Isoleucine is usually well-tolerated, but some people may experience gastrointestinal upset (e.g., nausea, diarrhea).[1] Importantly, these side effects have been observed when isoleucine was taken as part of a BCAA supplement; the potential side effects of isoleucine when supplemented alone are undetermined.
Observational studies have found that increased levels of isoleucine in the blood are associated with certain health conditions, including insulin resistance,[2] type 2 diabetes,[3][2] cardiovascular disease,[4][5] and obesity.[6][7] Further research is needed to determine whether these elevated levels of isoleucine are a cause or an effect of these conditions.