How do you carb load before a race?

To carb load effectively, eat 8-12 grams of carbohydrate per kilogram of bodyweight per day (10-12 g/kg is the textbook full-load dose) for 1-3 days before your race, paired with a tapering training load so your muscles have the chance to bank what you're eating as glycogen.

Trained endurance athletes don't need the old "depletion phase", they can skip straight to the high-carb, low-training window. Lower fat and lower fibre intake (especially if you have a sensitive stomach) to make room for the extra carbohydrate, and don't panic when the scales tick up: each gram of glycogen is stored with around 3 grams of water, which is why most athletes gain a kilo or two (2-4lbs) during the loading window.

Key takeaways

• Trained athletes can skip the depletion phase. A 1-3 day high-carb taper is enough to maximise muscle glycogen.
• Aim for 8-12g of carbohydrate per kg of bodyweight per day (3.6-5.5g per lb per day) in the final 1-3 days. For a 70 kg (154lb) athlete that's 560-840 g of carbohydrate per day.
• Carb loading is most effective for events lasting longer than ~90 minutes. Below that, your stored glycogen is usually already enough assuming you've not been under-eating.
• Cut fat and (if needed) fibre to make room for the carbs , not by overeating on top of your normal diet. The proportion of calories from carbohydrate is what changes.
• Expect to gain ~1-2 kg of bodyweight during loading. That's stored glycogen pulling water with it, a sign it's working, not water retention to worry about.
• If eating that much food is hard , simple-carb drinks (like Carb Only Drink Mix), gels and chews are useful tools to top up without bloat.

Why does carb loading work?

Carbohydrate is the fuel your body reaches for first when exercise gets hard. You store it as glycogen (chains of glucose molecules) in your muscles (~500g) and liver (~100g). That's a finite reserve, and once it runs low, your pace drops sharply. This is the dreaded bonk (or "hitting the wall").

The trick of carb loading is a phenomenon called glycogen supercompensation: when you combine a high-carbohydrate diet with a training taper, your muscles store more glycogen than their normal baseline. You don't just refill the tank, you fill it past everyday levels.

The concept goes back to 1920, when Krogh and Lindhard (1920) showed that exercise was metabolically more efficient and felt easier on a carbohydrate-rich diet than on a fat-based one, the first hint that pre-fueling matters. By the 1960s, Scandinavian researchers (Bergström, Hermansen, Hultman & Saltin, 1967) using needle-biopsy techniques had described supercompensation directly: a few days of low carbs, followed by a few days of high carbs, produced glycogen stores well above baseline.

The approach reached endurance athletes via British marathon runner Ron Hill, who used a version of the so-called Bergström or Scandinavian diet (after the 1967 Bergström/Hermansen/Hultman/Saltin protocol) before the 1969 European Championships marathon in Athens. Hill won gold in 2:16:47 on the notoriously tough Marathon-to-Athens course, having tested the carb-loading approach in training. He later described this in The Long Hard Road (1981), word spread, and pasta parties on the eve of marathons have been a thing ever since.

A 2025 meta-analysis (Solem, Clauss & Jensen, Frontiers in Physiology; 30 studies, 319 participants) confirmed what athletes had long suspected: a taper plus high carbohydrate intake reliably increases muscle glycogen stores, by ~270 mmol/kg dry weight in cyclists and ~157 mmol/kg dry weight in runners, on average.

How many days before a race should you start carb loading?

For modern, well-trained endurance athletes, 1 to 3 days is enough. You do not need to spend a week on it.

The original ("classic") protocol was a six-day affair:

• Days 1-3: glycogen-depleting phase. Hard training plus very low carbohydrate intake.
• Days 4-6: taper plus high carbohydrate intake.

The depletion phase was the unpleasant bit. Athletes felt flat, irritable, and fragile. It was believed that this depletion made the muscles biochemically hungrier for carbs by causing an up-regulation in glycogen synthase enzymes. But the good news for everyone since: trained athletes don't need it. Endurance training itself is a strong stimulus for enhanced glycogen storage, so well-conditioned muscles already supercompensate readily when given a high-carb diet and a chance to rest.

In our own 2025 fueling survey of PF&H customers, 18% of athletes who carb load told us they start at least a week in advance. That's overkill. Starting too early increases the risk of unwanted weight gain and sluggishness without adding glycogen.

The practical sweet spot for most endurance athletes is:

• 2-3 days before the race , increase the proportion of carbohydrate in your meals. Include carb-rich snacks and drinks between meals to spread intake out if you struggle with big portion sizes.
• Taper your training in parallel (most athletes do this naturally before a key race).
• The night before the race, eat a familiar carbohydrate-rich meal at a familiar time, race-eve isn't the moment to try anything new.

How many grams of carbohydrate per kilo/pound of bodyweight do you need?

The widely accepted target is 8-12 grams of carbohydrate per kilogram of bodyweight per day for the final 1-3 days. Smaller-built athletes and those who train less typically sit at the lower end; bigger, harder-training athletes sit at the upper end.

In absolute terms:

That's a lot of food. For a 70kg (154lb) athlete, 700g of carbohydrate is roughly the equivalent of:

• Porridge with banana and honey for breakfast (~120g)
• A bagel with jam mid-morning (~70g)
• A large pasta lunch with bread (~180g)
• Rice cakes, bananas and a sports drink in the afternoon (~120g)
• A rice-based dinner with a sweet pudding (~210g)

Hitting the upper end of the range almost always means cutting fat and fibre to make room, not piling extra calories on top of your normal diet (more on that below).

What should you eat (and avoid) when carb loading?

The aim is to shift the ratio of your daily calories toward carbohydrate, not to overeat. Practically, that means:

Eat more of:

• White rice, white pasta, white bread, bagels
• Potatoes (boiled or mashed)
• Cereal, porridge, toast
• Bananas, dried fruit, fruit juice
• Honey, jam, maple syrup
• Sports drinks, gels and chews if appetite is the bottleneck (see below)

Eat less of:

• High-fat foods (cream, butter, cheese, oily sauces, fried foods, large amounts of meat)
• High-fibre foods if you have a sensitive stomach (see the next section)
• Anything new or unfamiliar

A simple way in for athletes new to carb loading: in your last couple of days before the race, add a moderate extra portion of carbohydrate to each meal at the expense of richer or fattier foods. White rice, potato, pasta, or a glass of Carb Only Drink Mix alongside your meal works well. If you can't face another plate of pasta, PF 30 Gels and PF 30 Chews are easy ways to add 30g of carbohydrate without bulk.

Why do you gain weight when carb loading?

Each gram of glycogen is stored alongside roughly 3 grams of water (Olsson & Saltin, 1970). So when you add a few hundred grams of glycogen to your muscles and liver, you can put on 1-2 kg (2.2-4.4lbs) of bodyweight in a couple of days.

Don't panic when the scales tick up. This is the loading working, not water retention to worry about. The water comes with the glycogen and is released as the glycogen is burned during the race. Many athletes notice they look a little fuller in the legs the day before a big race. That's the same effect.

If you're competing in a sport where weight matters (running, climbing, anything graded), the maths still favours the loaded athlete: 1-2 kg of extra mass is a small penalty for an extra 60-90 minutes of high-quality fuel in the tank.

Should you carb load for shorter events?

No, not in the formal sense. If your event is shorter than about 90 minutes, your normal glycogen stores are more than enough to get you through (assuming you've been tapering and eating adequately), and the supercompensation effect is unlikely to translate into a meaningful performance gain.

That said, you should still:

• Eat normally and definitely not skimp on carbs in the day or two before the race.
• Have a familiar carb-rich meal 2-3 hours before the start.
• Top up with a small carb-rich snack 30-60 minutes before the gun if it sits well with you.

The full carb-loading protocol (8-12 g/kg/day over a few days) pays off when you're going to be working hard for longer than 90 minutes. Doing a Marathon, IRONMAN 70.3®, IRONMAN®, ultra, gravel race or sportive? You should be carb-loading. Doing a 5k or 10k run or a sprint triathlon? You may benefit from carb-loading, but it's less critical.

How do you carb load if you have a sensitive stomach?

Two changes are usually enough to make carb loading kinder on a delicate gut:

1. Reduce fibre intake the day or two before the race. Drop the wholegrain bread, brown rice, raw veg, beans and pulses, and switch to white rice, white pasta, white bread, peeled potatoes, ripe bananas, and easily-digested fruit.
2. Lean on simple, predictable carbohydrate sources like sports drinks, gels and chews to top up without bulk or fibre.

Liquid carbohydrate (e.g. a serving of Carb Only Drink Mix or a bottle of fruit juice) is one of the easiest ways to add 30-60g of carbohydrate without taking up much room in the stomach.

James Phillips, PF&H's Head of Athlete Support and an experienced endurance athlete, describes his own approach:

As I'm tapering down for a big event, usually ~3-7 days depending on the priority of the race, I start to remove some of the fat and fibre from my diet and replace this with more simple carbohydrate, with the view that it's less likely to result in GI distress on race day and to slightly increase my overall carb intake.

I feel that when my training load has dropped ~50%, I'm burning less carbohydrate in training, so indirectly I know I'm storing more anyway. This seems to have worked for me in the past as I (touch wood…) rarely end up with stomach issues and don't feel overly bloated come race day.

If you've been on a lower-carb diet day-to-day, don't try to go from 4g/kg to 12g/kg cold the week of your race. Start practising on long training days a few weeks out, so your gut gets used to processing the volume. The same gut-training principles that apply to fuelling during exercise (Jeukendrup, 2017) apply to fuelling around it: regular exposure builds tolerance.

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Frequently asked questions

How many days before a race should I carb load? For most trained endurance athletes, 1-3 days is enough. The classic 6-day "deplete then load" protocol isn't necessary. Endurance training itself is a strong enough stimulus for glycogen storage so you can go straight to a high-carb taper.

How much carbohydrate per kg of bodyweight should I eat? Aim for 8-12g of carbohydrate per kg of bodyweight per day for 1-3 days. A 70kg (154lb) athlete should target 560-840g of carbohydrate per day, with smaller athletes at the lower end and bigger athletes at the upper end.

Will I gain weight when I carb load? Yes, typically 1-2kg (2.2-4.4lbs) over a couple of days. Each gram of glycogen is stored with around 3 grams of water, so the weight gain is mostly stored fuel and the water that comes with it. It's a sign the loading is working, not retention to worry about.

Should I carb load for a half-marathon or 10K? For events under about 90 minutes, formal carb loading isn't needed, your normal glycogen stores cover it. Just eat normally, don't cut carbs, and have a familiar, carb-rich pre-race meal. For a half-marathon at hard intensity, lightly increasing carbs in the final 1-2 days can help, but the full 8-12 g/kg protocol is probably overkill.

Do I need to do a "depletion phase" first? No. Trained endurance athletes don't benefit from the depletion phase used in the classic 1960s protocol, the supercompensation effect occurs without it. Skip straight to the high-carb taper.

What if I have a sensitive stomach? Lower your fibre intake in the 1-2 days before the race (white rice, white pasta, peeled potatoes, ripe bananas), lean on liquid carbohydrate sources to top up without bulk, and avoid trying any new foods during the loading window.

Can I just use sports nutrition products to carb load? You can lean on them when appetite or schedule limits how much real food you can eat. A serving of Carb Only Drink Mix adds 60 g of carbohydrate per litre, and gels and chews give you 30 g hits. But real food should still do most of the work; sports nutrition products fill gaps rather than replacing meals.

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Further reading

• How much carbohydrate do athletes need per hour?
• What should you eat before training and races?
• What happens when you 'bonk'? (And how to avoid it!)
• How to train your gut to handle more carbohydrate
• How to taper before a race

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References

1. Cox GR, Clark SA, Cox AJ, Halson SL, Hargreaves M, Hawley JA, Jeacocke N, Snow RJ, Yeo WK, Burke LM. Daily training with high carbohydrate availability increases exogenous carbohydrate oxidation during endurance cycling. Journal of Applied Physiology . 2010;109(1):126–134. doi: 10.1152/japplphysiol.00950.2009
2. Impey SG, Hearris MA, Hammond KM, Bartlett JD, Louis J, Close GL, Morton JP. Fuel for the work required: a theoretical framework for carbohydrate periodization and the glycogen threshold hypothesis. Sports Medicine . 2018;48(5):1031–1048. doi: 10.1007/s40279-018-0867-7
3. Podlogar T, Wallis GA. New horizons in carbohydrate research and application for endurance athletes. Sports Medicine . 2022;52(Suppl 1):5–23. doi: 10.1007/s40279-022-01757-1
4. Solem K, Clauss M, Jensen J. Glycogen supercompensation in skeletal muscle after cycling or running followed by a high carbohydrate intake the following days: a systematic review and meta-analysis. Frontiers in Physiology . 2025;16:1620943. doi: 10.3389/fphys.2025.1620943
5. Krogh A, Lindhard J. The relative value of fat and carbohydrate as sources of muscular energy. Biochemical Journal . 1920;14(3-4):290–363. doi: 10.1042/bj0140290
6. Bergström J, Hermansen L, Hultman E, Saltin B. Diet, muscle glycogen and physical performance. Acta Physiologica Scandinavica . 1967;71(2-3):140–150. doi: 10.1111/j.1748-1716.1967.tb03720.x
7. Sherman WM, Costill DL, Fink WJ, Miller JM. Effect of exercise-diet manipulation on muscle glycogen and its subsequent utilization during performance. International Journal of Sports Medicine . 1981;2(2):114–118. doi: 10.1055/s-2008-1034594
8. Bussau VA, Fairchild TJ, Rao A, Steele P, Fournier PA. Carbohydrate loading in human muscle: an improved 1 day protocol. European Journal of Applied Physiology . 2002;87(3):290–295. doi: 10.1007/s00421-002-0621-5
9. Olsson KE, Saltin B. Variation in total body water with muscle glycogen changes in man. Acta Physiologica Scandinavica . 1970;80(1):11–18. doi: 10.1111/j.1748-1716.1970.tb04764.x
10. Jeukendrup AE. Training the gut for athletes. Sports Medicine . 2017;47(Suppl 1):101–110. doi: 10.1007/s40279-017-0690-6
11. Hill R. The Long Hard Road (autobiography, 2 vols). Hampden Press; 1981.