How and when to carbohydrate load (AKA carb loading)

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For many marathon races, carbohydrate loading is critical to success and quite easy. However, for shorter races carbohydrate loading can be detrimental and for longer races it has marginal benefits.

1 What is carbohydrate loading?

Running requires energy, and that energy comes from a mixture of carbohydrate, fat and Protein. The supply of fat is practically unlimited, but there is a limited supply of carbohydrate. This carbohydrate is stored as Glycogen in the liver and muscles. The liver Glycogen can travel via the blood to any of the muscles as well as the brain, but muscle Glycogen cannot leave the muscle fibers. Typically, there is sufficient Glycogen for about 20 miles of running at marathon pace, making the final stages of the marathon tough. Carbohydrate loading increases the Glycogen stores beyond their normal levels.

2 Does carbohydrate loading work?

For most people it does, but carbohydrate loading does not always dramatically increase Glycogen store, with results varying between 25% and 100%. Also, increased Glycogen stores do not always translate to improved performance. Factors such as the type of carbohydrate taken, the timing, the type of competition and other factors are probably to blame.

3 The downside to carbohydrate loading

The biggest downside to carbohydrate loading is that Glycogen is stored with additional water. Generally, 2-5 grams of water are stored with each gram of Glycogen, so carbohydrate loading adds extra weight. If the race does not require the extra Glycogen supplies, then the extra weight will degrade performance.

4 The golden rule of racing

Remember the The Golden Rule of Racing, which is 'Never do something in a race you have not practiced in training'. Carbohydrate loading can be practiced in training. Choose a week some time out from your race and try the technique out. You may gain a little extra weight due to the extra calories, but it's worth the trade off to understand how carbohydrate loading works for you. You can combine this with a small taper to get a sense of how your body responds to that technique as well.

5 Carbohydrate loading in different races

The value of carbohydrate loading is dependent on the length of the race and to some extent the runner. This is because while running a given distance uses roughly the same number of calories regardless of the pace, at a higher intensity a higher percentage of those calories come from Glycogen. So, if you burn 100 calories to run a mile, at 60% of V̇O2max you may get 40% from Glycogen, but at 80% of V̇O2max you would get 65% of calories from Glycogen.

5.1 Carbohydrate loading and the marathon

For runners who run for vast majority of the marathon distance and are pushing themselves for fast time, carbohydrate loading is critical. This is because the marathon is raced at a relatively high intensity, which burns Glycogen quickly and the standard supply will not be sufficient. I believe this to be the case for runners finishing in 2 hours or 5 hours. Taking Walking Breaks can reduce the intensity sufficiently that Glycogen may not be a limiting factor. Likewise, those who run the marathon at an easier pace than they are capable of can complete the distance without Glycogen being a factor.

5.2 Carbohydrate loading and the half marathon

The half marathon is a borderline race from a carbohydrate loading perspective. While many authorities put the boundary for needing carbohydrate loading at 90 minutes, this may not be quite accurate. Runners with relatively large leg muscles and low body fat are likely to have sufficient Glycogen without needing carbohydrate loading. However, runners with smaller leg muscles and higher body fat may be limited by their Glycogen supplies and benefit from carbohydrate loading.

5.3 Carbohydrate loading and shorter races

In races shorter than the half marathon, carbohydrate loading will increase body weight without providing any benefit.

5.4 Carbohydrate loading and the ultramarathon

Ultramarathon races are generally run at a significantly lower intensity than the marathon and racers refuel a lot more than shorter distances. So while carbohydrate loading may be of benefit, it's probably marginal. The exception would be the 50K distance, which for some runners is raced like a marathon.

6 How to carbohydrate load

There are various approaches to carbohydrate loading, and we'll discuss several of them.

6.1 Simple carbohydrate loading

The simple and common approach to carbohydrate loading is to eat a high carbohydrate diet just before the race. Traditionally the high carbohydrate intake is maintained for 3 days, but it has been shown[1] that 24 hours are sufficient.

  • How much? Take 10 grams of carbohydrate per Kg body weight in a day. This amount has been shown to increase Glycogen storage by 90%. This is a lot of carbohydrate; for a 70 Kg athlete that's 700g (see next bullet point.)
  • What form? Getting that much carbohydrate can be a challenge. High carbohydrate foods like rice, pasta or potatoes can work well but tend to be a lot of bulk and can cause digestive problems. 100g of rice contains 28g of carbohydrate, so you'd need 2,500g of rice to get 700g of carbohydrate. The best option is drinks containing Maltodextrin, as this has a high Glycemic Index and is easily digested. You can buy it in bulk cheaply, or use a drink like Hammer HEED. It is best to avoid Fructose as much as possible, so don't carb load with sugar or honey.

6.2 Preloading glycogen depletion

The original work on carbohydrate loading in the 1960s used a bout of prolonged exercise to deplete the Glycogen stores. This was then either followed by three days of low carbohydrate intake and three days of high carbohydrate intake, or just the final three days of high carbohydrate intake. This depletion of the Glycogen stores has been shown not to be necessary, but it does seem to improve the level of Glycogen storage. However, the 3 days of low carbohydrate intake after the Glycogen depletion exercise tend to be unpleasant both physically and mentally, with depression, irritability and reduced mental capacity. This process also tends to interfere with Tapering. This approach should be given careful consideration, but I would not generally recommend it.

6.3 Preloading fat adaptation

By following a low carbohydrate, high fat diet for a few days or weeks, it is possible to improve the body's ability to burn fat and spare Glycogen. This has obvious benefits akin to carbohydrate loading itself. The fat adaptation phase is less drastic than the Glycogen depletion approach, and with the diminished training load of Tapering can have minimal impact. Personally, I've found it easier to avoid gaining weight while tapering by following a low carbohydrate, high fat diet during the taper period. Following a single day carbohydrate loading approach as described above does not negate the fat adaptation.

6.4 High intensity exercise

A 2002 study used 150 seconds of high intensity exercise (130% V̇O2max) followed by an all-out 30 second sprint before carbohydrate loading. They found that doing that level of exercise followed by 10 grams of carbohydrate per Kg body weight in a day increased Glycogen storage by 82%. Sadly, this study did not have a control to see if the exercise actually helped. In fact, the results are slightly worse than taking 10g/Kg carbohydrate without the exercise! In addition, the exercise is likely to leave you sore for the race. This is not a recommended approach

7 Other factors

7.1 Carbohydrate loading for women

Few studies have looked at Carbohydrate loading in women, but some useful information is available. One key conclusion is the importance of taking in carbohydrate based on body weight rather than changing the percentage of carbohydrate. This typically gives a woman more carbohydrate calories than their usual total calorie intake. Maintaining this calorie intake for a day should not cause a significant gain in body fat, but longer periods may be problematic.

7.2 Menstrual Cycle and Oral Contraceptives

Women tend to have higher levels of Glycogen than men under normal (non-carbohydrate loading) conditions. Women are able to carbohydrate load more Glycogen in the luteal phase (ovulation to period) than the follicular phase (period to ovulation), but this difference tends to disappear if oral contraceptives are taken.

7.3 Gluconeogenesis

Gluconeogenesis is the creation of glucose from non-carbohydrate sources. While Gluconeogenesis can turn trace amounts of fat into glucose, this is insignificant. The primary mechanism of Gluconeogenesis is the creation of glucose from Protein. A high Protein diet may produce some Glycogen replenishment, carbohydrate should be used for pre-race loading.

7.4 Fat Loading

Most studies of 'fat loading' actually look at 'preloading fat adaptation', using a high fat intake for several days or weeks before competition. There are few studies looking at taking a high fat meal the day before a race. However, there is anecdotal evidence of improved performance from taking a high fat meal the night before a race.

8 Further Reading

9 References

  1. Carbohydrate loading in human muscle [Eur J Appl Physiol. 2002] - PubMed - NCBI

10 See also