What is protein?

Protein is one of the three main nutrients (alongside carbohydrates and fat) and a central building block of life. It consists of amino acids, which are necessary for the growth, repair, and function of all body cells.

About 15–20% of the human body is made up of protein—primarily in muscles, skin, organs, enzymes, hormones, and the immune system. Without an adequate protein intake, the body cannot form new cells, maintain muscles, or carry out important metabolic processes.

Protein structure and types

Proteins consist of amino acids linked together by peptide bonds. There are 20 proteinogenic amino acids in total, of which 9 are essential—meaning they must be obtained through food because the body cannot produce them itself.

Essential amino acids:
Leucine, isoleucine, valine, lysine, methionine, phenylalanine, threonine, tryptophan, histidine

Non-essential amino acids:
Can be produced by the body itself (e.g., alanine, glutamine, glycine).

High-quality proteins contain all essential amino acids in optimal proportions (e.g., from animal sources or certain plant-based combinations).

Important sources of protein

Animal sources:

  • Meat, fish, eggs

  • Dairy products (quark, yogurt, cheese)

  • Poultry

Plant sources:

  • Legumes (lentils, beans, chickpeas)

  • Soy and tofu

  • Quinoa, amaranth

  • Nuts, seeds, whole grains

By combining foods wisely—for example, grains + legumes (such as rice with beans or bread with hummus)—the biological value of plant proteins can be significantly increased.

How does protein work in the body?

1. Building and maintaining muscle:
Proteins are the basic building blocks of muscle. They support muscle growth, maintenance, and recovery, especially after physical activity. The amino acid leucine plays a key role because it activates muscle protein synthesis.

2. Structure and cell formation:
Protein is a component of skin, hair, nails, bones, tendons, and connective tissue. It provides strength, elasticity, and stability to all body structures.

3. Enzymes and hormones:
Many enzymes and hormones are made of proteins. They regulate vital processes such as digestion, metabolism, cell division, and signaling.

4. Immune system:
Antibodies (immunoglobulins) are made of proteins and are essential for defending against pathogens.

5. Transport and storage:
Proteins transport nutrients (e.g., hemoglobin transports oxygen, transferrin iron) and store important substances.

6. Energy source (in an emergency):
If energy intake from carbohydrates or fats is insufficient, protein can be used for energy production – but at the expense of muscle mass.

Benefits of adequate protein intake

  • Promotes muscle growth and recovery

  • Supports the maintenance of muscle mass during weight loss

  • Provides long-lasting satiety and stable blood sugar levels

  • Strengthens the immune system, skin, hair, and nails

  • Supports hormone and enzyme production

  • Contributes to healthy cell formation and wound healing

Recommended daily protein intake

According to the German Nutrition Society (DGE):

  • Adults: 0.8 g of protein per kg of body weight

  • Physically active people: 1.2–2.0 g/kg

  • Muscle building or dieting: up to 2.2 g/kg

Example:
A person weighing 70 kg needs an average of about 56 g of protein per day, or up to 140 g with regular training.

Possible effects of deficiency or excess

Protein deficiency:

  • Muscle weakness and loss

  • Delayed wound healing

  • Brittle nails, hair loss

  • Weakened immune system

  • Fatigue and difficulty concentrating

Excessive intake:

  • Generally harmless for healthy kidneys

  • Very high amounts over the long term can strain the kidneys, especially if fluid intake is too low

  • A balanced diet remains essential

Conclusion

Protein is the body's most important building material – essential for muscles, organs, skin, hormones, and the immune system. Adequate protein intake supports performance, recovery, metabolism, and satiety. What matters is not just the amount, but also the quality and variety of protein sources. Combining animal and plant proteins ensures optimal nutrition and long-term health.

Sources

  1. German Nutrition Society (DGE). (2020): Reference values for nutrient intake – protein.

  2. Phillips SM, et al. (2016): Dietary protein for athletes: from requirements to metabolic advantage. Appl Physiol Nutr Metab, 41(5), 565–572.

  3. Paddon-Jones D, et al. (2008): Role of dietary protein in the sarcopenia of aging. Am J Clin Nutr, 87(5), 1562S–1566S.

  4. Wu G. (2016): Dietary protein intake and human health. Food Funct, 7(3), 1251–1265.