Kidney Strain in Strength Athletes: Prevention, Diagnosis and When to See a Doctor
"Too much protein will wreck your kidneys" is one of the most persistent gym myths, while genuinely risky habits — routine painkillers, severe dehydration, hormonal drugs — get far less attention. Our editorial team explains what actually puts stress on the kidneys of active people, how kidney function is tested, why athletes' results are often misread, and when you should get checked.
What the kidneys do and what "strain" means
Each kidney contains around a million filtering units called nephrons. Together they filter the blood many times a day, removing waste products such as urea and creatinine, regulating fluid and electrolyte balance, controlling blood pressure and producing hormones that stimulate red blood cell production and activate vitamin D.
"Kidney strain" is not a medical diagnosis. In practice, people use the term to describe anything from a temporary rise in creatinine after a hard session to genuine kidney injury. Clinicians distinguish acute kidney injury (AKI), which develops over hours or days, from chronic kidney disease (CKD), defined by abnormalities persisting for more than three months.
The kidneys have considerable reserve, and healthy people can lose a substantial proportion of function before noticing any symptoms. This is both reassuring and a reason for caution: damage can accumulate silently.
For people who train, the practical concern is avoiding situations that cause AKI and recognising longer-term risk factors such as high blood pressure, which is one of the leading causes of CKD in the UK.
Real risks versus gym myths
High-protein diets have been studied extensively. A systematic review and meta-analysis by Devries and colleagues found that kidney function did not differ meaningfully between healthy adults on higher-protein and normal-protein diets. A one-year crossover trial in trained men by Antonio and colleagues likewise found no harmful effects. The picture is different for people who already have CKD, who may need to moderate protein under medical advice.
Creatine supplementation raises blood creatinine slightly, because creatine is converted to creatinine. This can make a routine eGFR look worse without any actual change in kidney function. The ISSN position stand by Kreider and colleagues concluded that creatine monohydrate is safe for healthy people at recommended intakes.
| Factor | Real effect on kidneys | Level of concern for healthy people |
|---|---|---|
| High-protein diet | No meaningful harm shown in healthy adults | Low |
| Creatine monohydrate | Raises creatinine reading, not damage | Low (inform your doctor) |
| Regular NSAID use (ibuprofen, naproxen) | Reduces kidney blood flow, can cause AKI | Moderate to high, especially when dehydrated |
| Severe dehydration, extreme heat | AKI risk | Moderate to high |
| Rhabdomyolysis | Muscle proteins can block kidney tubules | High |
| Anabolic steroids | Associated with FSGS and high blood pressure | High |
Rhabdomyolysis, the rapid breakdown of muscle tissue, deserves special attention. It can follow unaccustomed, extreme exercise, particularly high-repetition eccentric work, training in heat, or exercise combined with stimulants or certain drugs. Bosch and colleagues describe how myoglobin released from damaged muscle can cause acute kidney injury.
Anabolic steroid use has been linked to focal segmental glomerulosclerosis (FSGS), a form of kidney scarring. Herlitz and colleagues described a series of bodybuilders who developed FSGS and heavy protein loss in the urine after long-term steroid use. Associated weight gain and high blood pressure add to the burden.
How kidney function is assessed
The standard tests are a blood test for creatinine, from which the laboratory calculates the estimated glomerular filtration rate (eGFR), and a urine test for albumin, usually reported as the albumin-to-creatinine ratio (ACR). NICE guideline NG203 and the international KDIGO guidelines use both to classify CKD.
Here lies a common trap for muscular people. eGFR equations assume average muscle mass. Someone with a lot of muscle, or who takes creatine or eats a lot of meat before the test, will produce more creatinine, so their eGFR may be underestimated. A one-off reading in the G2 or even G3a range in a heavily muscled athlete does not automatically mean disease.
Doctors can clarify the picture with a cystatin C blood test, which is much less affected by muscle mass, and by repeating tests after avoiding heavy exercise and high meat intake for a day or two. Urine testing for albumin and blood, plus a blood pressure check, completes the basic assessment.
Protecting your kidneys
Most kidney protection comes down to everyday habits. The following points apply to healthy adults; people with known kidney disease should follow individual medical advice.
- Drink enough to keep urine pale yellow, adjusting for heat and sweat losses.
- Avoid regular use of anti-inflammatory painkillers to train through pain, especially before endurance events.
- Build up training volume gradually, particularly with new high-repetition or eccentric exercises.
- Check your blood pressure periodically; treat hypertension early.
- Avoid anabolic steroids and unregulated "kidney support" or detox products.
- Tell your doctor about creatine and other supplements before blood tests.
Be cautious with very high doses of vitamin C, which can increase oxalate and kidney stone risk in susceptible people, and with herbal products of uncertain composition. Some herbal remedies have caused serious kidney damage when contaminated or misidentified.
Finally, people with diabetes, high blood pressure, a family history of kidney disease or of certain ethnic backgrounds at higher risk should ask their GP whether periodic kidney tests are appropriate.
Warning signs and when to see a doctor
Kidney problems are often silent, but certain signs should prompt medical review. Book a GP appointment if you notice persistently foamy urine, swelling of the ankles or around the eyes, unexplained high blood pressure readings, or blood test results flagged as abnormal.
Seek urgent care (NHS 111 or A&E) if you develop dark brown or "cola-coloured" urine after intense exercise, especially with severe muscle pain and weakness. This may indicate rhabdomyolysis and requires prompt treatment with intravenous fluids.
A sharp reduction in urine output, confusion, vomiting or severe flank pain also needs urgent assessment. Visible blood in the urine should always be checked by a doctor.
If you use or have used anabolic steroids, tell your doctor and ask about blood pressure, urine albumin and kidney function testing. Early detection of protein leakage gives the best chance of slowing progression.
Conclusions of the editorial team
For healthy people, high-protein diets and creatine are not proven threats to the kidneys, although creatine can distort creatinine-based tests.
The real risks are dehydration, regular NSAID use, rhabdomyolysis from extreme sessions, untreated high blood pressure and anabolic steroids.
eGFR in muscular people should be interpreted carefully, with cystatin C and urine albumin as useful additional tests. Dark urine after extreme exercise is an emergency.
You might also like our articles on rhabdomyolysis in CrossFit-style training, painkillers and sport, and blood pressure in bodybuilders.
References
- Kidney Disease: Improving Global Outcomes (KDIGO) CKD Work Group. KDIGO 2012 clinical practice guideline for the evaluation and management of chronic kidney disease. Kidney Int Suppl. 2013;3(1):1–150.
- National Institute for Health and Care Excellence. Chronic kidney disease: assessment and management (NG203). London: NICE; 2021.
- Devries MC, Sithamparapillai A, Brimble KS, et al. 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. J Nutr. 2018;148(11):1760–1775.
- Antonio J, Ellerbroek A, Silver T, et al. A high protein diet has no harmful effects: a one-year crossover study in resistance-trained males. J Nutr Metab. 2016;2016:9104792.
- Kreider RB, Kalman DS, Antonio J, et al. International Society of Sports Nutrition position stand: safety and efficacy of creatine supplementation in exercise, sport, and medicine. J Int Soc Sports Nutr. 2017;14:18.
- Herlitz LC, Markowitz GS, Farris AB, et al. Development of focal segmental glomerulosclerosis after anabolic steroid abuse. J Am Soc Nephrol. 2010;21(1):163–172.
- Bosch X, Poch E, Grau JM. Rhabdomyolysis and acute kidney injury. N Engl J Med. 2009;361(1):62–72.
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