Weight gain is associated with an increase in intrahepatic triglycerides (IHTGs), and is the primary cause of nonalcoholic fatty liver disease in obese individuals. We combined imaging and stable isotope tracer techniques to evaluate the physiologic mechanisms of weight gain−induced steatosis in 27 obese people. Weight gain appeared to increase IHTG content by generating an imbalance between hepatic fatty acid availability and disposal, and resulted in increased hepatic de novo lipogenesis, decreased intrahepatic fatty acid oxidation, and inadequate increases in IHTG export via very low-density lipoprotein secretion.