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What is a Carbohydrate?

Carbohydrates are smaller or larger molecules that play a major role in our energy provision. Read our deep dive into how carbohydrates serve as fuel for the body.

Carbohydrates are found in foods and they serve as a fuel in the body. The name carbohydrate is derived from the fact that these are molecules that contain carbon (carbo) and hydrogen (hydrate). In reality these compounds can be found in most foods, both plant based and animal based and they can be found in our bodies. In our body they are an important energy source for many processes and the main energy source for intense physical exercise, as well as a fuel for the brain. Often carbohydrates are divided into simple and complex, but as we will see this terminology can sometimes be confusing.


how do carbohydrates fuel us

What is the function of carbohydrate?

Carbohydrates have various function in the body but the main function is energy provision. In most conditions and all cells in the body, carbohydrate (and specifically glucose) is the preferred fuel. This is especially true for the brain which relies heavily on the supply of glucose. If the brain is deprived of glucose we will first develop symptoms hypoglycemia (the brain is not getting enough fuel and dizziness, nausea, weakness and feeling cold are common symptoms). If glucose concentrations in the blood would drop too much this can be life-threatening and therefore the body has many mechanisms to make sure that blood glucose is maintained within a relatively narrow range. Glucose is stored in the liver (in the form of glycogen) and the liver releases glucose when needed.

Glucose and muscle function

Glucose is also used as a fuel in the muscle and it is the most important fuel especially during activities that are moderate to high intensity. The muscle has its own stores of glucose (muscle glycogen). Any activity where it is not easy to talk will rely heavily on carbohydrates. Carbohydrate in our diet is also important because it refills the depleted glycogen store and thus is a key component of recovery.

What are simple carbohydrates?

The smallest carbohydrates are called monosaccharides. Glucose is the most abundant one of these in nature. Fructose (also known as fruit sugar) is another one. Carbohydrates can occur as a monosaccharide, but can also be combinations of two or more monosaccharides. For example, two glucose molecules together will form maltose (malt sugar), glucose plus fructose is sucrose (also known as table sugar) and glucose and galactose forms lactose (milk sugar).

Maltodextrin, starches and glycogen

When multiple glucose molecules are combined (usually 10-20), this forms a maltodextrin (see infographic). Many sports nutrition products use maltodextrins as a carbohydrate source. If we combine even more glucose molecules together we get starch. Most starches are very large molecules of that consist of hundreds of glucose molecules linked together. Often these structures are branched like a tree, which will allow enzymes to break these carbohydrates down more rapidly (during digestion). One of the largest and most branched types of carbohydrate is glycogen. Glycogen is the storage form of carbohydrate in our bodies (in muscle and liver).

What are complex carbohydrates?

The small carbohydrates (mono and disaccharides) are often referred to as simple carbohydrates or sugars. The larger carbohydrate structures are the complex carbohydrates. There is a lot of confusion around these terms though because in the context of foods these definitions are used slightly differently. Complex carbohydrates are often plant based foods that are also high in fibre and high in micronutrients. Simple carbohydrates are often used as added sugars and contain few other nutrients. Therefore, the general advice is to ingest more complex carbohydrates and fewer simple carbohydrates. However, although in general complex carbohydrates may be more nutrient rich and simple sugars nutrient poor, this is not always the case. If we take fruit as an example. Fruits contain simple sugars but also a lot of other nutrients. In contrast, some starch rich foods contain few micronutrients. There is another oversimplification that is often made that causes confusion. Simple carbohydrates are often referred to as fast and complex carbohydrates as slow. This is simply incorrect. Some simple sugars are slow (fructose is an example). Some complex carbohydrates are just as fast as glucose (maltodextrins, some starches).

What food sources contain carbohydrate?

Sugars like sucrose are found in many baking products, chocolate, candy/sweets etc. Starchy foods are our main source of carbohydrate. Starchy foods, such as potatoes, bread, rice, pasta, and cereals have an important role in a healthy diet. They should make up just over a third of the food you eat and in athletes usually more. Sports nutrition products like gels, sports drinks, energy bars, chews often contain glucose, maltodextrin or sucrose. These products are usually designed to deliver energy fast. So it is not possible to call these foods good or bad as is often done, because on the purpose of use and the context they are used in. Fast carbs can mean the difference between finishing a race or not finishing, or certainly between finishing string and a struggle. But excess intake of sugars is discouraged for people who are not active. 

Carbohydrate foodsHow are carbohydrates digested?

Digestion starts as we chew our food. By chewing we cut up and grind the food into smaller pieces and at the same time the food mixes with saliva that contains enzymes (amylase) that start the breakdown of larger carbohydrates. When the food is swallowed it first enters the stomach and the stomach coordinates the release of the food into the small intestine. This is where the next steps of digestion take place. All carbohydrates have to be broken down to sugars before they can be absorbed.

How are carbohydrates absorbed?

Our intestine contains various transporters to take up the carbohydrate from the intestine across the cell wall of the intestine into the blood. These transporters play a crucial role, in effective delivery of fuel during exercise, as we will discuss in this article on what are multiple transportable carbohydrates?


Jeukendrup AE, Gleeson M. Sport Nutrition. Human Kinetics Champaign IL 2018.

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