A controlled trial demonstrated that regular use of BCAAs by adolescents and young adults with PKU improved performance on some tests of mental functioning. This outcome makes sense because BCAAs may compete with phenylalanine, reducing its toxic effects. In tardive dyskinesia , phenylalanine levels have also been reported to be elevated. Dairy products and red meat contain the greatest amounts of BCAAs, although they are present in all protein-containing foods. Whey protein and egg protein supplements are other sources of BCAAs.
BCAA supplements provide the amino acids leucine, isoleucine, and valine.
Only an individual deficient in protein would become deficient in BCAAs, because most food sources of protein supply BCAAs. Few people in Western societies are protein deficient. Side effects have not been reported with the use of BCAAs, except in the one study of ALS described above. At high intakes, BCAAs are simply converted into other amino acids, used as energy, or converted to fat for storage. However, individuals with kidney or liver disease should not consume high intakes of amino acids without consulting their doctor.