Essential amino acid supplementation

Amino acids can be supplemented in either free or isolated form. In other words, in unitary molecules of a single amino acid, or in groups of bonded amino acids known as peptides.

Role in metabolism

Proteins are linear molecules made up of amino acids (AA) linked by peptide bonds. They contain carbon, hydrogen, oxygen, and nitrogen, and sometimes also sulfur and phosphorous. Once ingested, they are broken down in digestion, with the amino acids being the micronutrients absorbed by the body. Absorption of these amino acids depends on proper protein digestion which starts with adequate chewing, and acid production in the stomach. Pancreatic enzymes and a healthy gut microbiota also influence the process. The nervous and digestive systems are closely related. Acute and chronic stress affects digestion and the composition of the microbiota.

The most important functions of each essential amino acid are the following:

• BCAAs (leucine, isoleucine, and valine) are essential amino acids and basic micronutrients, and they account for 15% to 25% of total daily protein intake. They act in multiple metabolic processes that stimulate protein synthesis, formation of proteins, precursors of alanine and glutamine synthesis, and maintenance of glucose homeostasis. In strenuous exercise, taking BCAAs seems to decrease muscle degradation and delay muscle glucogen degradation. BCAAs can cross the blood-brain barrier.
• Threonine is a basic micronutrient and a glycine precursor. Glycine is the main inhibiting neurotransmitter in the spinal medulla. But glycine alone hardly penetrates the central nervous system; threonine, on the other hand, has better penetration and stimulates glycine synthesis in the brain. Increased glycine synthesis in the brain counteracts the negative effects of excitatory neurotransmitters such as glutamate.
• Phenylalanine is a basic micronutrient and an essential amino acid whose metabolic fate (apart from its incorporation in proteins) is exclusively to be transformed into tyrosine.
• Tyrosine is a basic micronutrient and has high metabolic responsibility, as it is a precursor of various indispensable substances such as catecholamines (dopamine, adrenaline, and noradrenaline), the thyroxine hormone, and other products like melanin, the pigment responsible for hair and skin coloration. If we tried to cover all those needs exclusively with phenylalanine, we would be overloading the phenylalanine-to-tyrosine conversion reaction (through phenylalanine hydroxylase), a complex reaction that depends on the tetrahydrobiopterin coenzyme. When liver or kidney function are affected, or in situations of acute stress, the conversion of phenylalanine to tyrosine can be deficient. Low levels have been associated with low blood pressure, low body temperature, and hypothyroidism. Coenzyme Q10 deficiencies can be related to tyrosine deficiencies.
• Methionine and cysteine are interconvertible basic micronutrients and considered “nutritionally equivalent amino acids.” Cysteine is necessary for glutathione and taurine synthesis. Three amino acids are needed to form glutathione (cysteine, glutamate, and glycine), and the limiting amino acid is normally cysteine because it is less abundant in the diet.
• Tryptophan is a basic micronutrient and an essential amino acid that is present in concentrations of 1% to 2% in many plant and animal proteins. A typical diet contains 0.5–2.0 g of tryptophan. The body absorbs tryptophan from protein sources in the diet and first converts it to 5-HTP, then to serotonin, and finally to melatonin. Depletion of endogenous tryptophan can cause a relapse in a treated depression and trigger depressive symptoms in patients with a family history of depression, as well as in healthy individuals. Tryptophan and tyrosine are two key amino acids in mood disorders.
• Lysine is a basic micronutrient and an essential amino acid that is necessary for carnitine synthesis and fat metabolism. It is an amino acid that incorporates proteins that are often cross-linked, such as collagen and elastin. Copper deficiency, therefore, can result in imperfections in collagen or elastin. Lysine antagonizes growth of the herpes simplex virus (HSV), and this effect can be clinically significant. Arginine antagonizes this effect. A frequently limiting amino acid, the lysine content in grains is low and easily altered by cooking and freezing foods. Vegan diets are often deficient in lysine.

Foods with essential amino acids

All protein foodstuffs, whether of animal or plant origin, supply essential amino acids to a greater or lesser degree.

Chicken, turkey, meat, and eggs supply all of the essential amino acids.

Not all plants, on the other hand, with the exception of soybeans, supply essential amino acids in adequate amounts. For example, lentils have a limited amount of the essential amino acid methionine, and grains such as brown rice contain hardly any lysine or threonine.

Therefore, if we combine lentils (rich in lysine, low in methionine) with brown rice (rich in methionine, low in lysine), we create a dish with a complete protein.