Branched fatty acid esters of hydroxy fatty acids (FAHFAs) are a class of endogenous lipids whose levels are modulated by fasting and high-fat diets, and they play a role in insulin sensitivity. These compounds consist of a fatty acid—either a C-16 or C-18, such as palmitoleic, palmitic, oleic, or stearic acid—esterified to a hydroxylated C-16 or C-18 lipid. One notable FAHFA, 9-PAHSA, features an ester linkage between palmitic acid and 9-hydroxy stearic acid. PAHSAs, with 9-PAHSA being the most prevalent isomer, are significantly found in the serum and both white and brown adipose tissues of glucose-tolerant AG4OX mice, which express the Glut4 gene in adipose tissue, enhancing insulin sensitivity. Additionally, 9-PAHSA is abundant in wild type and AG4OX mice and present in humans, though at reduced levels in those with insulin resistance. 9-PAHSA is associated with improved glucose tolerance, enhanced insulin secretion, and anti-inflammatory effects in mice. The compound 19-PAHSA^13C4 represents an isotopically enriched form of this polyunsaturated fatty acid.