Background: Consumption of a diet high in protein can cause glomerular hyperfiltration, a potentially maladaptive response, which may accelerate the progression of kidney disease. Study Design: An ancillary study of the OmniHeart trial, a randomized 3-period crossover feeding trial testing the effects of partial replacement of carbohydrate with protein on kidney function. Setting & Participants: Healthy adults (N=164) with prehypertension or stage 1 hypertension at a community-based research clinic with a metabolic kitchen. Intervention: Participants were fed each of 3 diets for 6 weeks. Feeding periods were separated by a 2- to 4-week washout period. Weight was held constant on each diet. The 3 diets emphasized carbohydrate, protein, or unsaturated fat; dietary protein was either 15% (carbohydrate and unsaturated fat diets) or 25% (protein diet) of energy intake. Outcomes: Fasting serum creatinine, cystatin C, and ?2-microglobulin levels, estimated glomerular filtration rate (eGFR). Measurements: Serum creatinine, cystatin C, and ?2-microglobulin collected at the end of each feeding period. Results: Baseline cystatin C-based eGFR was 92.0�16.3 (SD) mL/min/1.73 m2. Compared with the carbohydrate and unsaturated fat diets, the protein diet increased cystatin C-based eGFR by ?4 mL/min/1.73 m2 (P < 0.001). The effects of the protein diet on kidney function were independent of changes in blood pressure. There was no significant difference between the carbohydrate and unsaturated fat diets. Limitations: Participants did not have kidney disease at baseline. Conclusions: A healthy diet rich in protein increased eGFR. Whether long-term consumption of a high-protein diet leads to kidney disease is uncertain.