Amino acids are the building block of proteins. They are organic compounds composed of an amino group (-NH2) and a carboxylic acid (-COOH) functional group.
In nature, there are hundreds of amino acids but only 20 are needed to make all the proteins found in the human body as well as hormones, muscles and other cellular processes. These 20 amino acids are divided into non-essential and essential amino acids; non-essential amino acids, also known as dispensable amino acids, are those that the human body can synthetise, so they are excluded from the diet. On the other hand, the essential amino acids, also known as indispensable amino acids, cannot be synthetised by the human body. Therefore, they are supplied from the diet.
The nine essential amino acids include phenylalanine, valine, tryptophan, threonine, isoleucine, methionine, histidine, leucine and lysine.
Phenylalanine: this amino acid is typically found in mother’s milk, meat, fish, cottage cheese, peanuts and sesame seeds. It is the precursor of dopamine, which is a neurotransmitter that plays a key role in regulating mood, motivation, satiation and sex drive.
Research studies have established that phenylalanine has antidepressant effects. When the body is deprived from this essential amino acid such as in the genetic disorder called phenylketonuria (PKU), dopamine synthesis is reduced. As a result, low dopamine levels cause depression and lethargy. This is why phenylalanine has been shown to have antidepressant activity.
Valine: is found in meat, fish, eggs, dairy, beans and tempeh. These food sources typically contain 40-60 mg/g of valine. Valine deficiency is a result of malnutrition, and the consequences are growth failure, loss of muscle mass and organ damage.
Tryptophan: is found in tuna, turkey, chicken and cheese. It is used to synthetise serotonin via a chemical reaction in which tryptophan is converted into 5-hydroxytryptophane and then into serotonin. Serotonin is a very important hormone responsible for regulating our mood. For this reason, as tryptophan is the precursor of serotonin, is used for the treatment of depression, insomnia, stress and behavioural disorders.
Threonine: is found in high quantity in red meat such as beef, lamb and pork. It is very important for the formation of collagen, elastin and muscle tissue. It helps to strengthen bones and speed up wound healing.
Isoleucine: is found in meat, fish, eggs, cheese, nuts and seeds. It is critical for some physiological functions such as growth, immunity and glucose transportation. Several studies have shown that isoleucine improves the immune system and restores the effect of some pathogens on the health of humans.
Methionine: is predominately found in red meat. Lower methionine foods are rice, wholegrains, nut butter, sunflower seeds and chia seeds. Its function is to form cartilage in the body and therefore prevent hair loss and nail damage. It is important to consume an adequate amount of methionine from the diet (17-27 mg/g of protein), but not too much methionine, as this amino acid stimulates cancer cell growth and metabolism.
A growing body of evidence shows that a low methionine diet helps starve cancer cells. This has been shown in the laboratory: both cancer cells and normal cells were grown on a Petri dish. It was observed that in absence of methionine normal cells thrived, whereas cancer cells died. This was called “absolute methionine dependency”. The reason why cancer cells were not surviving was shown to be related to their need of sulphur compounds, and methionine is the only sulphur compound among the nine essential amino acids. Sulphur compounds are molecules that produce a compound called SAM that alters the DNA, causing cancer cell growth. As a result, it was concluded that a low methionine diet based on the consumption of more plant sources helps to prevent cancer-related diseases.
Histidine: is found in yogurt, sauerkraut, kimchi, kombucha and fermented meat. It is also found in breast milk. This amino acid is used to treat rheumatoid arthritis and anaemia in patients with chronic renal failure. It is also administrated to prevent fatigue during strenuous exercise.
Leucine: is found in high amount in red bell peppers. It displays anabolic effects on cell signalling and protein synthesis in the muscle. Therefore, it stimulates skeletal muscle anabolism, preventing loss of muscle mass. It is recommended to those with lower protein intake and particularly the elderly, as they are more likely to suffer from reduced muscle mass (sarcopenia) or muscle atrophy.
Lysine: is found in red meat, fish, dairy and chickpeas. It is important for the growth, and it plays a key role in the production of carnitine, a nutrient responsible for converting fatty acids into energy and helping lower the cholesterol. Several studies have also shown that it slows or stops the growth of viruses, preventing the sexual transmission of the herpes virus.
A protein that contains all of these amino acids is defined as complete. Complete proteins are mainly found in animal sources such as meat, fish, milk and eggs, but they can also be found in some plant sources such as soya (tofu is an example), quinoa, buckwheat, hemp, chia seeds, spirulina, tempeh and amaranth. These are also known as superfoods.
Nevertheless, the majority of plant proteins are incomplete. For example, rice, corn and wheat are all deficient in lysine; beans, peas and other legumes are deficient in methionine. For this reason, vegan and vegetarians are recommended to combine different protein sources in order to obtain a complete protein.
Common combinations are rice with beans, hummus with pita bread, refried beans with tortilla or toast with nut butter.
To sum up, within this article, we want to empathize the importance of consuming a balanced diet that includes complete protein sources. Whether you are omnivorous or vegetarian, there are complete protein sources that we recommend you to live a happy, long and healthy life.