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name of the term : Protein

Short introduction

  • Protein belongs to the group of macronutrients, together with fat and carbohydrates.
  • The body is composed of many different types of cells, each with its own structure and function. Body cells vary in size, but all are very small. Groups of cells that have a similar shape and function form tissues. Body functions are carried out by organs, which are is made of several types of tissue and therefore several types of cells.
  • Protein is the major structural and functional component of body cells, required for their growth, maintenance and renewal (repair). Existing body cells are constantly renewed and protein is broken down and replaced by new body protein.
  • Proteins consist of amino acids linked together by peptide bonds.
  • Proteins vary in length and complexity, based on the number and type of amino acids that compose the chain.
  • Amino acid > polypeptide > protein. Amino acids make up the polypeptide chain and polypeptide chains (folded in a three-dimensional structure) make up the protein – protein is the functional product.
  • For example: haemoglobin (blood protein) is composed of 4 polypeptides, each with 146 amino acids, a total of 584 amino acids.
  • Peptides are short chains of amino acids: a dipeptide consists of 2 amino acids, a tripeptide of 3 amino acids and an oligopeptide contains maximal 10 amino acids.
  • 22 amino acids are important to human health: 9 essential amino acids, 7 non-essential amino acids and 6 conditionally essential amino acids.
  • The body cannot make essential amino acids; they must be obtained from food. Non-essential amino acids can be made within the body from other molecules. Conditional essential amino acids are only essential in times of rapid growth (e.g. in infancy) or illness.
  • Essential amino acids: histidine, isoleucine, leucine, lysine, methionine, phenylalanine, threonine, tryptophan and valine.
  • Animal protein contains almost the same proportion of each essential amino acid as human protein and is called complete protein; high biological value (BV).
  • Vegetable proteins are deficient in one or more essential amino acids and termed incomplete proteins: low biological value (BV).

Main natural sources

  • Animal protein (complete protein): e.g. meat, fish, milk and dairy products, eggs.
  • Vegetable protein (incomplete protein): e.g. bread, grains, vegetables, legumes, nuts, seeds.

Main function

  • Proteins are responsible for nearly every task of cellular life, including cell structure and inner organisation, the formation of compounds and waste clean-up, as well as routine maintenance.
  • Proteins also receive signals from outside the cell and activate intracellular responses (a subsequent reaction from the inside). Proteins serve a variety of functions within cells. Some are involved in structural support and movement, others in enzymatic activity or interaction with the outside cell-world.
  • Major structural and functional component of body cells. Needed for growth, maintenance and renewal (repair) of all body cells. Existing body cells are constantly renewed and protein is broken down and replaced by new body protein.
  • Functions as enzyme. Some enzymes help to break down large nutrient molecules (protein, fat, carbohydrates), into smaller units. Other enzymes guide the small units through the intestinal wall into the bloodstream.
  • Protein forms antibodies that help prevent infection and disease. Antibodies identify and assist in destroying antigens such as bacteria and viruses.
  • Functions as hormone. Insulin is a protein hormone that regulates blood sugar and secretin assists in the digestive process by stimulating the production of digestive juices.
  • Major element in the transportation of molecules. Haemoglobin is a protein that transports oxygen throughout the body.
  • Storage of molecules. For example, ferritin is a protein that combines with iron for storage in the liver.
  • Most neurotransmitters are made from amino acids obtained from protein. These are brain chemicals responsible for mediating sensory and emotional responses. For example: the amino acid tryptophan helps make the neurotransmitter serotonin, responsible for mood control, appetite and sleep.
  • Source of energy: 1 g of protein provides 4 kcal (17 kJ).

Deficiency disease

  • Poor protein intake influences growth, development and immunity. Signs and symptoms include: weight loss, frequent infections, fluid retention, loss of muscle mass, skin pigmentation, diarrhoea, changes in colour or texture of hair, bulging belly, rashes, fatigue, irritability and apathy. Growth failure is evident in children.
  • At risk: pregnant women (risk of low birth weight), children (growth failure), vegetarians and vegans (it is difficult to obtain all nine essential amino acids in a plant-based diet).
  • Protein Energy Malnutrition (PEM) is fairly common worldwide in children and adults.
    • Kwashiorkor: protein deficiency with adequate energy intake.
    • Marasmus: inadequate energy intake in all forms, including protein.

Recommended daily intake

Latest Dietary Reference Intakes  (DRIs) 
Institute of Medicine (IOM)

Age categoryPer day
71 g (RDA) – 0,88 g/kg/day
71 g (RDA) – 1,05 g/kg/day
Infants 6 – 12 months11 g (RDA) – 1 g/kg/day
 1 – 3 years
 4 – 8 years
13 g (RDA) – 0,87 g/kg/day
19 g (RDA) – 0,76 g/kg/day
 9 – 13 years
14 – 18 years
19 – 30 years
31 – 50 years
50 – 70 years
> 70 years

34 g (RDA) – 0,95 g/kg/day
52 g (RDA) – 0,85 g/kg/day
56 g (RDA) – 0,80 g/kg/day
56 g (RDA) – 0,80 g/kg/day
56 g (RDA) – 0,80 g/kg/day
56 g (RDA) – 0,80 g/kg/day

 9 – 13 years
14 – 18 years
19 – 30 years
31 – 50 years
50 – 70 years
> 70 years
34 g (RDA) – 0,95 g/kg/day
46 g (RDA) – 0,85 g/kg/day
46 g (RDA) – 0,80 g/kg/day
46 g (RDA) – 0,80 g/kg/day
46 g (RDA) – 0,80 g/kg/day
46 g (RDA) – 0,80 g/kg/day


Based on g protein per kg of body weight for the reference body weight, e.g. for adults 0.8 g/kg body weight for the reference body weight.

RDA = Recommended Dietary Allowance.

Synonyms : Proteins

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