Isoleucine

Studies indicate Isoleucine can strengthen the immune system, particularly by helping produce β-defensins, which are peptides that fight off infections and support both innate and adaptive immunity.
Isoleucine is crucial for making muscle proteins, which helps with muscle growth and recovery after exercise. It also seems to help with how our muscles use glucose, potentially aiding energy levels and reducing muscle soreness post-workout.
There's evidence suggesting Isoleucine helps muscles take in glucose and break it down for energy, which might help manage blood sugar levels.
Enhances immune function by inducing the expression of host defense peptides, specifically β-defensins.
Reduces post-exercise muscle soreness and markers of muscle damage when taken as part of BCAAs, though it is less effective than Leucine.

Muscle Growth & Recovery	Essential for muscle protein synthesis, reduces post-exercise muscle soreness and damage when part of BCAAs, but less effective than leucine. Optimal synthesis requires all nine essential amino acids.
Glucose Metabolism	Enhances glucose uptake by muscle cells, increases glucose breakdown into energy, reduces hepatic gluconeogenesis in rodents. Rat dosages of 0.3–0.45 mg/kg effective for reducing blood glucose, extrapolated to human dose of 48–72 mg/kg (3.3–4.9 g for 150 lb person). Inconsistent human trial results.
Other Functions	Acts as a signaling molecule, regulates protein, glucose, lipid metabolism, glucose transport, and immune function.
Potential Negative Associations	Increased blood levels associated with insulin resistance, type 2 diabetes, cardiovascular disease, and obesity in observational studies. Cause-effect unclear.
Side Effects	Gastrointestinal upset (nausea, diarrhea) when taken as part of BCAAs. Side effects of isoleucine alone undetermined.
Recommended Intake	RDA: 19 mg/kg bodyweight daily. No strong evidence for supplementing isoleucine alone above RDA.