Leucine and isoleucine are essential amino acids found in many protein-rich plant and animal foods, and are important for tissue growth and repair. They have different chemical formulas and functions in the body, and are synthesized differently in plants. Leucine stimulates protein manufacturing in muscle cells, while isoleucine is only made by plants and some microorganisms.
Leucine and isoleucine are two different amino acids, substances that are found naturally in the foods humans eat and are the main components of dietary protein. Both are considered essential amino acids, which means that the body cannot manufacture these compounds on its own, and therefore they must be consumed in food. Additionally, both leucine and isoleucine are readily available in many protein-rich plant and animal foods, including walnuts, almonds, soybeans, eggs, meat, and milk, and are also popular in supplement form. It is important to note that the two have different chemical formulas, are derived from different source compounds, require the action of different enzymes to be synthesized, and have slightly different functions in the body.
Isoleucine is only made by plants and some microorganisms, a process that involves the conversion of pyruvic acid and alpha-ketoglutaric acid as an intermediate. Pyruvic acid is a substance made naturally as part of carbohydrate and fat metabolism, while alpha-ketoglutaric acid is another metabolic compound. Both facilitate aerobic respiration in cells. Leucine, although it is also synthesized by microorganisms and plants and involves the metabolism of pyruvic acid, requires the participation of different intermediate substances, namely alpha-ketoisovaleric acid.
Leucine and isoleucine also differ in the enzymes required to facilitate their synthesis in plants. To create isoleucine from pyruvic acid, four enzymes or substances that speed up chemical reactions are needed: acetolactate synthase, acetohydroxy acid isomeroreductase, dihydroxy acid dehydratase, and valine aminotransferase. In contrast, the synthesis of leucine requires the first three in addition to alpha-isopropylmalate synthase, alpha-isopropylmalate isomerase, and leucine aminotransferase.
Plant foods that synthesize a large amount of leucine include soybeans, peanuts, wheat germ, and almonds. Plant foods that are dense with isoleucine include soybeans and algae. Farm animals tend to eat a lot of plant foods that contain amino acids, such as corn, wheat, and soybeans. Similarly, farmed fish are fed algae. For this reason, a large amount of leucine and isoleucine are transferred to animal foods eaten by humans, including eggs, poultry, beef, lamb, and fish.
These two amino acids also perform slightly different functions in the human body once consumed. Along with the amino acid valine, leucine and isoleucine are known as branched-chain amino acids (BCAAs), meaning they have an extra carbon-based side chain in their molecular structure. BCAAs are associated with tissue growth and repair, making them popular as a supplement among bodybuilders and other weightlifters. Furthermore, both are stored by the body in the liver, in the muscles, and in stored body fat. However, only leucine has been shown to stimulate protein manufacturing in muscle cells, which is necessary for muscle hypertrophy or growth.
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