Discover carb loading for runners: how it works, who benefits, what to eat and how to increase carbohydrates before a marathon without stomach discomfort.
Carb loading for runners involves increasing carbohydrate intake before a long-distance race to maximise the energy stored in the muscles. But it requires more than eating an enormous bowl of pasta the night before. Here’s how carb loading works, who benefits and how to do it without overwhelming your stomach.
Carb loading is often misunderstood as permission to eat an enormous bowl of pasta the night before a race. In reality, it is a planned increase in carbohydrate intake during the final one or two days before a long endurance event.
The aim is to maximise the glycogen stored in your muscles and liver before you reach the start line. These stores provide an important source of energy during running, particularly as the intensity rises.
Done properly, carb loading can help delay fatigue and support your ability to maintain pace. Done badly, it can leave you feeling uncomfortably full, worried by a temporary increase in body weight or dealing with stomach problems on race morning.
This guide explains who needs to carb load, how much carbohydrate is recommended and how to make the process practical.
What is carbohydrate loading?
Carbohydrate loading – often shortened to carb loading – is a sports-nutrition strategy used before prolonged endurance exercise.
During the final 36-48 hours before an event, the runner:
- Reduces training volume as part of their taper
- Increases carbohydrate intake substantially
- Spreads that intake across meals, snacks and drinks
- Chooses familiar foods that are easy to digest
The combination of lighter training and greater carbohydrate availability allows the muscles to store more glycogen than they would under normal training conditions. This is known as glycogen supercompensation.
Older carb-loading methods included a demanding workout followed by several days of very low carbohydrate before the high-carbohydrate phase. This depletion stage is not necessary for most runners. A short taper combined with a high carbohydrate intake can increase glycogen stores without adding unnecessary fatigue or dietary restriction during race week.
Why does glycogen matter for running?
Carbohydrates are broken down into glucose. Some glucose is used immediately, while some is stored as glycogen in the liver and skeletal muscles.
Muscle glycogen provides fuel directly to the working muscles. Liver glycogen helps maintain blood glucose, supporting the brain and the rest of the body during exercise.
The body can also use fat to produce energy, especially at lower intensities. However, fat cannot provide energy as quickly as carbohydrate. As the effort becomes faster or harder, the body relies more heavily on carbohydrate to meet the rising demand for energy.
This matters in races because glycogen stores are limited. If muscle glycogen and blood glucose availability fall too far, runners may experience:
- A sudden increase in perceived effort
- Difficulty maintaining pace
- Heavy or unresponsive legs
- Poor concentration and decision-making
- A strong urge to slow down or stop
This is commonly described as “hitting the wall.” Glycogen depletion is not the only cause of late-race fatigue – muscle damage, dehydration, pacing, heat and central fatigue can also contribute – but inadequate carbohydrate availability is an important part of the problem.
What does carb loading actually do?
Carb loading raises the amount of glycogen available at the start of an event. It does not make you fitter or increase your VO₂ max. Instead, it gives your existing fitness a larger store of readily available fuel.
Think of it as filling the tank before a long journey. A full tank does not make the engine more powerful, but it reduces the likelihood of running out of fuel before the destination.
Research suggests that increasing starting glycogen can delay fatigue and improve performance in prolonged endurance events. The greatest benefit is expected when exercise lasts longer than approximately 90 minutes, because this is when limited carbohydrate stores are more likely to affect performance.
Who should consider carb loading?
Carb loading is most relevant for runners preparing for:
- Marathons
- Ultramarathons
- Longer trail races
- Half marathons expected to take more than approximately 90 minutes
- Long-distance triathlons or similar endurance events
- Other sustained endurance competitions lasting longer than 90 minutes
It is usually unnecessary for a short easy run, most 5K and 10K races, or a brief gym session. Normal meals and an appropriate pre-run snack can provide enough carbohydrate for these activities.
A half-marathon runner finishing well under 90 minutes may not need a full loading protocol, although arriving well fuelled still matters. A runner expecting to be on the course for two or three hours is more likely to benefit.
How much carbohydrate do runners need when carb loading?
Sports-nutrition guidance commonly recommends approximately 10-12 grams of carbohydrate per kilogram of body mass per day for 36-48 hours before an endurance event lasting longer than 90 minutes.
Some practical plans use a broader range of 8-12 g/kg per day, particularly when individual appetite, body size, gastrointestinal tolerance and event demands are considered. The upper end is a large amount of food, so the strategy needs to be planned rather than improvised.
| Body mass | 8 g/kg | 10 g/kg | 12 g/kg |
|---|---|---|---|
| 50 kg | 400 g | 500 g | 600 g |
| 60 kg | 480 g | 600 g | 720 g |
| 70 kg | 560 g | 700 g | 840 g |
| 80 kg | 640 g | 800 g | 960 g |
These figures refer to grams of carbohydrate, not the total weight of the food. For example, 100 grams of cooked rice does not contain 100 grams of carbohydrate.
The recommended quantities can look surprisingly high. This is why relying on one evening meal rarely works. Carbohydrate needs to be distributed across the day, often with a combination of meals, snacks and carbohydrate-containing drinks.
Smaller or less experienced runners may find the upper end difficult to tolerate. A registered sports dietitian can help tailor the calculation when body composition, medical needs, previous stomach symptoms or a difficult relationship with food make a standard body-mass formula inappropriate.
What should runners eat while carb loading?
Carb loading does not require special supplements. Familiar carbohydrate-rich foods can do the job:
- White rice
- Pasta or noodles
- Bread, bagels and wraps
- Potatoes without large amounts of skin
- Pancakes or waffles
- Lower-fibre breakfast cereal
- Bananas and other tolerated fruit
- Yoghurt or flavoured milk
- Rice pudding
- Pretzels, crackers and cereal bars
- Fruit juice, smoothies or sports drinks
- Jam, honey or syrup
The best choices are foods you already know you tolerate. Race week is not the time to test a new supplement, unusual meal or food that has previously caused symptoms.
Why runners often reduce fibre and fat
Fibre and dietary fat are valuable parts of a healthy everyday diet. During a short carb-loading period, however, very high amounts can make it harder to eat enough carbohydrate and may increase fullness or gastrointestinal discomfort.
Temporarily choosing lower-fibre options – such as white rice instead of brown rice or a plain bagel instead of dense seeded bread – can reduce food volume and leave less material in the digestive system on race morning.
Keeping fat slightly lower can also help because high-fat meals digest more slowly and are very energy dense without contributing much carbohydrate. The goal is not to eliminate fibre and fat completely. It is to make the high-carbohydrate intake more comfortable for one or two days.
Protein should remain part of meals, but enormous portions can crowd out the carbohydrate you are trying to prioritise. Choose normal servings of familiar, relatively lean protein sources.
Example carb-loading day for a 60 kg runner
A 60 kg runner using a target of 10 g/kg would aim for approximately 600 grams of carbohydrate across the day. The following example shows how that amount might be distributed.
| Meal | Example | Approximate carbohydrate |
| Breakfast | Two plain bagels with jam, one banana and fruit juice | 150-170 g |
| Morning snack | Cereal bar and a sports drink | 50-70 g |
| Lunch | White-rice bowl with a normal serving of lean protein, a bread roll and yoghurt | 120-150 g |
| Afternoon snack | Pancakes with syrup or jam | 80-100 g |
| Dinner | Pasta with a lower-fat tomato sauce, bread and fruit juice | 120-150 g |
| Evening snack | Lower-fibre cereal with milk or rice pudding | 50-70 g |
Exact values depend on portion size and product. Nutrition labels and a short-term food-tracking calculation can help during planning, but you do not need to weigh every meal throughout normal training.
Liquid carbohydrates can be useful because they add carbohydrate without the same fullness as another large plate of food. They should support the plan rather than replace all normal meals.
A simple 48-hour carb-loading plan
Two days before the race
- Begin increasing carbohydrate at breakfast rather than waiting until dinner.
- Reduce training according to your taper.
- Build each meal around rice, pasta, bread, cereal or another familiar carbohydrate.
- Add carbohydrate-based snacks between meals.
- Drink normally and include fluid with meals.
- Keep fibre and fat moderate if you are prone to stomach problems.
The day before the race
- Continue spreading carbohydrate across the entire day.
- Eat the largest meals earlier if a heavy evening meal affects your sleep.
- Avoid using dinner as a last-minute attempt to reach the target.
- Choose familiar foods with predictable portion sizes.
- Prepare breakfast and race fuel for the following morning.
- Stop eating when the plan is complete; carb loading is not an eating challenge.
What should you eat on race morning?
Carb loading fills the muscles, but liver glycogen falls overnight because it helps maintain blood glucose while you sleep. A carbohydrate-rich breakfast helps restore liver glycogen and provides fuel before the start.
General sports-nutrition guidance recommends 1-4 g of carbohydrate per kilogram of body mass in the 1-4 hours before exercise. The amount should rise with the time available for digestion.
In practice, many runners use approximately 2-3 g/kg two to three hours before a long race, then add a small snack or drink closer to the start if needed.
Possible race-morning meals include:
- Bagel or toast with jam, plus a banana
- Porridge made with tolerated milk, honey and fruit
- Lower-fibre cereal with milk and juice
- Pancakes with syrup and yoghurt
- White rice with eggs or another familiar savoury option
Keep the meal familiar and reasonably low in fat and fibre. If nerves reduce your appetite, a combination of solid and liquid carbohydrate may be easier.
Carb loading does not replace fuel during the race
Even fully loaded glycogen stores are limited. For long races, carbohydrate consumed during the event can help maintain blood glucose, provide additional fuel and delay fatigue.
Common targets are:
- Exercise lasting 1-2.5 hours: approximately 30-60 g of carbohydrate per hour
- Exercise lasting more than 2.5 hours: up to approximately 90 g per hour when tolerated
Higher intakes generally require products containing more than one type of carbohydrate, such as glucose and fructose, because the intestine uses different transport routes to absorb them.
These are not targets to try for the first time on race day. Practise your chosen gels, drinks, chews or foods during long runs. Begin with an amount you tolerate and progress gradually.
This process is sometimes called gut training. Repeated practice can improve confidence, help the digestive system handle carbohydrate during running and reveal which products or timings cause problems.
What about hydration and electrolytes?
Carbohydrate loading and hydration are connected, but drinking excessive water does not improve glycogen storage or guarantee better performance.
Fluid needs vary with body size, sweat rate, weather, pace and event duration. Arrive normally hydrated, drink regularly with meals and use a race strategy based on experience rather than forcing a fixed amount.
Long races, warm conditions and high sweat losses may also increase sodium requirements. However, sodium and fluid plans should be individualised. Both dehydration and excessive fluid intake can be harmful.
Sports drinks can provide carbohydrate, fluid and sodium together, making them useful during carb loading or racing. They are an option, not a requirement.
Why does body weight increase during carb loading?
Glycogen is stored with water. Research suggests that every gram of muscle glycogen is associated with at least around three grams of water.
As glycogen stores rise, body mass may temporarily increase – often by approximately one or two kilograms, although the response varies. This is not the same as gaining that amount of body fat. It reflects stored fuel, associated water and, in some cases, a greater volume of food in the digestive system.
For a long endurance race, the extra stored fuel is normally more valuable than the disadvantage of carrying a small amount of additional mass. Avoid restricting fluid or carbohydrate to make the scale fall before the event.
Do female runners need a different approach?
Female athletes have historically been underrepresented in sports-nutrition research, so there are still gaps in the evidence.
Current carbohydrate-loading guidance can be used by female runners, but total energy intake matters. If carbohydrate is added without eating enough overall – or if other foods are simply removed to keep calories low – the runner may fail to reach the amount needed to maximise glycogen.
The priority is adequate energy availability throughout training and race preparation. Menstrual-cycle changes may affect appetite, fluid balance and how some runners feel, but they do not justify avoiding carbohydrate before endurance events.
Persistent fatigue, disrupted periods, repeated injuries, declining performance or anxiety around increasing food intake may indicate low energy availability or another issue that deserves support from a doctor and registered sports dietitian.
Common carb-loading mistakes
Waiting until the final dinner
One meal cannot comfortably provide an entire day’s carbohydrate target. Begin earlier and distribute intake.
Treating it as unlimited eating
The goal is more carbohydrate, not simply more food. Very high-fat takeaway meals may add plenty of energy while providing less carbohydrate than expected.
Eating far more fibre than usual
Huge portions of beans, vegetables, whole grains and bran cereals may increase bloating or the need to use the toilet. Keep the diet familiar and consider lower-fibre substitutions temporarily.
Trying new foods or supplements
Race expos and social media can introduce tempting new products. Use only what has already worked in training.
Forgetting the race-morning meal
Carb loading does not make breakfast irrelevant. Liver glycogen has been used overnight and should be topped up.
Starting in-race fuelling too late
Do not wait until energy levels crash. Follow the timing practised during long runs, often beginning carbohydrate intake early in the race.
Panicking about weight gain
A short-term increase is expected because glycogen binds water. It is evidence that fuel and fluid are being stored, not a reason to restrict intake.
Copying somebody else’s exact plan
A 50 kg runner and an 80 kg runner have different calculated needs. Appetite, speed, event duration and gastrointestinal tolerance also vary.
The takeaway
Carb loading is not one enormous pasta meal. It is a planned increase in carbohydrate across the final 36-48 hours before a prolonged endurance event.
For races lasting longer than approximately 90 minutes:
- Aim for a high carbohydrate intake appropriate to your body mass and tolerance.
- Spread food across meals, snacks and drinks.
- Choose familiar, easy-to-digest carbohydrate sources.
- Reduce fibre and fat slightly if they cause fullness or stomach problems.
- Eat a practised carbohydrate-rich breakfast on race morning.
- Continue fuelling during the race according to its duration.
- Rehearse the complete strategy during training.
The purpose is not to eat as much as possible. It is to begin the race with more usable fuel and a plan your stomach already understands.
Fill the tank. Protect your pace. Run stronger.
References and further reading
- Burke, L. M., Hawley, J. A., Wong, S. H. S. and Jeukendrup, A. E. (2011). Carbohydrates for training and competition. Journal of Sports Sciences, 29(Suppl 1), S17-S27. https://pubmed.ncbi.nlm.nih.gov/21660838/
- Thomas, D. T., Erdman, K. A. and Burke, L. M. (2016). Nutrition and athletic performance: position of the Academy of Nutrition and Dietetics, Dietitians of Canada and the American College of Sports Medicine. Medicine & Science in Sports & Exercise, 48(3), 543-568. https://pubmed.ncbi.nlm.nih.gov/26891166/
- Burke, L. M. (2007). Nutrition strategies for the marathon: fuel for training and racing. Sports Medicine, 37(4-5), 344-347. https://pubmed.ncbi.nlm.nih.gov/17465604/
- Podlogar, T. and Wallis, G. A. (2022). New horizons in carbohydrate research and application for endurance athletes. Sports Medicine, 52(Suppl 1), 5-23. https://pmc.ncbi.nlm.nih.gov/articles/PMC9734239/
- Jeukendrup, A. E. (2017). Training the gut for athletes. Sports Medicine, 47(Suppl 1), 101-110. https://pubmed.ncbi.nlm.nih.gov/28332114/
- Fernández-Elías, V. E. et al. (2015). Relationship between muscle water and glycogen recovery after prolonged exercise in the heat in humans. European Journal of Applied Physiology, 115, 1919-1926. https://pubmed.ncbi.nlm.nih.gov/25911631/
- Sims, S. T. et al. (2023). International Society of Sports Nutrition position stand: nutritional concerns of the female athlete. Journal of the International Society of Sports Nutrition, 20(1), 2204066. https://pubmed.ncbi.nlm.nih.gov/37221858/
This article provides general educational information rather than individual nutrition or medical advice. Runners with diabetes, gastrointestinal conditions, food allergies, a history of disordered eating or other medical needs should seek personalised guidance from an appropriate doctor and registered sports dietitian.
