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Carbohydrates are the most important nutrient in our diet in terms of quantity. They are one of the three macronutrients in our diet and provide the body with energy. They occur in many different forms, such as sugar, starch or fiber, and in many different foods, such as whole grains, fruits and vegetables.

However, not all carbohydrates are the same. Depending on how complex they are, their health value varies. Carbohydrates are our body's energy supplier and the most important fuel for muscles and brain. Around fifty percent of the calories we eat every day come from carbohydrates.

Functions and effect in the body

• Next to fat, carbohydrates play the most important role in meeting energy needs, although in direct comparison they provide only about half the calories. One gram of carbohydrates provides around 4 calories or 17kJ of energy and is thus far lower in calories than fat (9 calories per gram).
• Under the influence of the hormone insulin, they are preferentially burned for energy or converted into their storage form glycogen.
• The quantity and quality of carbohydrates in the diet influence satiety and metabolic parameters such as blood glucose, blood insulin and cholesterol concentrations.
• They are components of biomolecules such as nucleic acids and glycoproteins, which are responsible for structure in cell membranes, for example^.1

Types carbohydrates

Chemically, carbohydrates are a compound of carbon (C), hydrogen (H), oxygen (O) and correspond to the composition Cn(H2O)n. Basically, carbohydrates consist of sugar units (molecules) and can be classified depending on how many sugar units are combined in one molecule^.2

Carbohydrates are also known by the following names, which usually refer to specific carbohydrate groups^:3

• Sugar
• simple and complex carbohydrates
• probiotic resistant starch
• Dietary fiber
• glycemic and whole grain products
• Intrinsic and added sugars

In addition to the type of carbohydrates, the German Nutrition Society (DGE) recommends that no more than half to two-thirds of the daily diet should be covered by carbohydrates^.4 Thus, the recommendation has decreased significantly in recent years.

Groups of carbohydrates

A carbohydrate can consist of one to several thousand sugar molecules. A distinction is made between the following types of carbohydrate, depending on the number of sugars:

Simple sugar (monosaccharide): This sugar consists of only one molecule. The most important representatives are glucose and fructose. (Fig. 1 shows the simple sugar D-glucose).

They enter the blood quickly and cause the blood sugar level to rise rapidly. As a result, insulin secretion shoots up, causing the blood sugar level to drop again quickly. Blood sugar and energy levels are like a roller coaster. The rapid drop in blood glucose levels often leads to feelings of hunger or cravings.

It is also "empty nutrients" that your body gets. This means that your body takes in calories, but gets little valuable nutrients in proportion. You quickly get hungry again and may even eat more calories than necessary.

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Dual sugars (disaccharides): In the case of disaccharides, two simple sugars are always linked together. These include sucrose (white household sugar), maltose (malt sugar) and lactose (milk sugar).

Multiple sugars (polysaccharides): As the name implies, polysaccharides are made up of many, up to ten thousand, simple sugars that combine to form one large molecule. They are also called complex carbohydrates or long-chain carbohydrates. Starch from plants belongs to this group, as do some dietary fibers (e.g., cellulose)^.6

Due to their many complex molecules, the body takes longer to digest them. They also regulate your blood sugar level evenly, so that it does not rise quickly. This ensures a long feeling of satiety and prevents cravings. Complex or long-chain carbohydrates are found in all whole grains, oatmeal, rice, corn, millet, legumes, flax, hemp and chia seeds, potatoes, almonds, buckwheat and vegetables^.7

The great thing about complex carbohydrates is that they often contain other nutrients such as B vitamins, folic acid, magnesium, calcium, protein and fiber^.8

Good to know: Sugar - and thus carbohydrates - now have over 60 different names. On the packaging, however, we recognize them by their ending: All types of sugar end in "-ose." We can use this clue to identify hidden sugars in foods.

Processing of carbohydrates in the body

Carbohydrates must first be broken down again into simple sugars, i.e. glucose, in the digestive tract before they can enter the bloodstream. This breakdown and comminution takes place with the help of enzymes in our small intestine, so that the glucose can be absorbed into the bloodstream via the mucous membrane of the small intestine.

The carbohydrates pass through the blood to the liver, which sends them to the various cells and organs according to their respective needs. The pancreas assists in this process by producing the messenger substance insulin. This ensures that the cells can absorb the sugar. After all organs have been supplied, the body stores the remaining energy in the muscles as glycogen, a long-chain carbohydrate.

The brain and also blood cells can only absorb energy in the form of glucose. However, you do not need to ingest simple sugars for this: Multiple sugars are broken down into glucose during their processing and are thus available to the brain. Even for reading this article, your brain uses glucose! 

If the blood glucose level falls below a certain threshold value during a brief period of starvation, the liver controls the blood glucose level by breaking down glycogen. The liver produces glucose via the breakdown of body protein. This type of synthesis ensures that the necessary blood glucose level can be maintained to supply the brain (so-called gluconeogenesis)^.9

Unused glucose can be converted into glycogen, which is found in the liver and muscles and can be made available here as an energy store. For example, the ATP that serves as an energy source during muscle work is provided by the breakdown of glycose. If the body is supplied with more glucose than it can store in the form of glycogen, this is converted into fat for long-term energy storage. We refer to this as 'depot fat'. Your body stores energy in stock^.10

Sources: 

1. The nutrients - building blocks for your health, Deutsche Gesellschaft für Ernährung e.V., Edition 6, 2022
2. Biesalski, H. K., Grimm, P., Nowitzki-Grimm, S. (2020). Pocket atlas of nutrition. Georg Thieme Verlag.
3. Carbohydrate terminology and classification - PubMed (nih.gov).
4. Carbohydrates - DGE
5. Berg J.M., Tymoczko J.L., Stryer L. (2013) Carbohydrates. In: Stryer Biochemistry. Springer Spektrum, Berlin, Heidelberg.
6. Carbohydrates: What are they & what are they in? | health.com
7. Carbohydrate quality and health: distilling simple truths from complexity - PubMed (nih.gov).
8. [Carbohydrates and dietary fiber] - PubMed (nih.gov).
9. Biesalski, H. K., Grimm, P., Nowitzki-Grimm, S. (2020). Pocket atlas of nutrition. Georg Thieme Verlag.
10. Biesalski, H. K., Pirlich, M., Bischoff, S. C., & Weimann, A. (Eds.). (2017). Nutritional medicine: following the curriculum nutritional medicine of the German Medical Association. Georg Thieme Verlag.
11. Souci, SW; Fachmann, W; Kraut, H. (2009): The little Souci specialist Kraut. Food table for practice, 4th edition. Wiss. Verlagsges, Stuttgart.
12. Biesalski, H; Grimm, P; Nowitzki-Grimm, S (2017): Pocket atlas of nutrition, 7th edition. Thieme, Stuttgart, New York.
13. Cummings, JH; Stephen AM (2007): Carbohydrate terminology and classification. European Journal of Clinical Nutrition 61(Suppl 1):5S-18S.