Leucine and isoleucine are two distinct amino acids, substances occurring naturally in the foods humans eat that are the main components of dietary protein. Both are considered essential amino acids, meaning that the body cannot manufacture these compounds on its own and therefore they must be consumed in food. Also, both leucine and isoleucine are readily available in many protein-rich plant and animal foods like walnuts, almonds, soybeans, eggs, meat, and milk and are popular in supplement form as well. It is important to note 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 manufactured by plants and some microorganisms, a process involving the conversion of pyruvic acid and involving alpha-ketoglutaric acid as an intermediary. Pyruvic acid is a substance made naturally as a part of the metabolism of carbohydrates and fats, while alpha-ketoglutaric acid is another metabolic compound. Both facilitate aerobic respiration in the cells. Leucine, while also synthesized by microorganisms and plants and involving the metabolism of pyruvic acid, requires the involvement of different intermediary 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 jumpstart chemical reactions, are needed: acetolactate synthase, acetohydroxy acid isomeroreductase, dihydroxy acid dehydratase, and valine aminotransferase. By contrast, leucine synthesis 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 soy, peanuts, wheat germ, and almonds. Plant foods that are dense with isoleucine include soy and seaweed. Animals farmed for food tend to consume a lot of amino-acid-containing plant foods, including corn, wheat, and soy. Likewise, farmed fish are fed seaweed. For this reason, plenty 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 that 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 with bodybuilders and other weightlifters. In addition, both are stored by the body in the liver, in muscle, and in stored body fat. Only leucine, however, has been shown to encourage the manufacture of protein in muscle cells, which is necessary to muscle hypertrophy, or growth.