The protein balance
Since humans are evolutionarily designed for scarcity , they initially try to metabolize everything. In addition to large amounts of water and some minerals, the human body must consume carbohydrates, fats, and proteins.
Carbohydrates and fats are, of course, essential for more complex structures, but are primarily used for energy —whether stored or burned. Proteins, on the other hand, are more of a building material for the body, although they are occasionally used for energy (as during fasting).
Protein metabolism – an overview
Proteins are also called proteins, Greek for “first-class” (proteno) – the name is of course no coincidence and suggests the importance given to proteins.
Protein makes up on average around 20% of the body, clearly number 1 (besides the 60% water!).
After a meal, dietary protein first enters the stomach, where it is "broken down" (denatured) with the help of stomach acid. The next stop is directly connected to the stomach—the duodenum. This is also where the secretions of the pancreas and gallbladder empty. The secretions of the pancreas contain numerous digestive enzymes that break down the proteins into individual amino acids.
The free amino acids are then absorbed by the intestinal cells and released into the bloodstream—this occurs largely in the small intestine, by the way. The total protein loss during digestion, including the body's own dead cells, amounts to no more than 10%... our body is most stingy with proteins.
Around 130 g of free amino acids from the intestine enter the bloodstream daily
After breakdown in the gastrointestinal tract and subsequent absorption into the bloodstream, the amino acids are quickly made available. Many amino acids are absorbed directly by the cells or initially form a so-called amino acid pool .
The amino acid pool consists of available amino acids. These are either absorbed externally, as described above, or partially replenished by the breakdown of the body's own proteins. The daily turnover, i.e., the synthesis and breakdown of protein, is approximately 300 grams.
Through protein metabolism and recycling , free amino acids are regularly available. Unlike carbohydrates and fats, the body therefore has no need to store amino acids —except, of course, in the form of muscle.
A diet that provides a person’s daily energy needs usually also contains enough protein to cover the actually rather small protein requirement
Proteolysis – the breakdown of protein
Each amino acid has a nitrogen-containing side chain, the so-called amino group.
Amino acids are the smallest components of proteins and are formed when they are broken down. However, amino acids can and must be broken down and metabolized by the body. Before an amino acid is broken down, the amino group must be removed. This initially produces ammonia for a very short time.
This ammonia is also a cell toxin and is therefore rapidly metabolized. In the liver, where most protein metabolism takes place, ammonia is converted into non-toxic urea.
This process costs energy: about a quarter of the energy provided by the protein is immediately used up again in protein metabolism, which is often used as an “effect” in high-protein weight loss diets.
Urea is excreted in the urine as an end product of protein metabolism. Our entire blood flows through the kidneys—our body's own wastewater treatment system—approximately every five minutes. In this process, waste products like urea are removed from the blood and can be excreted in the urine.
The kidneys and liver therefore largely control protein metabolism.
Protein balance during fasting
As described at the beginning, energy can also be generated from protein. This is the case when the usual energy sources, i.e., fats and carbohydrates, are no longer available in sufficient quantities – for example, during fasting.
During fasting, so-called “ketone bodies” are quickly produced from fat – this alternative “super fuel” provides the majority of energy after about 48-72 hours.
Until ketone body production is fully underway, a small amount of protein is initially converted into energy.
To minimize protein loss during fasting, you should definitely avoid a zero-calorie diet or zero-calorie fast. The resulting protein breakdown is unnecessary, and many studies have shown that protein loss can be significantly reduced with the right meals.
In established fasting clinics and in the Salufast fasting boxes, small amounts of food are always consumed.