Chemically speaking, an amino acid is a complex molecule which has a carboxyl group (formula COOH) and a functional amine group (formula NH2 or NH). Almost 500 have been identified in the living world, classified into different categories depending on their configuration.
Among them are the proteinogenic amino acids which play a crucial role as they constitute the building blocks of proteins. Common to all living beings, there are precisely 20 of them in the human body and they combine together to form peptide bonds (1).
Involved in the synthesis of structural, contractile and transport proteins, as well as immunoglobulins, enzymes and hormones (such as insulin), they play an important role in the body’s integrity and growth: maintaining homeostasis, supporting muscle development and immunity… (2)
They are distinct in having (with one exception) the same fundamental structure of H2N–HCR–COOH. In addition to the usual amino and carboxyl groups, they have a carbon atom, a hydrogen atom and a variable R chain. It’s the latter which distinguishes one amino acid from another and determines its chemical and functional properties (3).
Of these proteinogenic amino acids, nine are considered ‘essential’. In other words, our bodies are unable to synthesise them and we need to obtain them from our daily diet (4).
It should be noted that not all amino acids produced by the body are not necessarily integrated into proteins. They do, however, participate in equally crucial physiological processes. This is the case with citrulline, present in abundance in the Cucurbitaceae or gourd family, which is a key intermediate in the urea cycle (5).
The 20 amino acids present in human proteins are (6):
Leucine acts as a trigger in building muscle at a cellular level and is involved in energy production (7). It is found in meat, fish, dairy products and soya. Many athletes benefit from it by taking branched-chain amino acid supplements (BCAA) to support their performance (8).
Another of the three branched-chain amino acids in BCAA, isoleucine is found in hemoglobin and modulates how the body uses blood sugars (9). In addition to animal-source products, it is in nuts, spirulina, sunflower seeds and chickpeas.
A tyrosine precursor, phenylalanine is involved in the synthesis of catecholamines (adrenaline, noradrenaline) (10). Good sources include meat, fish, soya, oleaginous fruits and potatoes, and to a lesser degree, whole grains.
Tryptophan is used to make serotonin (‘happiness hormone’ and by extension, melatonin (‘sleep hormone’) (11). Deficiency can therefore cause sleep problems or mood disorders. Wholegrain rice, dairy products, nuts and chocolate are all good sources, which are absorbed even more effectively in the presence of sugars.
If you’re feeling irritable or down, or are not getting enough restful sleep, it might be wise to boost your dietary intake by taking a tryptophan supplement (such as the powerful L-Tryptophan) (12).
Contributing to the formation of connective tissue (elastin and collagen), threonine also plays a key role in digestion (13). Soya beans, veal, turkey and cod all contain significant amounts.
In competition with arginine, lysine is involved in many biological bone and connective tissue functions, in carbohydrate metabolism and in the production of antibodies (14). Abundant in the animal kingdom, it is also found at appreciable levels in pulses, corn and fermented foods.
While it’s rare to be lacking in lysine, prolonged intense stress or malnutrition can cause deficiency (taking a lysine supplement, such as L-Lysine, can help boost your intake) (15).
The third of the amino acids in BCAA, valine supports good physical and muscle recovery after exercise (16). It’s primarily found in cheese, pumpkin seeds, chia seeds and sunflower seeds.
Different from the other amino acids in its sulfur-containing structure, methionine underpins liver function and strong defense mechanisms (17). It requires a simultaneous intake of vitamin B12 to be fully absorbed. Nuts, beef, parmesan, fish and whole grains are all good sources.
Considered essential in children and pregnant women, histidine is an amino acid which can be partially produced by adults in general and for them it is thus regarded as semi-essential. It acts as a precursor of histamine (released in allergic reactions), and is also found in pancreatic enzymes and in hemoglobin where it neutralizes the blood pH (18). The best food sources are parmesan, chicken, bacon, tofu and steak.
Generally speaking, amino acids are found in animal protein (meat, fish, eggs, dairy products) and plant protein (grains, legumes, nuts…) (19).
However, it’s animal protein which contains the 9 essential amino acids in sufficient amounts (giving it a higher biological value). This is rarely the case for plants except for soya and its derivatives, quinoa, and buckwheat (20-21).
Grains are generally deficient in lysine and legumes in methionine (22-23). These amino acid deficits can be problematic as they limit protein synthesis.
Vegetarians and vegans are thus advised to ingest both at the same meal (rice with kidney beans, semolina with chickpeas… ) (24). Combining in this way is perfectly achievable for adults over a day.
As we’ve seen with histidine, certain amino acids are considered to be semi-essential: they can theoretically be produced by the body but only under certain conditions. Endogenous production may prove insufficient, especially in early childhood or in the case of underlying disease (kidney failure, for example).
Arginine, cysteine, glutamine, glycine, proline and tyrosine all fall into this category and should therefore be provided by the diet when metabolism requires it (25-26). Supplementing with certain semi-essential amino acids can provide a useful boost in some cases: click on the name of a particular amino acid in those listed above to find the corresponding supplement (27-28).
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