SUMMARY The frequency of kidney stones, of which idiopathic calcium oxalate (CaOx) kidney stones are the most common, is increasing in the United States, affecting 9% of the population. Environmental factors, lifestyle factors, diet and metabolic or endogenous factors are known to play a role in the risk of kidney stone disease. Urinary oxalate excretion influences risk of CaOx kidney stone formation and is derived from both dietary oxalate intake and endogenous oxalate synthesis. Studies have addressed the contribution of dietary oxalate to urinary oxalate excretion in healthy subjects, and demonstrated that dietary contribution can vary between 20 to 50%. However, studies examining the importance of endogenous oxalate synthesis are lacking for patients with CaOx stone disease. Obesity is linked to CaOx kidney stone risk and may influence urinary oxalate excretion, although the underlying mechanisms are unknown. Strategies for treatment and prevention of CaOx kidney stones and research into new treatments would benefit from a better understanding of the sources of endogenous oxalate synthesis. The current proposal aims to determine if calcium oxalate kidney stone formers and subjects with obesity have increased rates of endogenous oxalate synthesis compared with healthy volunteers with normal BMI. An accurate measurement of endogenous oxalate synthesis will be performed using primed, steady-state, continuous intravenous infusion of the stable isotope of oxalate, 13C2-oxalate, and analysis by ion chromatography coupled with mass spectrometry, in subjects on a controlled low oxalate diet. 24-hr urinary oxalate excretions collected on a low oxalate, controlled diet will be compared with the measurement of the endogenous oxalate synthesis rate calculated by the continuous 13C2-oxalate intravenous infusion method. This comparison will determine if the utilization of a low oxalate diet and collection of 24 hour urine is an appropriate and easier approach to assess endogenous oxalate synthesis. The results of this pilot study will form the basis for a future R01 application to, 1), test the hypothesis that CaOx kidney stone formers have increased endogenous oxalate synthesis, 2), test the hypothesis that obesity enhances endogenous oxalate synthesis, and, 3), identify sources of endogenous oxalate synthesis that are altered in obese individuals and those with CaOx kidney stone disease, which in turn may lead to the development of novel therapeutic approaches to decrease urinary oxalate excretion.