Effect of glycemic index and carbohydrate intake on kidney function in healthy adults

Background: Replacing carbohydrate with protein acutely increases glomerular filtration rate (GFR) but is associated with faster, long-term kidney disease progression. The effects of carbohydrate type (i.e. glycemic index, GI) on kidney function are unknown. Methods: We conducted an ancillary study of a randomized, crossover feeding trial in overweight/obese adults without diabetes or kidney disease (N = 163). Participants were fed each of four healthy, DASH-like diets for 5 weeks, separated by 2-week washout periods. Weight was kept constant. The four diets were: high GI (GI >= 65) with high % carb (58 % kcal) (reference diet), low GI (<= 45) with low % carb (40 % kcal), low GI with high % carb; and high GI with low % carb. Plasma was collected at baseline and after each feeding period. Study outcomes were cystatin C, beta 2-microglobulin (beta 2M), and estimated GFR based on cystatin C (eGFRcys). Results: Mean (SD) age was 52 (11) years; 52 % were women; 50 % were black. At baseline, mean (SD) cystatin C, beta 2M, and eGFRcys were 0.8 (0.1) mg/L, 1.9 (0.4) mg/L, and 104 (16) mL/min/1.73 m(2). Compared to the high GI/high %carb diet, reducing GI, % carb, or both increased eGFRcys by 1.9 mL/min/1.73 m(2) (95 % CI: 1.1, 2.7; P < 0.001), 3.0 mL/min/1.73 m(2) (1.9, 4.0; P < 0.001), and 4.5 mL/min/1.73 m(2) (3.5, 5.4; P < 0.001), respectively. Increases in eGFRcys from reducing GI were significantly associated with increases in eGFRcys from reducing % carb (P < 0.001). Results for cystatin C and beta 2M reflected eGFRcys. Conclusions: Reducing GI increased GFR. Reducing % carb by increasing calories from protein and fat, also increased GFR. Future studies on GI should examine the long-term effects of this increase in GFR on kidney injury markers and clinical outcomes.