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Dietary Protein – Functions, Requirements and Sources

Dietary protein is an essential macronutrient obtained from food, composed of amino acids that are vital for muscle building, immune defense, enzyme production, and overall metabolism.

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Things worth knowing about "Dietary Protein"

Dietary protein is an essential macronutrient obtained from food, composed of amino acids that are vital for muscle building, immune defense, enzyme production, and overall metabolism.

What is Dietary Protein?

Dietary protein is a vital macronutrient consumed through food. Proteins are made up of long chains of amino acids – organic compounds that serve as the fundamental building blocks of the human body. There are 20 different amino acids in total, nine of which are considered essential, meaning the body cannot synthesize them on its own and they must therefore be obtained through the diet.

Biological Functions

Dietary protein fulfills a wide range of critical functions in the human body:

Muscle growth and maintenance: Proteins are the primary structural component of muscle tissue, essential for growth, repair, and preservation.
Enzyme production: Nearly all enzymes in the body are proteins that catalyze and regulate biochemical reactions.
Immune function: Antibodies used by the immune system to fight pathogens are protein-based molecules.
Hormone synthesis: Many hormones, including insulin and growth hormone, are composed of protein.
Transport and storage: Proteins such as hemoglobin carry oxygen in the blood, while others store essential nutrients.
Structural roles: Collagen, keratin, and elastin provide structure to skin, hair, nails, and connective tissue.
Energy supply: When needed, the body can use protein as an energy source (approximately 4 kcal per gram).

Dietary Sources

Animal-Based Protein Sources

Animal foods typically provide complete proteins, meaning they contain all essential amino acids in sufficient quantities:

Meat (beef, poultry, pork)
Fish and seafood
Eggs
Dairy products (milk, yogurt, cheese, cottage cheese)

Plant-Based Protein Sources

Plant proteins are often considered incomplete, as individual sources may lack one or more essential amino acids in adequate amounts. However, by combining different plant foods, a complete amino acid profile can be achieved:

Legumes (lentils, beans, chickpeas, soybeans)
Grains (oats, quinoa, amaranth)
Nuts and seeds (almonds, hemp seeds, chia seeds)
Tofu, tempeh, and other soy-based products

Recommended Intake

The World Health Organization (WHO) recommends a daily protein intake of 0.8 g per kilogram of body weight for healthy adults. Protein requirements may be higher in certain life stages or circumstances:

Pregnancy and breastfeeding
Infants, children, and adolescents during growth phases
Older adults (65 years and above): 1.0–1.2 g/kg body weight
Athletes and physically active individuals: 1.2–2.0 g/kg body weight
Recovery after illness, surgery, or injury

Protein Deficiency

Protein deficiency is uncommon in high-income countries but can occur due to restrictive diets, eating disorders, poverty, or underlying medical conditions. Signs and symptoms of protein deficiency may include:

Muscle wasting and general weakness
Hair loss and brittle nails
Impaired wound healing
Weakened immune response
Edema (fluid retention, particularly in the abdomen and legs)
In severe cases: kwashiorkor (a form of severe malnutrition most common in children)

Excessive Protein Intake

Chronically high protein intake can place a burden on the kidneys, as excess nitrogen from protein metabolism must be excreted via the urinary system. Individuals with pre-existing kidney disease should have their protein intake monitored by a healthcare professional. In otherwise healthy individuals, a moderate intake above the baseline recommendation is generally considered safe.

Protein Quality

Not all proteins are nutritionally equivalent. Biological value (BV) indicates how efficiently a dietary protein can be converted into body protein, with whole egg protein used as the reference value (100). Additional scoring systems recommended by the FAO/WHO include the PDCAAS (Protein Digestibility Corrected Amino Acid Score) and the DIAAS (Digestible Indispensable Amino Acid Score), which provide more comprehensive assessments of protein quality.

References

World Health Organization (WHO) / Food and Agriculture Organization (FAO): Protein and Amino Acid Requirements in Human Nutrition. WHO Technical Report Series 935, Geneva 2007.
Richter M. et al.: Revised Reference Values for the Intake of Protein. Annals of Nutrition and Metabolism, 2019; 74(3): 242–250.
Stokes T. et al.: Recent Perspectives Regarding the Role of Dietary Protein for the Promotion of Muscle Hypertrophy with Resistance Exercise Training. Nutrients, 2018; 10(2): 180.

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