3-Aminoisobutyric acid is a non-protein amino acid formed during thymine catabolism.¹ It induces browning in primary adipocytes, increasing the expression of uncoupling protein 1 (UCP-1) and CIDEA when used at a concentration of 5 µM. It also increases the expression of PPARα in primary adipocytes and in mouse inguinal white adipose tissue (WAT) in vivo and increases β-oxidation in hepatocytes. Plasma levels of 3-aminoisobutyric acid are increased in mice following exercise and, when administered at a dose of 100 mg/kg per day, it decreases weight gain and body fat percentage without decreasing food intake or increasing energy expenditure, and improves glucose tolerance. Levels of 3-aminoisobutyric acid are elevated in the plasma, urine, and cerebrospinal fluid of patients with β-ureidopropionase deficiency, a disorder characterized by an inborn error of pyrimidine degradation.²