In this trial, both cystatin C and β2M were affected by OMNICARB diets in a similar fashion, that is, lower %carb/higher protein or lower GI decreased plasma concentrations of both cystatin C and β2M. In contrast, creatinine did not decline with lower GI and further increased during the low %carb (higher protein)/high GI diet. In absence of measured GFR, this could be interpreted as either non-GFR effects on both cystatin C and β2M or non-GFR effects on creatinine. These observed effects had important ramifications on estimated GFR, which was significantly increased when estimated using cystatin C, but null or decreased when estimated using creatinine. Given the independent biosynthetic pathways of each of these three markers, it is more probable that non-GFR effects would influence one rather than two markers [10], that is in this case, creatinine versus both cystatin C and β2M. This underscores the value of non-traditional markers in better estimating changes in GFR [10, 44].