Leucine and isoleucine, along with valine, make up the branched-chain amino acids. These amino acids are of interest to athletes, as they may aid in muscle recovery after a strenuous workout, according to Dr. Yoshiharu Shimomura in the February 2006 volume of “The Journal of Nutrition.” On the other hand, avoiding these amino acids is critical if you suffer from maple sugar urine disease, a disorder in which your body does not properly metabolize the branched-chain amino acids. While many protein-rich foods contain leucine and isoleucine, certain foods are particularly abundant in these amino acids.
Soy products offer a unique source of protein because they are the only plant-based foods containing nutritionally complete proteins. In other words, soy proteins supply all the essential amino acids your body cannot synthesize and must receive from your diet, explains Dr. Aaron Michelfelder of Loyola University Chicago Stritch School of Medicine. Soy protein isolate, a concentrated form of soy protein found in protein supplements and in meat and cheese substitutes, provides 1.9 g of leucine and 1.5 g of isoleucine per 1-oz. serving, making this soy product one of the richest food-based sources of branched-chain amino acids. Tofu and soy milk, in comparison, each contain less than 0.2 g of leucine and isoleucine per ounce.
Meats and Fish
Meats and fish contain nutritionally complete proteins with plentiful branched-chain amino acids. A 1-oz. portion of beefsteak provides nearly 0.7 g of leucine and 0.4 g of isoleucine, and the same amount of pork roast supplies slightly less leucine and the same amount of isoleucine. Tuna fish is also abundant in these amino acids, with 0.5 g of leucine and 0.3 g of isoleucine per ounce. Roast chicken and roast turkey each contain 0.4 g of leucine and almost 0.3 g of isoleucine in a 1-oz. serving.
Dairy Products and Eggs
The leucine and isoleucine content of cheddar cheese mimics that of beefsteak, with 0.7 and 0.4 g of each, respectively, per ounce. Low-fat cottage cheese is also a rich source of these amino acids, containing 0.4 g of leucine and 0.2 g of isoleucine in a 1-oz. portion. Ounce for ounce, eggs contain a similar amount of branched-chain amino acids as cottage cheese, with the bulk of these nutrients located in the egg white. In comparison, milk and yogurt offer 0.2 g of leucine and 0.1 g of isoleucine per ounce.
Legumes, while relatively protein-rich, do not supply nutritionally complete proteins to your diet. However, they do offer moderate amounts of the branched-chain amino acids. Lentils, black beans and pinto beans provide just under 0.2 g of leucine and nearly 0.1 g of isoleucine per ounce. While most animal sources of proteins provide greater amounts of branched-chain amino acids than plant sources of proteins, legumes contain a fair amount of these nutrients in a fiber-rich, low-fat package.