Everything about protein. Function and effects of protein

What Are Proteins?

Proteins, also called eiweiss in technical terms, are organic compounds that, like carbohydrates and fats, contain the elements carbon (C), hydrogen (H) and oxygen (O), but additionally also nitrogen (N). Some proteins also contain sulphur (S). Proteins determine the function and structure of the human body to a decisive extent. They are therefore not only an indispensable building material of human cells, but are also involved in numerous metabolic processes in a wide variety of ways.

The building blocks of proteins are amino acids. Depending on the number of amino acids that make up a protein, a distinction is made between oligopeptides with fewer than ten amino acids, polypeptides, which are composed of 10-100 amino acids, and proteins with more than 100 amino acids. The sequence of amino acids needed to produce proteins is stored in DNA. Theoretically, an infinite number of proteins can therefore be formed, since amino acids can be combined and strung together in any way. The human body, by contrast, produces "only" 30'000 proteins, which perform a multitude of functions in the body.

In the human organism, 20 different amino acids are needed for protein synthesis. Nine of them are essential. Essential amino acids cannot therefore be produced by the body itself and must be supplied in sufficient quantities through food. These nine also include the three so-called branched-chain amino acids valine, leucine and isoleucine. These branched-chain amino acids are also called BCAAs. Four further amino acids are conditionally essential, i.e. under certain conditions (e.g. in infancy, during high physical stress) they cannot be produced by the body itself in sufficient quantities and therefore become essential in such situations.

Essential amino acids

• Valine
• Leucine
• Isoleucine
• Histidine
• Lysine
• Methionine
• Phenylalanine
• Threonine
• Tryptophan

Conditionally essential amino acids

• Tyrosine
• Cysteine
• Glutamine
• Arginine

Sources of protein

Meat, fish, milk and dairy products as well as eggs are protein sources of animal origin, while cereal and soy products, legumes and nuts are plant sources. Plant protein sources are lower in terms of biological value (see below) than animal proteins.

Digestion, absorption and breakdown of proteins

Proteins are digested in the acidic environment of the stomach by enzymes formed by the stomach wall, which serve to split the protein chains into shorter chains. The stomach enzymes are rapidly inactivated in the small intestine, and the already shortened protein chains are broken down into individual amino acids by the enzymes of the pancreas. At the end of digestion, the amino acids are absorbed into the intestinal cells through the intestinal wall with the help of transporters, from where they enter the blood and finally the liver. There, the amino acids can be assembled into proteins and released back into the blood, enabling their use by other organs such as the muscles.

The body's own proteins are constantly built up and broken down. Ammonia is produced when amino acids are broken down in the cells. It is toxic to the body and must therefore be removed. The ammonia produced is therefore converted into urea with great energy expenditure and excreted via the kidneys with the urine. Urea is therefore relatively non-toxic in contrast to ammonia and very readily soluble in water, which is why the organism allows itself such a wasteful urea cycle.

When protein intake increases, more urea is formed, which, because of its good water solubility, binds a large amount of water. You therefore lose more fluid along with a larger amount of urea. If you eat a protein-rich diet and do not drink enough fluids, the kidneys are unnecessarily strained.

Functions of proteins in the body

Proteins occur in the human organism as a component of:

• Hormones (peptide or proteohormones)
• Enzymes
• Membrane proteins of the cell wall (e.g. receptors or transport proteins)
• Structural and framework proteins (e.g. collagen, keratin or elastin)
• Contractile proteins (e.g. actin and myosin filaments as contractile elements of the muscle)
• Plasma proteins (e.g. albumin)
• Transport proteins (e.g. haemoglobin, myoglobin and certain plasma proteins)
• Blood clotting factors
• Antibodies
• In energy supply, only a reserve function

In energy supply, protein only has significance in exceptional cases (e.g. with very low energy intake or during several hours of endurance exercise).

Proteins have the same energy value as carbohydrates (17 kJ/g (= 4 kcal/g)) and are clearly below that of fats (39 kJ/g (= 9 kcal/g)).

Daily protein requirement

The protein requirement of athletes, both in strength and endurance sports, is higher, at around 1.2-1.8 g/kg body weight daily. Once the requirement is met, an even higher protein intake brings no advantages. However, with an optimal intake timing, the protein synthesis rate can be maximised, which therefore has a positive effect on adaptation to training stimuli (e.g. building muscle mass).

Quality of proteins

If we believe the recommendations of most manufacturers of protein supplements, almost everything falls into the category of "high-quality protein". Unfortunately, this is objectively not the case.

Several "scores" exist for assessing the quality of different protein sources, which are used in practice. Besides the outdated (and often undefined) "biological value", this applies above all to the "Protein Digestibility Corrected Amino Acid Score" (PDCAAS), which, however, according to the recommendations of the "Food and Agriculture Organization of the United Nations" (FAO) and the "World Health Organization" (WHO), should consistently be replaced by the "Digestible indispensable amino acid score" (DIAAS).

This is defined as: DIAAS (%) = 100 x [(amount in mg of a digestible indispensable amino acid in 1 g of the food protein) / (amount in mg of the same digestible indispensable amino acid in 1 g of the reference protein)], whereby digestibility refers to digestibility in the small intestine (most precise assessment of digestibility). The DIAAS thus assesses a protein's ability to meet the needs of human metabolism.

Simply put, the following can be noted regarding the DIAAS:

• The quality of a protein source ultimately corresponds to "the supply" of indispensable amino acids to the body.
• The quality of the protein source is made up of its biological availability, its amino acid profile (distribution) and the amount of indispensable amino acids.
• DIAAS can exceed 100%. Milk protein (whole fraction) has a DIAAS of 122%.
• Foods of lower quality can achieve higher DIAAS through enrichment with high-quality protein sources or possibly free amino acids than in their unenriched state, and can thus be optimised.

What does this mean for supplements?

Well, one thing is certain: many protein supplements advertised as "high-quality" (e.g. pure plant proteins without added essential amino acids; collagen hydrolysates, etc.) are not optimal with regard to human metabolism. In addition, we can note that protein supplements based on milk proteins have high DIAAS and are therefore high-quality. However, if we go one step further with the scientific analyses and consider the direct effects of protein sources on protein synthesis and protein breakdown, we can further specify the benefit of supplements.

Protein deficiency

If protein supply is insufficient, physical and mental performance first declines. Furthermore, fertility and the immune system are impaired, which results in increased susceptibility to infectious diseases. An acceleration of ageing processes in the body can also occur in the context of a protein deficiency. In cases of severe protein deficiency, pronounced oedema, i.e. fluid retention in the tissue, can occur.

A very high protein intake (>2.0 g / kg body weight daily) leads, via increased urea formation, to a growing demand on the kidneys and liver. However, with healthy organs, this is not a problem.

Protein balance

All tissues of our body consist largely of protein. This (and therefore also our tissues, such as the muscles, skin, hair, connective tissue, etc.) is subject to permanent building and breakdown processes, so that our body must be supplied with fresh building materials incessantly. The building materials needed for tissue formation are called amino acids, which our body obtains from dietary protein during digestion. By consuming dietary protein, we therefore essentially supply our body with the building blocks needed for tissue formation.

The ratio between the building up and breaking down of body proteins is called protein balance. Changes in protein building and breakdown are triggered by both training and nutrition. These changes mean that, depending on the training and/or nutritional measures, the protein balance is increased or reduced within a very short time, and you ultimately build up net protein mass (positive protein balance) or break it down (negative protein balance). For example, strength training without food intake does lead to an increase in protein synthesis (protein building), but due to the simultaneously increased protein breakdown, it results in a negative protein balance (i.e. a net breakdown of body protein).

How does dietary protein affect the protein balance?

The above example shows that, even if a specific training stimulus increases protein synthesis, in the absence of dietary protein this does not result in an acutely positive protein balance and thus does not lead to the building of muscle mass.

If you supplement strength training with the intake of high-quality dietary protein in the necessary amount, this quickly leads to a positive protein balance. The reason is that the additionally supplied dietary protein further boosts protein synthesis, which ultimately outweighs protein breakdown. As a result, the body accumulates, on balance, the smallest amounts of protein. The sum of these extremely small "protein build-up surpluses" leads to measurably more muscle mass in the long term.

What is responsible for increasing protein synthesis?

Certain (essential) amino acids are responsible for the increase in protein synthesis through dietary protein. Since different protein sources contain these amino acids in different amounts and also in different compositions, different protein sources influence muscle protein synthesis to different degrees. More on this in a further article.

Central to increasing protein balance is, besides the protein source, also the amount of dietary protein consumed. Since muscle protein synthesis cannot be increased indefinitely and, at the same time, excessively high total protein amounts boost protein breakdown, there are biological upper limits to effective protein intake. It therefore makes no sense to consume amounts of protein that are too small or excessively large in order to increase muscle protein synthesis.

Protein intake during the day

Supplementary protein intake during the day (e.g. when protein requirements are increased due to physical activity or with "unbalanced" dietary patterns) therefore aims to maintain muscle mass, in that the increased protein synthesis offsets the "natural" protein breakdown. Central here is that just under 10 g of essential amino acids are sufficient to maximise muscle protein synthesis, while protein breakdown rises at protein portions > 20-25 g/portion.

You should therefore make sure to consume protein sources that are "highly concentrated" and contain as high a proportion of essential amino acids as possible. Thus, it makes little sense to consume protein supplements which, per serving, maximally increase protein synthesis but, based on the high total protein amount (for example 40 g per serving), also unnecessarily boost protein breakdown.

Protein sources vs. protein synthesis rate

In an illuminating study, Tang et al. (2009) examined the effects of whey protein, soy protein and casein with regard to digestion speed (shown as blood concentrations of, e.g., essential amino acids over time after protein intake), plasma insulin concentration and the mixed muscle protein synthesis rate. For this purpose they recruited 18 young, healthy men accustomed to muscle training who, rested, performed the two exercises knee extension and leg press to muscle failure ("intensive") with one leg on three different days with sufficient rest in between (the resting leg served as internal control). All study participants consumed, in random order and immediately after muscle training, either whey protein, soy protein or casein, with all protein portions containing around 10 g of essential amino acids (EAA).

Three hours after protein intake, the researchers took a muscle tissue sample from each of both thighs with the help of biopsy needles and determined the mixed muscle protein synthesis rate. In addition, they took blood samples from all study participants 30, 60, 90, 120 and 180 min after protein intake and examined the blood for the concentration of essential amino acids, insulin, L-phenylalanine and L-leucine.

Amino acid profiles of the protein drinks administered (protein dissolved in water):

The analyses showed clear differences between the various protein sources.

Blood concentrations ("digestion speed")

While whey protein led to an almost doubling of the EAA concentration just 30 min after consumption, and soy protein increased the EAA concentration in the blood to just under 1.5 times in the same period, the EAA blood concentration with casein rose by only about 50%. After 3 hours, the EAA concentration approached the baseline value again for all protein sources, while casein led to a less abrupt decline than whey protein and soy protein. With regard to the L-leucine concentration, whey protein led to an almost 3 times higher concentration increase (measured as the area under the curve) than casein and to an approximately twice as high increase as soy protein. With regard to blood insulin concentration, it was found that whey protein increased insulin concentration the most, closely followed by soy protein. In contrast, the blood insulin concentration did not change after casein.

Muscle protein synthesis rate

Intake of whey protein and soy led to a greater increase in the muscle protein synthesis rate than casein, both at rest (untrained leg) and after training. In addition, the increase after training was greater with whey protein than with soy.

In summary, the following points can be noted about proteins

• Whey protein, when consumed after training, leads to a greater increase in amino acid and insulin concentration in the blood than soy protein, and soy protein in turn than casein (whey > soy > casein).
• Whey protein increases the muscle protein synthesis rate more than soy protein, and this in turn more than casein.
• The "digestion speed" decisively determines the increase in the muscle protein synthesis rate (the faster and higher the rise, the greater the increase in the muscle protein synthesis rate).
• When the same amount of EAA is administered (around 10 g), the protein source with the higher L-leucine content increases muscle protein synthesis the most.

What does this mean in practice?

After training, drink about 20 g of whey protein.

After your muscle training, do not consume casein. So avoid milk-based (possibly heavily sweetened) UHT ready-made shakes. Mix your whey protein powder with water or drink a water-based whey drink. Casein is therefore more suitable for protein supply before going to sleep.

If you cannot or do not want to take whey protein, therefore supplement your soy protein shake with L-leucine or BCAAs.