Proteinogenic amino acids

Synonym(s): alanine, aspartic acid, cysteine, cystine, Glycine, histidine, L-alanine, L-Aspartic acid, L-cysteine, L-cystine, L-glycine, L-histidine, L-proline, L-serine, proline, serine

Nutrient group: Amino acids

Sources and physiological effects

Dietary sources
L-aspartic acid was first obtained from seedlings of legumes by hydrolysis of the contained asparagine. A good source of L-aspartic acid is asparagus. Potatoes, meat, eggs and alfalfa also contain the non-essential amino acid in significant quantities. L-Alanine is a non-essential proteinogenic amino acid that can be synthesized by the human body. In nutrition, the branched-chain amino acid can be found in gelatin, beef, pork, chicken egg and rice, L-cysteine is a sulfur-containing proteinogenic amino acid that can be produced from L-methionine in the liver of adults. The L-cysteine content of food is difficult to determine, which is why cystine, a dipeptide consisting of two molecules of cysteine, is usually measured instead. Dried soybeans, sunflower seeds, walnuts and whole wheat flour are good plant sources of L-cysteine. Animal L-cysteine sources are whey protein, chicken eggs, meat and salmon. Glycine is the smallest naturally occuring amino acid. It can be formed in the body from various precursors (serine, choline, threonine, glyoxylate) and is therefore not considered essential. In nutrition gelatin is notable for its high glycine content. Dried soybeans, lentils, pumpkin seeds, meat and eggs also contain significant amounts. Like all amino acids in general, glycine is better utilized on an empty stomach. L-histidine is a proteinogenic semi-essential amino acid. In certain situations, such as chronic kidney failure or infancy, the human body is dependent on an external intake. Dried soybeans, wheat germ, wholemeal flour are plant-based sources of histidine. Animal sources include meat, tuna, salmon and egg. L-proline is a non-essential proteinogenic amino acid, which is needed in the body to form collagen. L-proline is found in signifficant quantities in soybeans, certain cheeses (e.g. Emmentaler, Edam) and wholemeal products. L-serine is a non-essential proteinogenic amino acid whose name (lat. sericum: silk) drives from the fact that this amino acid was first isolated from silk protein. L-serine is found in various protein-rich foods in the diet. Sources include eggs, oats, corn, milk and dairy products. L-cystine is a non-essential sulfur-containing dipeptide, which consists of two molecules of cysteine and is found in high concentrations in skin, hair and immune cells. If necessary, the body can convert cystine and cysteine into each other. Foods with a significant cystine content include meat, eggs, milk and wholemeal products.
Physiological effects
Protein biosynthesis	L-proline influences the folding of proteins. L-cysteine contributes to the stabilization of protein structures by disulfide bridges. L-cysteine is required for taurine synthesis. Glycine is the central C2N unit for all purines (DNA).
Carbohydrate metabolism	L-alanine controls blood sugar levels by releasing glucagon and synthesizing glucose from glycogen in hypoglycemia. L-alanine stimulates the pancreas to release insulin.
Nervous system	L-aspartic acid acts as a transmitter in the central nervous system. Glycine is an inhibitory neurotransmitter of the central nervous system. L-histidine is involved in the synthesis of histamine (neurotransmitter and tissue hormone).
Mineral status	L-histidine is involved in the transport and uptake of zinc, copper and iron.
Skin and hair	L-proline is the precursor of L-hydroxyproline, which is formed with the help of vitamin C after incorporation into collagen and determines its mechanical properties. L-cystine is an important building block for the structure of hair keratin, which consists of 2 molecules of L-cysteine. L-cysteine is contained in structural proteins that give strength to connective tissue and hair.
Enzyme activity	L-serine plays an important role in the activation or inactivation of enzymes.
Antioxidant	L-cysteine and glycine are required for the synthesis of glutathione. L-histidine acts as scavenger of free radical oxygen.
Detoxification	L-Alanine regulates the removal of NH₃ from the muscle and promotes the synthesis of urea in the liver. L-aspartic acid is involved in the urea cycle.

Recommended intake

Nutrient needs and safety
Recommended intake	Adults with normal metabolism: 1 g/kg bw daily Adults with catabolic metabolism due to sport, stress or illness: 1 – 2 g/kg bw daily
Increased demand	Pregnancy and lactation, growth, seniors, sports, convalescence, after burns, alcohol abuse, chronic inflammatory bowel disease, short bowel syndrome, tumor cachexia
Special group at risk of deficiency	Convalescents, athletes, seniors, cancer patients
Nutrient safety	L-cysteine: Allowed in foodstuff as a flour treatment as additive (E 920) without maximum quantity restrictions (quantis satum) L-histidine: NOAEL (Maximum intake, with no observed adverse effect) 4 – 4,5 g (Norwegian Scientific Committee on Food Safety. http://www.vkm.no/dav/ba7a85274a.pdf)

Detailed information

Proteinogenic amino acids - basic building blocks of the human body

Proteins are the main component of organic macromolecules in the human body. Around 50,000 different proteins act as essential structural and functional carriers for all life processes. The basic building blocks of proteins are L-amino acids (α-amino carboxylic acids). For endogenous protein synthesis 20 proteinogenic amino acids are used - nine of them are considered essential and must be supplied through the diet (1) (2). In addition, there are amino acids which the body can synthesize in sufficient quantities under normal metabolic conditions, but which may become essential in certain situations, e.g. as a result of illness or after surgical interventions. These so-called "semi-essential" amino acids include L-arginine, glycine, L-cysteine, L-glutamine, L-tyrosine, L-serine and taurine (3).

Increase in biological value

Food protein is broken down by digestive processes into individual amino acids that are available to the body for the formation of endogenous proteins. Since the protein from food can be converted to body proteins at varying degrees due to its different amino acid composition, the actual protein requirement ultimately depends on how well the dietary protein meets the body's needs. In order to measure the nutritional quality of a protein, the concept of biological value was introduced. Biologically high-value protein contains all essential amino acids in an optimal ratio for the efficient production of body protein. This value remains relatively constant and does not change in the course of a lifetime (2). Above all, animal protein sources have a high biological value. Plant foods usually lack one or two essential amino acids, which makes them less valuable to the body. However, these so-called limiting amino acids can be supplied separately and thus increase the biological value of the food protein. A similar effect is achieved by a selected combination of different foods with different biological values.

The advantages of high biological value

If more protein with high biological value is supplied, the protein supply can be restricted. Since high protein consumption is generally associated with considerable amounts of purines, cholesterol and saturated fatty acids, which in turn promote the development of certain metabolic diseases, an optimization of protein intake is generally desired. With a high value, less degradation products of unneeded amino acids are produced (urea from ammonia). This relieves the liver and kidneys. In case of limited functionality of these organs, the dietary increase in the biological value of the dietary protein is also a therapeutic necessity.

Therapeutic use of amino acids in sarcopenia

Human energy requirements decrease with age, but the need for essential nutrients remains the same or increases. Since older people tend to eat less, it is necessary to increase the nutrient density and improve the biological value of the protein consume. Targeted nutrient supplementation is therefore of particular advantage for older people (4). Additionally, the individual amino acids demonstrated good results in the treatment of sarcopenia. Clinical studies have shown that balanced supplementation with essential amino acids can promote muscle growth in older people and thus counteract the age-associated reduction of muscle mass (5). New data show that long-term supplementation of essential amino acids, in particular leucine, can have a positive effect on muscle metabolism in old age and can be regarded as a therapeutic approach for the prevention and treatment of sarcopenia (6). The use of essential amino acids such as leucine also counteracts muscle breakdown in a generally low-protein diet (7).

L-cystine - essential for hair synthesis

The amino acid L-cystine is an important building block for the structure of hair keratin, which consists of 2 molecules of L-cysteine. 90 % of the dry weight of the hair cortex is formed from keratins (proteins) which are held together by covalent disulfide bridges between the cysteine residues and weaker dipole-dipole interactions. The keratins form filaments, which in turn form macrofibrils. In the case of hair formation disorders and hair loss, the supplementation of sulfur-containing amino acids such as cysteine or cystine in combination with other hair nutrients is recommended (8). Biotin, also known as vitamin H, promotes the storage of sulfur-containing amino acids, such as L-cystine, in the hair root and thus increases the proportion of keratin matrix proteins (8). A lack of biotin can manifest itself through hair loss (9).

Deficiency symptoms

Impact on	Symptoms
General well-being	Muscle weakness, performance weakness, depression, growth stunting
Skin	Wound healing disorders
Immune system	Increased susceptibility to infections, disorders of humoral and cellular immune defense
Muscles	Muscle degradation
Gastrointestinal tract	Atrophy of the intestinal mucosa

Indications

Effect	Indication	Dosage
Physiological effects at a low intake	To increase nutrient density and biological value of food; e.g. sarcopenia, in malnourished patients renal insufficiency	1 g protein/kg bw daily
	To cover the increased demand in athletes during growth, during diseases and during the convalescence phase	1 – 2 g proteins/kg bw daily

Administration

General mode of administration
When	Proteinogenic amino acids should be taken between meals.
Side effects
There are no known side effects to date.
Contraindications
There are no known contraindications to date.

Interactions

Drug interactions
None	No relevant interactions are known to date.
Nutrient interactions
None	No relevant interactions are known to date.

Description and related substances

Description of the micronutrient

Proteinogenic amino acids

References

1) Hahn, A. et al. Ernährung. Physiologische Grundlagen, Prävention, Therapie. 2005.
2) Reeds, P. J., Garlick, P. J. 2003. Protein and amino acid requirements and the composition of complementary foods. JNutr. 133(9):2953S-61S.
3) Breuille, D. et al. 2005. Beneficial effect of amino acid supplementation, especially cysteine, on body nitrogen economy in septic rats. Clin Nutr.
4) Li, P. et al. 2007. Amino acids and immune function. Br J Nutr. 98(2):237-52.
5) Volpi, E. et al. 2002. Essential amino acids are primarily responsible for the amino acid stimulation of muscle protein anabolism in healthy elderly adults. Am J Clin Nutr. 78(2):250-8.
6) Fujita, S., Volpi, E. 2006. Amino acids and muscle loss with aging. J Nutr. 136:277S-80S.
7) Sugawara, T. et al. 2008. Supplementation with dietary leucine to a protein-deficient diet suppresses myofibrillar protein degradation in rats. J Nutr Sci Vitaminol. 53(6):552-5.

References Interactions
Stargrove, M. B. et al. Herb, Nutrient and Drug Interactions: Clinical Implications and Therapeutic Strategies, 1. Auflage. St. Louis, Missouri: Elsevier Health Sciences, 2008.
Gröber, U. Mikronährstoffe: Metabolic Tuning –Prävention –Therapie, 3. Auflage. Stuttgart: WVG Wissenschaftliche Verlagsgesellschaft Stuttgart, 2011.
Gröber, U. Arzneimittel und Mikronährstoffe: Medikationsorientierte Supplementierung, 3. aktualisierte und erweiterte Auflage. Stuttgart: WVG Wissenschaftliche Verlagsgesellschaft Stuttgart, 2014.