Obesity is increasingly recognized as a risk factor for the development of end-stage renal disease. However, few studies have investigated the influence of dietary changes on kidney function in the obese. Friedman et al. report that dietary protein intake is unlikely to fully account for elevations in glomerular filtration rate and proteinuria observed in obesity. The relationship between diet and renal hemodynamics in obesity is complex, and more studies (including ones on the effect of caloric restriction) are needed.