Carbo Loading: what is it, and why is it important?

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

What does carbo-loading involve?

Carbo-loading involves increasing your daily consumption of carbohydrates in the build up to important races.

The most common approach is to increase your carbohydrate consumption over the final few days before an important competition. This is normally combined with a reduction in training volume.

Why is carbo-loading important?

Firstly, carbohydrate metabolism plays a key role in fuelling endurance exercise performance. Secondly, your body only stores enough carbohydrates (muscle and liver glycogen) to sustain approximately 90minutes of exercise. And once your glycogen stores become depleted endurance exercise performance suffers.

By carbo-loading, you are ensuring glycogen levels are maximised. This helps to delay the point where glycogen stores become limiting during longer endurance events.

When should you use Carbo-loading?

Carbo-loading is a useful approach in any events lasting longer than 90minutes.

Not surprisingly, carbo-loading is widely used in the build up to longer endurance events 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 if your event lasts less than 90 minutes?

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

Instead, focus on reducing training volume (by 25-50%) over the final 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

Some carbo-loading recommendations for events lasting longer than 90minutes.

Carbo-loading: 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 final week prior to competition.
  • Consume a high carbohydrate meal 3-4hours prior to competition.

Carbo-loading: 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:

Carbo-loading: High vs Low Glycemic Index (GI) Carbs

When carbo-loading there’s a range of different food options we can choose from. One important consideration is the glycemic index of different carbohydrates. Essentially, the glycemic index (GI) gives a measure of how quickly different sugars enter your blood stream.

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

  • High GI foods are fast releasing – they enter the blood stream quickly,
  • Medium GI foods are released more moderately – they enter the blood stream at a more steady rate.
  • Low GI foods are the slowest releasing foods – entering the blood stream more slowly than high or medium-GI foods.

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

So what’s the best carbs to use when carbo-loading? Whilst high, medium and low GI foods will all increase glycogen stores; from a health perspective, consuming medium and in particular low GI foods, is the best approach during the carbo-loading phase.

Why are low-glycemic foods better?

Low-glycemic foods are more healthy than high-glycemic foods for a number of reasons, but primarily they do not cause the same spikes in both 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, chick peas, lentils, kidney beans, milk, apples, pears, cherries, orange, strawberries

Carbo-loading: the last meal before competition

Why do you need to consume an extra high carb meal just prior to competition? Essentially, this helps to replace any muscle or liver glycogen that was depleted overnight. It also keeps glycogen levels topped up right until the start of your event.

How many carbs should you consume? Research, has shown that consuming a high carbohydrate meal (3-4g carbohydrate per kg of body mass), ~3-4 hours before exercise, ensures glycogen levels are full. It has also been shown to enhance 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;).

That being said, that’s quite a significant amount of carbs. For some athletes, that might be too much, especially if you’re a runner as this could lead to intestinal or digestive problems. My preference, has generally been to consume around 2g of carbs per kg.

Should you consume low of high-GI foods prior to competitions?

When it comes to consuming foods just prior to endurance competition; research is not completely clear as to whether high or low-GI foods are more beneficial (Jamurtas et al., 2011).

Some research has 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 a number of 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).

Carbo-loading: the benefits of low-GI foods before competition

Some studies have found that compared with high-GI carbs; 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 improvements 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 completely clear whether low or high-GI foods are better prior to exercise; research appears to be far more supportive with regards to low-glycemic carbs (Moore et al., 2009; Wee et al., 2005; Siu and Wong, 2004; DeMarco et al., 1999; Thomas et al., 1991).

Why consuming too much high-GI foods may be negative before exercise:

Consuming a large amount of high-GI carbs prior to exercise may negatively effect performance. Why is that? This is mainly due to the way High GI foods cause a rapid spike in blood sugar and insulin levels. This spike in insulin levels has two negative effects that may compromise endurance exercise performance:

Firstly, if insulin levels rise too high, it overcompensates for the actual amount of sugar in the blood. This then leads to a state of low blood sugar (hypoglycaemia).

Secondly, high insulin levels are known to have an inhibitory effect on the mobilization and use of fat as an energy source.

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

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

When considering all of this, it clearly makes sense to favour low-glycemic carbs, both during the initial carbo-loading phase and for the final pre-race meal.

Keeping Hydrated

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

Carbohydrate Loading Summary:

  • 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.
  • Carbo-loading involves increasing carbohydrate consumption – typically for a period of 3-4 days just prior to competition.
  • For shorter duration races ~30-90 minutes, or less, carbo loading may not be necessary. Instead, reducing your training volume should be sufficient to maximise glycogen levels.
  • Carbo-loading is often combined with a 25-50% reduction in training volume. This helps to enhance recovery and aids muscle glycogen storage.
  • A further high carbohydrate meal is often consumed 3-4 hours before competition. This can improve endurance exercise performance and time to exhaustion.
  • Low-GI foods appear to be more beneficial for carbo-loading and for the final pre-race meal. And appear to aid fat metabolism and spare 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.

Carbo-loading references:

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