SUMMARY Non-alcoholic fatty liver disease (NAFLD) affects about 25% of the general population and is associated with other common conditions including kidney stone disease. The amount of oxalate excreted in urine is a significant risk factor for developing calcium oxalate kidney stones and may also play a role in the progression of chronic kidney disease. Previous studies have shown that about 50% of the urinary oxalate pool is derived from endogenous oxalate synthesis. Ascorbic acid (AscA) turnover and glyoxylate metabolism are important components of endogenous oxalate synthesis. A recent report demonstrated in human steatotic liver biopsies that mRNA levels of various genes regulating oxalate synthesis from glyoxylate, including alanine gloxylate aminotransferase (AGXT), are downregulated in NAFLD. Furthermore, urinary oxalate was positively associated with the severity of hepatic steatosis in overweight or obese children and adolescents with biopsy-proven NAFLD. Oxidative stress which is prevalent in those with NAFLD could promote the conversion of the anti- oxidant, AscA to oxalate. The turnover of AscA to oxalate may be heightened in NAFLD, as the progression of NAFLD is marked by increased inflammation and oxidative stress, as well as decreased plasma AscA. Thus, increased endogenous oxalate synthesis from both glyoxylate and AscA could be a major driver of kidney stone risk in those with NAFLD. This proposal will test the central hypothesis that endogenous oxalate synthesis increases with the severity of NAFLD in non-kidney stone forming adults. Leveraging our expertise in controlled dietary studies and utilization of carbon-13 precursors to study metabolism, as well as the large pool of patients with NAFLD cared for at UAB, we will test the mechanistic hypotheses that AscA turnover and the metabolism of glycolate (a glyoxylate precursor) to oxalate are increased in individuals with more severe forms of NAFLD. We expect this pilot and feasibility study will generate important preliminary data for a future multi- disciplinary/multi-institutional NIH R01 application focused on mechanisms underlying endogenous oxalate synthesis in calcium oxalate kidney stone formers with and without NAFLD.