In 28 trials of 1,358 healthy adults filtration changed no faster on more protein. In 9,226 adults tracked up to 13 years the top intake quarter carried 1.32 times the risk.
The randomised evidence applies only to people whose kidneys were healthy at the start, and early kidney disease is common and silent. Diet in the cohort came from food frequency questionnaires, which are imprecise, and its top quartile is a moderate Korean intake rather than a deliberate high-protein diet. In the cohort, a rapid decline meant the estimated filtering rate falling by more than 3 mL a minute each year, a research threshold rather than a diagnosis of kidney disease. In a sixteen-week trial at 3.2 g per kg a day, kidney and liver markers rose within the high-protein groups even though the comparison between groups came out level.
Short trials show filtration rising without declining; a long cohort saw a signal but read kidney function off creatinine, which protein itself raises. Nothing here shows harm, and nothing establishes safety over decades.