Isoleucine is essential for muscle growth and may play a role in muscle recovery, similar to other essential amino acid, but it’s unclear whether supplementing isoleucine alone above the RDA provides any particular benefits.
Isoleucine is necessary for the process of muscle protein synthesis, and BCAAs in general are thought to be particularly important for stimulating this process while also minimizing protein breakdown.[1][2] However, compared to leucine, isoleucine does not stimulate muscle protein synthesis to a significant degree, and ultimately, research suggests that optimal muscle protein synthesis is achieved when all nine essential amino acid are present.[3][4] In terms of muscle recovery, when isoleucine is taken as part of a BCAA supplemen,t it seems to reduce post-exercise muscle soreness and markers of muscle damage, but may not impact the recovery of muscle performance.[5][6][7]
Animal studies suggest that isoleucine enhances the uptake of glucose by muscle cells and increases the breakdown of glucose into energy.[8] While theoretically this might be beneficial for exercise performance, studies providing isoleucine as part of a BCAA supplement have not consistently found a positive effect on performance, and isoleucine has generally not been studied alone.[7]