In our study, replacing carbohydrates with fat and protein increased GFR as estimated by cystatin C and β2M. These results are consistent with previous studies that test dietary effects on GFR [7]. Animal and human studies have indicated that dietary protein increases renal blood flow and glomerular filtration rates via increased intraglomerular pressures, leading to progressive glomerular sclerosis, particularly in the setting of decreased nephron mass [15–19]. These effects may be mediated by dietary effects on signaling molecules of the tubuloglomerular feedback system, responsible for arteriolar constriction [20–22]. It is also believed that dietary protein may increase neuronal nitric oxide synthase in the kidney cortex, which relaxes the afferent renal arteriole [23, 24]. Although fat was also higher in the low carbohydrate/high protein diet, there is much less evidence for a relationship between a higher fat diet and hyperfiltration [25], though some have hypothesized that such an association could be mediated through inflammation [26, 27].