld BJ, et al. International Society of Sports Nutrition position stand: nutrient timing. J Int Soc Sports Nutr. 2017.
- Burke LM, Hawley JA, Wong SH, Jeukendrup AE. Carbohydrates for training and competition. J Sports Sci. 2011.
- Jeukendrup A. A step towards personalized sports nutrition: carbohydrate intake during exercise. Sports Med. 2014.
- Burke LM, Ross ML, Garvican-Lewis LA, et al. Low carbohydrate, high fat diet impairs exercise economy and negates the performance benefit from intensified training in elite race walkers. J Physiol. 2017.
- Burke LM, Sharma AP, Heikura IA, et al. Crisis of confidence averted: Impairment of exercise economy and performance in elite race walkers by ketogenic low carbohydrate, high fat (LCHF) diet is reproducible. PLoS One. 2020.
- Burke LM, Whitfield J, Heikura IA, et al. Adaptation to a low carbohydrate high fat diet is rapid but impairs endurance exercise metabolism and performance despite enhanced glycogen availability. J Physiol. 2021.
- Volek JS, Freidenreich DJ, Saenz C, et al. Metabolic characteristics of keto-adapted ultra-endurance runners. Metabolism. 2016.
- American College of Sports Medicine; Sawka MN, Burke LM, Eichner ER, et al. American College of Sports Medicine position stand. Exercise and fluid replacement. Med Sci Sports Exerc. 2007.
- de Oliveira EP, Burini RC, Jeukendrup A. Gastrointestinal complaints during exercise: prevalence, etiology, and nutritional recommendations. Sports Med. 2014.
- NIH Office of Dietary Supplements. Potassium: Fact Sheet for Consumers.
Disclosure: OPA Nutrition sells supplements, including products mentioned in this article. This content is for education only and is not medical advice. Statements about dietary supplements have not been evaluated by the Food and Drug Administration. These products are not intended to diagnose, treat, cure, or prevent any disease. Talk to your healthcare provider before using any supplement, especially if you take medication or have a medical condition.
Written and reviewed by Michael Gonzales, Founder, OPA Nutrition · Last reviewed: · How OPA researches and writes
