Protein is found in foods of animal and plant origin. Protein provides 4 kcal/g energy.
By the intake of dietary protein, the body is supplied with amino acids and other nitrogenous compounds, which the human organism requires in order to synthesize endogenous proteins:
- Structural proteins: Components of cells and tissue (muscle fibers, skin, membranes, connective tissue)
- Nutrient resources for enzymes
- Nutrient resources for hormones such as insulin
- Nutrient resources for antibodies of the immune system
- Nutrient resources for clotting factors within the coagulation system
- Transport proteins for e.g. nutrients such as liposoluble vitamins or iron
The protein ABC:
Proteins are nitrogenous organic substances. They are composed of chains of different size, which are formed by the different amino acids. The proteins are split during digestion, whereupon amino acids are released and absorbed.
9 of the 20 different amino acids found in dietary protein are essential and have to be ingested with food. Also the other (non-essential and conditionally essential) amino acids are required by the organism in order to synthesize endogenous protein. The body is not or only limitedly capable of synthesizing conditionally essential amino acids during periods of illness or other situations of requirement.
Essential amino acids: Histidine, isoleucine, leucine, lysine, methionine, phenylalanine, threonine, tryptophan, valine
Non-essential amino acids: Alanine, asparagine, aspartic acid, glutamic acid, glycine, proline, serine
Conditionally essential amino acids: arginine, cysteine, glutamine, tyrosine
Amount of protein in various food:
10 g protein are contained in
1 ½ eggs
300 ml whole milk
40 g Gouda
50 g trout
50 g poultry
1,3 l whey
55 g chickpeas, dried
67 g quinoa
80 g noodles, egg-free, uncooked
154 g rice parboiled, uncooked
500 g potatoes (cooked with peel)
115 g rolls
120 g tofu
137 g rye wholegrain bread
How much protein should we eat?
Regarding a balanced diet, the percentage of dietary protein amounts to 8-10% of the daily energy requirement of adults, correlating to approximately 47 g per day for women and approximately 60 g per day for men. In order to avoid adverse effects of an immoderate protein intake in the long term, the average daily protein intake should not exceed a maximum of 120 g for women and 140 g for men.
Proteins
Proteins are basic elements for the body. They represent nutrients for cells and different tissues (such as muscles and organs). Furthermore, the body requires proteins for hormone production, the immune system and as transport substances. Proteins serve as source of energy only in case of necessity (e.g. during long periods of fasting).
Proteins cannot be stored by the body, therefore, a continuous dietary intake is essential for the synthesis and regeneration of body substance. A protein deficiency during adolescence may lead to underdevelopment, which occurs rarely in first world countries. In developing countries, however, protein-energy-deficiency is a frequent health issue, leading to weight loss, renal load and degradation of endogenous protein. In people with a specific genetic disposition, oversupply may trigger gout.
Biological value:
The biological value indicates the amount of endogenous protein (in grams), which can be synthesized from 100 g dietary protein. The amount and type of amino acids is different among proteins, therefore, the amount of essential amino acids varies between foods. The more balanced the composition of the essential amino acids, the higher is the biological value of the food.
Regarding proteins of animal and plant origin, sources for animal protein generally show a higher biological value. Nevertheless, it is important to maintain a balanced ratio between proteins of animal and plant origin, since animal foods contain also high amounts of fat, cholesterol and purines.
Biological value of some foods:
| Food | Biological value |
| Wheat | 59% |
| Milk protein | 85% |
| Egg protein | 100% |
| Soy milk | 84% |
| Meat | 85% |
| Potatoes | 96% |
Example: Wheat has a biological value of 59%, suggesting that our body can synthesize 59 g endogenous protein from 100 g wheat protein. Through the combination of wheat and milk, a biological value of 125% can be achieved.
Attainment of a complementary effect:
- Egg + potatoes
- Cereals + meat, milk, egg, fish
- Potatoes + meat, fish, egg
- Legumes + meat, fish, egg, milk
| Cereal products | Potatoes | Legumes |
| Bread with cheese, egg, sausage, curd cheese, fish | Boiled potatoes with fish | Bean salad with fish |
| Oats with milk | Fried potatoes with fried egg | Lentils with sausage |
| Semolina porridge with milk | Mashed potatoes with milk | Bean soup with bread |
| Pasta with meat, cheese | French fries with steak | |
| Rice with meat, egg, … | ||
| Yeast dough |
Table: High biological value by combination of foods
Combination of foods with a low biological value
- Grain + potatoes
- Legumes + vegetables
Questionaries:
Which other name is known for proteins? Egg white
Why is it called egg white? The name derives from the egg white of a hen’s egg. It was believed that the whole egg white was composed of protein.
Why do we need proteins? Proteins are basic elements for the body. They represent nutrients for cells and different tissues (such as muscles and organs). Furthermore, the body requires proteins for hormone production, the immune system, as enzymes and transport substances. Approximately 20% of the body is made from proteins.
Is an intake of proteins always necessary? Yes, proteins cannot be stored by the body, therefore, a continuous dietary intake is essential for the synthesis of body substance. A protein deficiency during adolescence may lead to underdevelopment.
Do proteins serve as a source of energy? Rarely only during long periods of fasting. Protein provides 4 kcal (17 kJ).
How are proteins formed? Proteins are synthesized from 20 different amino acids.
What are non-essential amino acids? They cannot be synthesized by the body.
What implies the L in front of the name of an amino acid, e.g. L-Leucine? L-amino acids are the naturally occurring form of amino acids. L stands for the respective spatial structure of the amino acid.
Which foods contain proteins? Proteins can be found in almost all foods. Animal products (e.g. low-fat milk and milk products, lean meat, fish, eggs), but also foods from plant origin represent good sources of protein (e.g. legumes, potatoes, bread).
What does biological value imply? Type and amount of amino acids of a protein determine its nutritional value. The biological value indicates to which amount a protein can be used for the synthesis of endogenous protein. The more similar the composition (amino acid pattern) of the dietary proteins to the endogenous proteins, the higher is the biological value.
Which biological value have different foods, with respect to proteins? Hen’s egg represents the reference food with a biological value of 100. A biological value higher than 100 indicates that the dietary protein is utilized better by the body, a value lower than 100 to a lesser extent, respectively.
Which biological value of proteins have some foods? Beef 92, tuna 92, cow’s milk 88, potatoes 76, beans 72, oats 60, turkey 70, poultry 80
Are there combinations, which result in a very high biological value of protein in food? Yes, potatoes (65%) and egg (35%) 137, hen’s egg (60%) and soy (40%) 122.
How much protein do I need per day? According to D-A-CH reference values, a daily protein requirement of 0,8 g per kilogram body weight is specified for adults.
How much protein should be consumed? Recent research suggests a recommendation of 1,0 to 1,2 g/kg/day. There is no indication of harmful effects by the consumption of 2 g/kg/day or more. If there is evidence of renal damage (potentially undetected), it would not be advisable to exceed 2 g/kg/day.
What happens to the proteins in our body? Proteins are absorbed as nutritional components. In the gastrointestinal tract the proteins are split into singular amino acids by enzymes such as proteases. These amino acids are introduced into the blood circulation by the small intestine.
What happens to the amino acids? The free amino acids, which have been ingested with food or which accumulate due to the degradation of endogenous proteins, are utilized either for synthesis of new proteins or fuel molecules such as glucose.
What happens with the amino acids in detail? Amino acids can be utilized for the synthesis of glucose (gluconeogenesis) or fats, suggesting a strong connection between carbohydrate and lipid metabolism.
How is a protein-rich nutrition defined? A protein-rich diet implies a daily intake of more than 0,8 g protein/kg body weight or more than 15% of the energy requirement from proteins.
Which effects has a protein-rich diet? Increased secretion of the appetite-suppressing hormones GIP and GLP-1, decreased secretion of the hormone ghrelin, which causes feelings of hunger, enhanced energy consumption during metabolism of food, improved glucose metabolism.
Why is urea produced during the degradation of amino acids? Removal of the amino group leads to the formation of ammonia, which is toxic to the cell and is excreted as urea.