What is Carbo Loading? — The science and recommendations

Carbo-loading is a strategy adopted by endurance athletes, to enhance endurance exercise performance by increasing liver and muscle glycogen storage. It’s regularly used in the build-up to important endurance races, principally when race duration is longer than 90minutes.

What does carbo-loading involve?

It involves increasing your daily consumption of carbohydrates in the build-up to long distance races like marathons.

The most common approach is to increase your carbohydrate consumption over the final few days before a big competition. We combine this with a reduction in training volume.

Why is it important?

First, carbohydrate metabolism plays a key role in fuelling endurance exercise performance.

Second, your body only stores enough carbohydrates (muscle and liver glycogen) to sustain approximately 90minutes of exercise.

Third, once your glycogen stores become depleted, endurance exercise performance suffers. 

By carbo-loading, we maximise glycogen levels. This helps to delay the point where they become limiting during longer endurance events.

When should you use it?

It’s a useful approach in the build-up to longer endurance events (those lasting more than 90-minutes) including:

  1. Marathons
  2. Ultra running events
  3. Triathlons (Olympic distance or longer)
  4. Duathlons (standard distance or longer)
  5. Cycling events
  6. Cross-country Skiing
  7. Endurance swimming

What about events lasting less than 90 minutes?

If your event lasts less than 90minutes, there’s really not a need to follow a specific carbo-loading strategy.

Instead, focus on reducing training volume (by 25-50%) over the last week before competition. If you do this whilst maintaining your normal intake of carbs, this will naturally lead to increased muscle and live glycogen storage.

Carbo-loading recommendations

Below are some recommendations for events lasting longer than 90minutes.

Endurance events lasting longer than 90minutes

  • Aim to increase carbohydrate intake, by ~100-200g per day, over the final 3-4 days before competition.
  • For most athletes, consuming 8-10g of carbohydrates per kilo of bodyweight is an effective approach to carbo-loading.
  • Ideally, combine this with a 25-50% reduction in training volume over the last week prior to competition.
  • Consume a high carbohydrate meal 3-4hours prior to competition.

Endurance events lasting less than 90 minutes:

  • For races of less than 90 minutes duration, glycogen stores should be more than adequate.
  • Continue with your normal dietary intake of carbohydrate (5-7g of carbs per kilo of bodyweight)
  • Combine this with a 25-50% reduction in training volume over the last week before the race.
  • Consume a carbohydrate based meal ~3-4hours prior to competition.

It’s also worth considering the type of carbohydrates that you are consuming:

High vs Low Glycemic Index (GI) Carbs

When carbo-loading, there’s various food options to choose from. One important consideration is the glycemic index (GI). This provides a measure of how rapidly different sugars enter your bloodstream.

The glycemic index allows us to categories foods into low, medium and high-GI foods.

  • High GI foods are fast releasing — they enter your bloodstream quickly,
  • Medium GI foods are released more moderately — they enter the bloodstream at a steady rate.
  • Low GI foods are slow releasing foods — entering the bloodstream more slowly than high or medium-GI foods.

Increasing your consumption of either high, medium, or low GI foods will increase glycogen storage.

So what are the best carbs to use for carbo-loading?… Whilst high, medium and low GI foods all increase glycogen stores; from a health perspective, consuming medium and low GI foods, is the best approach.

Why are low-glycemic foods better?

Low-glycemic foods are more healthy because they do not cause the same spikes in blood glucose and insulin levels.

Benefits of low-GI carbs:

  • Low-glycemic foods do not cause such a large rise in blood glucose levels.
  • They cause less of a spike in insulin levels.
  • Low-glycemic foods are less likely to result in the subsequent drop in blood glucose levels, that often accompanies consuming high-glycemic foods.

Examples of high, medium and low-glycemic foods:

High-glycemic foods (GI score of 70+): white bread, potatoes (boiled, mash), parsnips, white Rice, cornflakes, instant oat porridge, pretzels, watermelon

Medium-glycemic foods (GI score of 56-69): brown rice, basmati rice, baked potatoes, bananas, crisps, couscous, muesli, sweet potato, pineapple, porridge oats (rolled),

Low-glycemic foods (GI score of 1-55): spaghetti (white and wholemeal), long-grain rice, carrots, chickpeas, lentils, kidney beans, milk, apples, pears, cherries, orange, strawberries.

Eating before competition

Why do you need to consume an additional high carb meal just prior to competition?… Essentially, this tops up muscle and liver glycogen stores that start to deplete overnight. It also maintains glycogen levels right up until the start of competition.

How many carbs should you consume?… Research has established that consuming a high carbohydrate meal (3-4g carbohydrate per kg of body mass), ~3-4 hours before exercise, ensures glycogen levels remain full. This enhances exercise performance and time to exhaustion (Wee et al., 2005; Schabort et al., 1999; Chryssanthopoulos and Williams 1997; Sherman et al., 1989; Neufer et al., 1987;).

However, that’s a substantial amount of carbs. And for some athletes, that might be too much (especially if you’re a runner) and may lead to intestinal or digestive problems. My preference is to consume around 2g of carbs per kg bodyweight.

Should you consume low of high-GI foods before competitions?…

Research is not completely clear whether high or low-GI foods are more beneficial (Jamurtas et al., 2011).

Some have suggested that ingesting high-GI foods (3-hours prior to endurance exercise) may be more beneficial for increasing muscle glycogen levels than low-GI food (Wee et al., 2005). However, whilst high-GI foods can improve carbohydrate utilisation, they don’t appear to provide any real benefit over low-GI foods (Febbraio et al., 2000; Jamurtas et al., 2011). 

A point to consider here: although high-GI food can enhance carbohydrate utilisation, they can actually negatively affect fat metabolism (Little et al., 2009). 

In contrast, low-GI foods appear to have several benefits over high-GI foods:

  • Increased free fatty acid availability, 
  • Helps to maintain fat metabolism, 
  • Spares muscle glycogen,
  • May reduce muscle lactate levels (Wee et al., 2005).

The benefits of low-GI foods before competition

Some studies have found that low-GI carbs provide greater endurance performance benefits (Moore et al., 2009; DeMarco et al., 1999; Thomas et al., 1991).

When Researchers looked at pre-exercise low and high-GI consumption (30-60minutes prior to exercise); they found that low-GI carbs had the following benefits:

  • Greater increases in time to exhaustion (Thomas et al., 1991),
  • Increased time to exhaustion following a 2-hr submaximal cycle (DeMarco et al., 1999)
  • Improved 40km time trial performance (Moore et al., 2009).

Although, it is not altogether clear whether low or high-GI foods are better prior to exercise; research is more supportive regarding low-glycemic carbs (Moore et al., 2009; Wee et al., 2005; Siu and Wong, 2004; DeMarco et al., 1999; Thomas et al., 1991).

Why high-GI foods may be negative before exercise:

This is mainly because High GI foods cause a rapid spike in blood sugar and insulin levels. This has two negative effects that may compromise endurance exercise performance:

First, when insulin levels rise too rapidly, they overcompensate for the actual amount of sugar in the blood. This can lead to a state of low blood sugar (hypoglycaemia).

Second, high insulin levels have an inhibitory effect on the mobilisation and use of fat as an energy source.

The combination of inhibited fat metabolism, and reduced blood sugar levels, can increase depletion of glycogen stores. Why is that?… This is due to increased reliance on glycogen stores; because of a need to compensate for the lower blood sugar levels and reduced levels of fat metabolism.

If you start your race in this state, then carbohydrate metabolism will have to increase, leading to increased glycogen depletion.

With this in mind, it makes sense to favour low-glycemic carbs, both during the initial carbo-loading phase and for the final pre-race meal.

Keeping hydrated is essential

While carbo-loading, it’s important to ensure that you maintain an adequate intake of fluids. This is important because the storage of glycogen also requires additional fluid storage. In fact, your body stores ~3g of water for every gram of glycogen.


  • Carbo-loading is a strategy used to increasing muscle and liver glycogen levels.
  • It’s most effective when used for events lasting more than 90minutes.
  • It involves increased carbohydrate consumption – typically for 3-4 days just prior to competition.
  • For shorter duration races (less than 90 minutes), carbo loading may not be necessary. Instead, reducing your training volume will help to maximise glycogen levels.
  • Carbo-loading can be combined with a 25-50% reduction in training volume. This helps to enhance recovery and aids muscle glycogen storage.
  • Consuming an additional high carbohydrate meal (3-4 hours before competition), can enhance endurance exercise performance and time to exhaustion.
  • Low-GI foods are considered more beneficial for carbo-loading. And appear to aid fat metabolism and for maintaining muscle glycogen levels.
  • Consuming a meal that is too High GI, may negatively affect endurance performance by increasing insulin levels. This can lead to a spike in insulin, lower blood glucose levels, and may inhibit fat metabolism.
  • Hydration levels need to be sufficient throughout the carbo-loading phase.
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