beta-Aminoisobutyric acid is a non-protein amino acid originating from the catabolism of thymine and valine. The concentration of beta-aminoisobutyric acid is normally low in urine as beta-aminoisobutyric acid is further catabolized by beta-aminoisobutyrate aminotransferases to methylmalonic acid semialdehyde and propionyl-CoA. beta-Aminoisobutyric acid occurs in two isomeric forms and both enantiomers of beta-aminoisobutyric acid can be detected in human urine and plasma. In plasma, the S-enantiomer is the predominant type due to active renal reabsorption. In contrast, urine almost exclusively contains the R-enantiomer of beta-aminoisobutyric acid, which is eliminated both by filtration and tubular secretion. Persistently increased levels of beta-aminoisobutyric acid have been observed in individuals with a deficiency of R (-)-beta-aminoisobutyrate-pyruvate aminotransferase. In addition, transient high levels of beta-aminoisobutyric acid have been observed under a variety of pathological conditions such as lead poisoning, starvation, in total body irradiation, and in a number of malignancies. The S-enantiomer of beta-aminoisobutyric acid is predominantly derived from the catabolism of valine. It has been suggested that altered homeostasis of beta-alanine underlies some of the clinical abnormalities encountered in patients with a dihydropyrimidine dehydrogenase (DPD) deficiency. DPD constitutes the first step of the pyrimidine degradation pathway, in which the pyrimidine bases uracil and thymine are catabolized to beta-alanine and the R-enantiomer of beta-aminoisobutyric acid respectively. In normal individuals with an intact pyrimidine degradation pathway, R-methylmalonic acid semialdehyde can be synthesized directly from the catabolism of thymine. Hence, there might be less cross-over between the valine and thymine pathway, allowing the conversion of S-methylmalonic acid semialdehyde into S-beta-aminoisobutyric acid and the subsequent accumulation of S-beta-aminoisobutyric acid in plasma (PMID: 14705962, 14292857, 14453202).