L-Isoleucine

In muscle cells, isoleucine is oxidized by branched-chain aminotransferase and branched-chain ketoacid dehydrogenase; the carbon skeleton can proceed via succinyl-CoA and acetyl-CoA. During prolonged exercise, muscle preferentially uses this class of branched-chain amino acids. Eggs, whey, and poultry are common sources; the body cannot synthesize it, so intramuscular energy comes from this branched-chain residue.

Isoleucine can promote glucose transport in muscle cells, helping post-exercise blood glucose enter muscle and affecting glucose homeostasis. Among the three branched-chain amino acids, its role in glucose disposal is more prominent. After resistance training it still often appears with the other two in blends; isoleucine itself leans toward glucose disposal and muscle energy.