For adults without kidney disease, the trials so far have not shown harm. A 2018 meta-analysis of 28 trials found that eating about twice the protein did not change how kidney function shifted over the study compared with a normal intake3. People who already have reduced kidney function are a different case, and the evidence there points the other way1.
The certainty is limited, and most trials were short. This article sets out what was measured and where the gaps are. For anyone with a kidney condition or a question about their own kidneys, a doctor is the right person to ask.
Why people worry
The kidneys filter the blood. Eating more protein makes more waste (mainly urea) for them to clear, so they filter faster for a while. The concern is that filtering hard for years could wear them down3.
The key measure is GFR, glomerular filtration rate: how much blood the kidneys filter each minute. A falling GFR over time is how kidney decline is tracked. A rise after a protein-rich meal is expected; the question is whether it causes harm.
What was studied
The main evidence is a 2018 meta-analysis, a study that pools many trials into one estimate3.
| What | Number |
|---|---|
| Randomised trials | 28 |
| People | 1,358 adults aged 23 to 72 (average 49) |
| Who | Healthy adults and adults with obesity, type 2 diabetes or high blood pressure; no kidney disease |
| Higher-protein groups | about 1.81 g per kg of body weight a day |
| Comparison groups | about 0.93 g per kg a day |
| Trial length | 4 days to about 2 years |
The authors compared GFR in two ways. First, the value at the end of each trial. Second, the change from each person's own starting value, which accounts for differences at the start.
What the studies found
At the end, GFR was trivially higher on more protein. The standardised effect size was 0.19 (95% confidence interval 0.07 to 0.31)3. An effect size compares a difference with how much people normally vary; on the scale the authors used, anything under 0.2 is trivial. The confidence interval is the range the true value most likely sits in.
The change from the start did not differ. The effect size was 0.11, with an interval from −0.05 to 0.27 that crosses zero3. More protein went with a higher GFR at the end, but not with a different change over the trial.
Higher vs normal or lower protein: kidney filtering (GFR) · effect size
The authors read the rise as normal. The kidneys' ability to raise filtering after protein is called kidney functional reserve. The authors describe it as "a normal adaptive function" that "does not represent a risk factor" for kidney disease3.
A damage marker mostly did not move. Albumin, a protein that leaks into urine when kidney filters are damaged, was reported by 8 trials. Only 1 found an increase with more protein; the other 7 found no difference3.
In plain terms: in adults without kidney disease, more protein made the kidneys filter a little faster, as expected, without signs of harm over the trials' length.
In people who lift, at higher intakes
The meta-analysis mostly covered adults eating up to about twice the usual amount. Some trials in lifters went higher:
- One year, 2016. Fourteen resistance-trained men alternated their usual diet (about 2.5 g per kg a day) with a higher one (about 3.3 g per kg a day), each for about 6 months over a year. The authors found no harmful effects on measures of kidney function, liver function or blood lipids2.
- Sixteen weeks, 2023. Forty-eight trained men ate 1.6 or 3.2 g of protein per kg a day while training. Some markers of kidney and liver function rose within the 3.2 g groups, but they did not differ between groups4.
A 2024 review pushes back on how these studies are read. It points out that trials in lifters were designed to measure body composition, not kidney outcomes, so they were not built to detect harm, and argues that long-term very high intakes may still carry concern5.
In plain terms: short trials at very high intakes in young lifters found nothing alarming, but they were small and not designed as safety studies.
When kidney function is already reduced
The picture changes for people whose kidneys are not working fully. In a cohort of 1,624 women followed for 11 years, higher protein intake was not associated with faster GFR decline in women with normal kidney function1. In women with mild kidney impairment, each extra 10 g of protein a day was associated with a GFR decline of 1.69 mL/min per 1.73 m² more over the 11 years (95% CI −2.93 to −0.45).
GFR change over 11 years per extra 10 g of protein a day · mL/min per 1.73 m²
This is a cohort study: it observed eating patterns and outcomes, so it shows an association, not proof that protein caused the decline. Guidance for people with chronic kidney disease is set separately from guidance for healthy adults, and has itself changed over time3. That is a conversation for a doctor rather than an article.
What this does and does not mean
- It does not say more protein is harmless for everyone. The trials excluded people with kidney disease, type 1 diabetes or past kidney stones3.
- It does not cover decades. The longest trial in the meta-analysis lasted about 2 years.
- It does describe intakes common in training diets. The higher-protein groups averaged about 1.8 g per kg a day, similar to targets discussed in how much protein per kg.
The studies at a glance
| Study | Who was studied | Key result | Year |
|---|---|---|---|
| Knight et al., *Ann Intern Med*1 | Cohort of 1,624 women aged 42 to 68, 11 years | No link with decline at normal function; link with decline at mild impairment | 2003 |
| Antonio et al., *J Nutr Metab*2 | 14 trained men, 1-year crossover | About 3.3 g/kg a day: no harmful effects on kidney markers reported | 2016 |
| Devries et al., *J Nutr*3 | Meta-analysis of 28 trials, 1,358 adults without kidney disease | Change in GFR did not differ; end GFR trivially higher | 2018 |
| Bagheri et al., *JISSN*4 | 48 trained men, 16 weeks | 1.6 vs 3.2 g/kg a day: no between-group difference in kidney markers | 2023 |
| de Lorenzo et al., *Sports Med*5 | Review of studies in athletes and healthy adults | Lifter trials not designed to assess kidney outcomes | 2024 |
Limits worth knowing
- Low certainty. The 2018 authors rated the evidence "low to very low", because of poor reporting of how trials were randomised, unexplained differences between trials and imprecision3.
- Short trials. From 4 days to about 2 years.
- Mixed GFR methods. Some trials measured filtering directly; others estimated it from a blood marker (creatinine). The results did not differ between the two kinds3.
- Small lifter trials. 14 and 48 men, all young, all men.
- Cohort data. The 2003 study is observational and relied on food questionnaires.
- Abstract only. We read the 2003, 2016, 2023 and 2024 papers as abstracts and use only their stated numbers.
Latest research
The newest paper we opened is a 2024 review5. It argues that high protein intakes raise pressure inside the kidney's filters and that the reassuring trials in lifters were never designed to test kidney safety. It is a narrative review, not new data, so it does not overturn the 2018 trial pooling. It is a fair reminder that "no harm found in short trials" is not the same as "no harm". We found no newer meta-analysis of randomised trials on kidney function in adults without kidney disease.
The short version
- In adults without kidney disease, about twice the usual protein did not change how kidney function shifted in 28 trials.
- Filtering was a little higher at the end, which the authors read as a normal response.
- The evidence is low certainty and mostly short-term.
- In people with reduced kidney function, higher protein was linked with faster decline; that is a question for a doctor.
Educational overview only — not medical advice.
