Tasks of carbohydrates
Carbohydrates are the most important and primary source of energy for the body. 1g of carbohydrates provides 4 kcal of energy. Carbohydrates are the most important source of energy for most types of cells in the human body. They are particularly important for the brain and the red blood cells (hemoglobin), since they rely solely on glucose as an energy source*. This is also the reason why when switching to a ketogenic (low carbohydrate) diet, the first days of habituation leads to a significant loss in the ability to concentrate.
To make the human body more independent of a constant supply of carbohydrates, it has the ability to store carbohydrates in the form of glycogen in the muscles and the liver. An untrained person has a capacity of about 400g of carbohydrates, of which ¾ are stored in the musculature and about ¼ in the liver. Through regular training, however, it is possible to further expand this storage capacity, which is why an ambitious athlete can store 600g or more.
If the intake of carbohydrates is above the value that is currently required and, in addition, the depots in the liver and muscles are filled, the surplus is converted into fat and then stored in the fat deposits. However, approximately 30% of the energy is lost through this transformation process**.
On the other hand, if too little energy is supplied in the form of carbohydrates (but also fats), the body starts to take its required energy from its proteins. Therefore it can be said that carbohydrates have a protein-saving effect.
By the way, fiber is also made of carbohydrates. Until a few years ago it was assumed that they cannot be utilized by the body, so it is known today that some of the dietary fiber is fermented by enzymes and bacteria in the colon. In addition to the build-up of gases, short-chain fatty acids, which can be utilized by humans, are also produced. However, as the daily intake of fiber is quite low (according to studies often barely reached 30g), the partial fat production from them is negligible.
*red blood cells can also rely on lactate as an energy source
**energy is never lost (thermodynamics), but it is converted into heat, which is not a usable energy source for cells