Higher protein does not harm healthy kidneys, meta-analysis of 28 trials finds

The persistent belief that eating a lot of protein damages healthy kidneys is not supported by the evidence: a meta-analysis of 28 controlled trials found that higher-protein diets do not adversely affect kidney function in adults without kidney disease [s1]. The idea comes from conflating two very different situations — a healthy kidney, and a kidney that is already diseased — and the distinction is the whole answer.

The evidence in healthy people

The question matters because higher-protein diets are widely advocated — for building or preserving muscle, for managing weight, and, in older adults, for countering the age-related muscle loss called sarcopenia — which means a lot of people are deliberately eating more protein and wondering whether their kidneys will pay for it [s1]. The relevant synthesis is a systematic review and meta-analysis in The Journal of Nutrition that specifically asked whether high-protein intakes harm kidney function in healthy adults [s1]. It pooled randomised controlled trials comparing higher-protein diets — defined as at least 1.5 g/kg of body weight, or at least 20% of energy, or at least 100 g of protein a day — against normal- or lower-protein intakes, measuring glomerular filtration rate (GFR), the standard index of kidney filtering [s1]. Twenty-eight trials with 1,358 participants met the criteria [s1].

The headline finding is a non-finding, which is exactly the point. When researchers compared the change in GFR from before to after the diets, there was no significant difference between higher- and lower-protein groups (standardised mean difference 0.11, 95% confidence interval −0.05 to 0.27, P = 0.16) [s1]. A comparison of GFR measured only at the end of the trials did show a slightly higher GFR after high-protein diets, but the authors describe it as a "trivial effect" (SMD 0.19, 95% CI 0.07 to 0.31) — and a higher GFR is, if anything, the opposite of the kidney damage the myth predicts [s1]. Their conclusion is direct: high-protein intakes do not adversely influence kidney function in healthy adults [s1].

The distinction between those two comparisons is worth spelling out. The "end-only" snapshot showed a small linear relationship between protein intake and GFR (r = 0.332, P = 0.03), whereas the more informative before-and-after comparison — which controls for where each group started — showed no such relationship (r = 0.184, P = 0.33) [s1]. The authors read the pattern as a higher filtration rate that does not amount to adverse influence on kidney function, and they are careful to flag a real limitation of their own analysis: the risk of selection bias in the underlying trials was unclear, so this is a reassuring signal rather than the last word [s1].

Why the myth is sticky

The confusion is understandable, because protein restriction is a real and long-studied intervention — for people who already have chronic kidney disease. A Cochrane review examined low-protein diets in non-diabetic adults with established CKD, pooling 17 studies with nearly 3,000 participants who had CKD and were not yet on dialysis [s2].

Even there, the benefit is narrower than the fear implies. Compared with a normal-protein diet, a low-protein diet made little or no difference to the number of patients who died (risk ratio 0.77, 95% CI 0.51 to 1.18) or who reached end-stage kidney disease (RR 1.05, 95% CI 0.73 to 1.53) [s2]. A signal appeared only at the extreme: a very low-protein diet, compared with a merely low-protein one, probably reduced the number reaching end-stage kidney disease (RR 0.65, 95% CI 0.49 to 0.85; 165 fewer per 1,000; moderate-certainty evidence) [s2]. In other words, protein restriction is a targeted therapy for advanced kidney disease, with limited and graded evidence — not a general rule that applies to people whose kidneys are healthy.

What this leaves a reader with

For someone with normal kidney function, the trial evidence does not show that a higher-protein diet harms the kidneys [s1]. For someone who already has chronic kidney disease, protein intake becomes a genuine clinical variable that is managed with a clinician and often a dietitian, because the right amount depends on the stage of disease [s2]. Taking advice meant for the second group and applying it to the first is the error at the heart of the myth. Anyone unsure which group they are in can find out with the standard kidney tests rather than by cutting protein pre-emptively.

This article is informational and is not medical advice.

Sources

• Changes in Kidney Function Do Not Differ between Healthy Adults Consuming Higher- Compared with Lower- or Normal-Protein Diets: A Systematic Review and Meta-Analysis — The Journal of Nutrition, 2018-11-01
• Low protein diets for non-diabetic adults with chronic kidney disease — Cochrane Database of Systematic Reviews, 2020-10-29