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Carbohydrates and exercise: a Sweet connection

Carbohydrates are a vital source of energy during exercise, but not all carbohydrates are the same. In the context of exercise, they can be divided into two categories: those that are oxidised (or burned) rapidly and those that are oxidised more slowly.

Sucrose |Table sugar| Single Carbohydrate

Fast burning carbohydrates

up to 1 gram per minute [1]

Sugars such as glucose, sucrose, and oligosaccharides can be quickly absorbed in the gut and shuttled to muscles, providing swift energy during endurance exercise. How quick? Up to 1 gram per minute, so 60 grams per hour during exercise. In science speak, this is called the maximal exogenous carbohydrate oxidation rate (i.e. the maximum amount of carbohydrates that you can burn from food and drink).

Glucose, sucrose and oligosaccharides are found in foods like bananas and sports gels. Your everyday table sugar is made of sucrose.

Complex carbohydrates such as glucose polymers also fall into this group. They are less sweet than glucose or sucrose and allow for increased carbohydrate content without increasing osmolality (high osmolality in the gut can lead to gastrointestinal symptoms).

Maltodextrin is an example of a glucose polymer and can be found in many sports products such as drinks, energy bars and meal replacement shakes. It takes slightly longer to draw energy from maltodextrin because your body needs to break down the chains between glucose molecules first.

Apples and Pears| Fructose-Sugar| Roam, NZ

Slow burning carbohydrates

up to 0.6 grams per minute [1]

The intestines absorb fructose more slowly and it must be converted into glucose or lactate by the liver before it can be used as energy by muscles. This means that it is slower to provide your muscles with energy and therefore less effective in improving performance during prolonged exercise [2].

High fructose corn syrup is an example of this type of carbohydrate. Fructose can also be found in fruits such as apples and pears. Honey is an example of a food that contains both fructose and glucose.

This group of carbohydrates consumed from food or drinks can be burned at a maximum rate of 0.6 grams per minute during physical exercise.

However, don’t write off the importance of fructose. The gut uses a different transport mechanism to deliver carbohydrates from fructose to muscles than glucose or other fast burning carbs. What does this mean? You can burn more than 1.0 grams of carbohydrate per minute.

Our article on multiple transportable carbohydrates covers this in more detail.

Sports Nutrition| Osmolality concept| Roam, Australia

The importance of osmolality

Osmolality refers to the concentration of particles in a solution. In the context of sports nutrition, it’s about the concentration of sugars and other solutes (e.g. salts) in a drink or food item. If you imagine a glass of water with sugar dissolved in it, the more sugar you add means the higher the osmolality.

High osmolality drinks can also slow down the emptying of the stomach and energy absorption in the intestines. That means a delayed energy to your muscles. It can also cause gastrointestinal symptoms.

And if the osmolality of a drink is too high, it can actually pull water into your intestines from the bloodstream, reducing the bloods plasma volumes [3]. This can contribute to dehydration. Conversely, a drink with lower osmolality (at a level similar to the body’s fluids) can promote faster hydration.

From a drink standpoint, it is best to avoid fluids that are too high in fructose, as these can contribute to higher osmolality and are slower to be absorbed by the guts. If you are mixing a powder with water, make sure not to make your solution too concentrated.

Maximal Exogenous Carb Oxidation Rates Comparison| Roam NZ AU

The energy limit

We can burn up to 1,000 kcal per hour during intense exercise. Some quick math reveals that it’s very difficult to come close to consuming 1,000 kcal per hour.

If fast burning carbs can supply 60 grams of carbs per hour, that means we could only take on about 240kcal an hour. By combining fast and slow release carbohydrates, athletes can achieve levels of about 90 grams per hour (360kcal).  Better, but not amazing. The bottleneck is in our intestines – we simply can’t absorb carbohydrates quick enough.

That’s okay though. We can continue to improve our performance by training our guts to tolerate higher volumes of food/drink during exercise and increase absorption (although we’ll never hit 1000 kcal per hour). Plus our bodies are sophisticated machines. We are not just burning carbohydrates during exercise. Our bodies have a vast storage of fat to draw upon, and as our carbohydrate stores get depleted, your body will rely more on fat as fuel.

References

1 - Burke, L., Deakin, V. and Minehan, M. Clinical Sports
Nutrition. 2021;6 343-344

2 – Wagenmakers AJM, Brouns F, Saris WHM, et al. Oxidation
rates of orally ingested carbohydrates during prolonged exercise in man. J Appl Physiol 1993;75:2774-80

2 - Jeukendrup A. A step towards personalized sports nutrition: carbohydrate intake during exercise. Sports Med 2014;44 (1Suppl):25S 33S

3 – Evans GH, Shirreffs SM, Maughan RJ. Acute effects of ingesting glucose solutions on blood and plasma volume. Br J Nutr 2009;101:1503-8