Branched-chain amino acids and innate immunity in metabolic disease: Mechanisms, paradox, and therapeutic opportunities

Metabolic diseases such as obesity, type 2 diabetes mellitus (T2DM), dyslipidaemia, and metabolic dysfunction-associated steatotic liver disease (MASLD) are increasingly recognised as chronic inflammatory conditions driven in part by innate immune dysregulation. Among the metabolic factors implicated in this process, branched-chain amino acids (BCAAs) have emerged as key regulators linking nutrient sensing to immune cell function. Circulating BCAA concentrations are consistently elevated in these metabolic diseases. However, experimental and clinical studies also show that BCAA supplementation can improve metabolic and immune outcomes in specific contexts, revealing a paradoxical relationship between BCAA metabolism and inflammation. This narrative review examines how dysregulated BCAA metabolism and accumulation of branched-chain keto acids (BCKAs) shape the functional programming of innate immune cells across these conditions, including monocytes/macrophages, granulocytes, dendritic cells, and natural killer cells. Evidence indicates that the immunometabolic effects of BCAAs depend not solely on circulating concentrations but on the efficiency of their intracellular catabolism. When BCAA oxidation is preserved, these amino acids support mitochondrial metabolism and immune competence. Conversely, impaired catabolism leads to the accumulation of branched chain ketoacids, which activate inflammatory pathways and contribute to metabolic dysfunction. Resolving this paradox requires the need of targeting catabolic flux restoration rather than simple BCAA restriction or supplementation, and requires stratifying patients by enzymatic capacity, BCAA/BCKA ratios, and disease stage. Pharmacological modulators, including BCKDK inhibitors and BCAT1-targeted agents, show promise in simultaneously addressing metabolic and immune dysregulation.