The glycerol phosphate shuttle was first characterized as a major route of mitochondrial hydride transport in the flight muscles of blow flies. It was initially believed that the system would be inactive in mammals due to the predominance of lactate dehydrogenase activity over Glycerol-3-phosphate dehydrogenase 1 (GPD1) until high GPD1 and GPD2 activity were demonstrated in mammalian brown adipose tissue and pancreatic ß-islets.