Metabolism of the main nutrients Part III — Proteins
Proteins are an important part of our diet. Understanding protein metabolism requires some basic knowledge: Protein is the building material of our body par excellence. The individual building blocks of a protein are called amino acids (AS). There are 20 amino acids, 8 of which are essential. This means that they cannot be produced by the body’s own biosynthesis and must therefore be ingested with food. Although only 20 AS are involved in the construction of body proteins, they differ from person to person and even from organ to organ.
Structurally, aliphatic AS, which include the branched-chain AS (BCAA), can be distinguished from aromatic AS. Functionally, glucoplastic AS, which can be converted into glucose (gluconeogenesis), can be distinguished from ketoplastic AS, which can be converted into ketone bodies. Meanwhile, proteins have found their way into our diet in a wide variety of forms. According to a 2018 study, between 2013 and 2017, protein-rich products increased by 62.7 percent. As a result, among all other food trends, it has recorded by far the largest increase over the years. However, what should you know before increasing protein to your body?
Protein metabolism in minutes !
- The individual building blocks of a protein are called amino acids (AS).
- Although only 20 AS are involved in building the body’s proteins, proteins vary from person to person and even from organ to organ.
- According to a 2018 study, between 2013 and 2017, the supply of high-protein products increased by 62.7 percent. As a result, among all other food trends, it recorded by far the largest increase over the years.
- Digestion for animal proteins is completed after about three hours, while plant proteins take slightly longer because they are not as easily or quickly absorbed as animal proteins.
- According to the 30% increase in protein intake proposed by the WHO in 1985, a minimum amount of 450mg, i.e. 0.45 grams per kg body weight, is therefore calculated.
- Adults lose about 20% of their body protein between the ages of 25 and 65, which corresponds to the loss of about one percent of existing muscle mass per year.
- Egg protein serves as the reference protein and has the biological value of 100, followed by milk and meat, etc. animal protein is generally more biologically valuable than plant protein because it usually contains more essential amino acids.
- Increasing the biological value above 100 is possible by combining foods (classic example: egg plus potato).
How are proteins digested?
Protein digestion begins in the acidic environment of the stomach. The inactive enzyme precursors in the gastric juice that are responsible for the digestion of proteins (pepsinogens) are converted into the active digestive enzyme pepsin under the action of hydrochloric acid at a pH of 2–4. This endogenous protease splits the food proteins into longer fragments (polypeptides and oligopeptides).
The protein-partially digested food pulp is passed from the stomach in portions at regular intervals into the small intestine, where digestion is completed by the alkaline enzymes of the pancreas. The poly- and oligopeptides formed in the stomach are further broken down there by the enzymes trypsin and chymotrypsin. The broken down nutrients in the form of free amino acids, di- and tripeptides can now be absorbed by the small intestinal wall. This process is completed after about three hours for animal proteins, while plant proteins take somewhat longer because they cannot be absorbed as easily or as quickly as animal proteins.
Even though most of the digestion and resorption work of proteins takes only three hours, this does not mean that protein must be supplied every three hours. Due to the constant build-up and breakdown of proteins, there is nevertheless always a steady blood level of AS — regardless of food intake — which ensures the supply of the muscles and other body tissues. Man is a “postprandial being” (post=after, prandial=concerning the meal or during the meal). Even the usual night’s rest is too short to cause “starvation” of the organism — unless one follows an extreme catabolic metabolic situation such as the “zero diet” or “therapeutic fasting”.
How does muscle mass loss occur?
Under normal physiological conditions, muscle protein is not used for energy production. In other words, the fear of many athletes that they will “lose” the muscle mass they have built up again if they do not consume protein in excess is unfounded. On the contrary, it is precisely in the time between training sessions, i.e. in the regeneration phase, that the compensation of the temporarily catabolic metabolic situation during an intensive training session takes place. The compensating anabolic reaction results in protein neosynthesis and thus muscle growth, which is also ensured with “conventional” food intake.
How is protein metabolism regulated?
The blood transports the AS between the individual organs. In this process, the irregular, “intermittent” influx of amino acids after food intake is balanced by regulatory mechanisms, so that a steady blood level of amino acids always results. The center for this regulation is the liver, which compensates for fluctuations in the concentration of amino acids in the blood by breaking down and converting amino acids and by protein synthesis.
Protein metabolism is regulated by the current availability of amino acids and by hormonal regulatory circuits. The liver releases mainly branched-chain amino acids (valine, leucine, isoleucine), which then accumulate in the skeletal and cardiac muscles as well as in the brain and kidney. It can thus be seen that the branched-chain AS are of great importance for muscle metabolism. The peptide hormones insulin and glucagon are involved in the regulation of absorptive and postabsorptive amino acid flux. Some amino acids, especially arginine and the branched-chain AS, stimulate insulin secretion.
Other amino acids such as asparagine, glycine, serine, and cysteine stimulate the secretion of glucagon. Insulin promotes the uptake of amino acids into muscle cells and thus muscle protein synthesis. Glucagon, on the other hand, promotes uptake into the liver and stimulates gluconeogenesis. Both hormones thus prevent an excessive increase in plasma amino acids after their absorption, thus avoiding unnecessary losses via the urine.
What is an amino acid pool?
Proteins ingested with food are broken down in the digestive tract and absorbed in the form of free AS, i.e. taken up into the bloodstream and subsequently enter interstitial (connective tissue) and intracellular stores. Only 0.05% of all amino acids present in the body are in free form.
A much higher concentration of amino acids is present in cells than in plasma. The highest concentration of e.g. glutamine is found in muscle cells. The composition of free AS in different tissues varies. Furthermore, the composition of the AS pool as well as the concentration of individual amino acids varies from species to species. Therefore, it is not reasonable to extrapolate study data from animal experiments to humans. Skeletal muscle is the largest reservoir of amino acids. 70–80% of the amino acid pool is in muscle, while free amino acids in blood are a small fraction of the total pool. Not only diet, but also age, gender as well as disease are factors that influence the composition of the amino acid pool.
How much protein does the body need?
Protein turnover refers to both the formation and breakdown of proteins. Normally, there is a “steady state”, i.e. a flow equilibrium and the synthesis rate balances the degradation rate.
Let’s assume a person weighing about 70 kg who consumes a protein amount of about 100 grams of protein per day. In addition to the 100 grams of dietary protein, about 70 grams of protein are initially excreted from the intestinal cells themselves in the small intestine. Of these 170 grams of protein, about 160 grams are reabsorbed at a later point in the intestine, and the small remainder is excreted in the stool.
The average turnover of body protein is 300–400 grams per day. The difference between protein intake and turnover proves the recycling of amino acids released in protein metabolism. The high protein turnover is mainly determined by the daily renewal of the cells of the intestinal mucosa, muscle metabolism, the breakdown and build-up of plasma proteins, and also by the formation of hemoglobin and white blood cells (immune defense). The most common “blood protein” albumin can be used to assess malnutrition.
Protein metabolism is of course also dependent on the amino acids available in the amino acid pool. Theoretically, there are always sufficient free amino acids available for 8 hours of normal protein synthesis. In reality, however, there are differences in the availability of individual amino acids that must be taken into account when calculating the true protein turnover. The amino acids that are always in limited supply in cells are the branched-chain and aromatic AS.
Does more protein really get you more?
In bodybuilding, muscle mass gain plays the decisive role. In addition, the fear of losing muscle mass is constantly present and established the belief that one needs 3 to 4 grams of protein per kg of body weight for muscle hypertrophy. However, such a high protein intake is actually counterproductive. Apart from the energy content associated with it, which, like basically any excessive intake of a macronutrient, means an energy intake in excess of the requirement and thus favors an increase in the body fat percentage, a performance-reducing hyperammonemia is additionally induced, endogenous glutamine synthesis is reduced and, moreover, the development of cellular insulin resistance is risked. It should be known that an increased intake of protein automatically causes an increased oxidation of amino acids, so that these are not — as desired — available for the building metabolism, but for excretion.
The minimum protein requirement can be estimated by measuring the total nitrogen losses during a protein-free diet. It is assumed that these losses are equal to the requirement. Factors of mandatory daily nitrogen loss (urine: 37mg/kg, feces: 12mg/kg, skin 5–8mg/kg, and other small losses) are included in the calculation.
The total daily loss is 54mg nitrogen or 340mg protein per kg body weight. In accordance with the 30% increase in protein intake proposed by the WHO in 1985, this calculates to a minimum of 450mg, or 0.45 grams per kg body weight. This recommendation is confirmed today by most national and international commissions.
In 1985, the WHO coined the term “safe intake”, i.e. ensuring an adequate supply of amino acids, after studies directly determined the minimum amount of a high-quality protein to maintain a balanced nitrogen level. It was 0.6g/kg body weight for the young adult. By adding a “safety margin” to compensate for individual variations, the “safe intake” increases to 0.75g/kg. The minimum protein requirement for all other age groups was also set in this way. An infant in the first month of life has the highest protein requirement at 2.4g/kg. With increasing age, the protein requirement decreases, although the requirements of older people have not yet been sufficiently researched.
Adults lose about 20% of their body protein between the ages of 25 and 65, which corresponds to the loss of about one percent of existing muscle mass per year. Whether protein intake can slow down the aging process is still unclear.
One thing is certain, however: a loss of muscle mass can be prevented by regular physical activity and especially by strength training (one session per week is sufficient for this purpose), even at an advanced age. A protein intake of 1.2g/kg is sufficient to build up muscle mass. Ultimately, it is the training stimulus and not the diet that is decisive.
Which protein sources provide the best protein?
Nutrient analyses and nutrition tables indicate the protein content in grams per 100 grams of food. From this, it is also possible to calculate the energy content of protein relative to the total energy content of the food (1 gram of protein provides approximately 4.2 kcal). A balanced diet should provide 15% of energy in the form of protein. However, the indication of the protein content of a food alone does not say anything about its quality.
Biological value used to be defined as how many grams of body protein can be built up by 100 grams of dietary protein. In other words, how much of the absorbed protein is “retained” in the body. Thus, ignoring digestibility, one measures the protein retained relative to the protein consumed. This index is called NPU = “net protein utilization”. Egg protein serves as the reference protein and has the biological value of 100, followed by milk and meat, etc. Animal protein is generally more biologically valuable than vegetable protein because it usually contains more essential amino acids. Increasing the biological value above 100 is possible by combining foods (classic example: egg plus potato, see the table below) .
Food: Biological value
Egg: 100
Milk: 75–90
Meat: 70–90
Fish: 70–90
Soy: 75–85
Potatoes: 50–70
Bread (cereals): 50–70
Lentils, beans: 40–50
Food combination: biological value
36% whole egg + 64% potato protein: 136
75% milk + 25% wheat flour protein: 125
60% whole egg + 40% soy protein: 124
68% whole egg + 32% wheat flour protein: 123
76% whole egg + 24% milk protein: 119
51% milk + 49% potato protein: 114
88% whole egg + 12% corn protein: 114
78% beef + 22% potato protein: 114
35% whole egg + 65% bean protein: 109
How are proteins excreted?
The end products of protein metabolism are excreted in the urine. In urine, these would be mainly urea, but also creatinine, ammonia and uric acid. When protein intake is reduced, the amount of urea in the urine also decreases. Non-absorbable dietary protein and a small amount of protein secreted into the intestine are eliminated with the stool. The body also loses small amounts of protein with skin particles, hair, semen and menstrual blood.
Text-Sources:
(1) “Ernährungsmedizin“, Thieme
(2) Amboss — Fachwissen für Mediziner: Aminosäuren und Proteine
(3) Amboss — Fachwissen für Mediziner: Aminosäurenstoffwechsel
(4) Deutsche Gesellschaft für Ernährung e.V.
(5) dockcheck.dee: Proteinstoffwechsel
(6) dockcheck.de: Phenylketonurie
(7) Jordan M. Joy u.a. (2013): The effects of 8 weeks of whey or rice protein supplementation on body composition and exercise performance. Nutrition Journal, 12: 86.
(8) Solon-Biet u. a. (2019): Branched chain amino acids impact health and lifespan indirectly via amino acid balance and appetite control. Nature metabolism, 1(5):532–545.
(9) Song (2016): Association of Animal and Plant Protein Intake With All-Cause and Cause-Specific Mortality. JAMA Internal Medicine, 1;176(10):1453–1463
Image-Sources:
