Kidney Stress in Athletes: Causes and the Link to Performance Drugs
Kidneys rarely get attention in the gym until a routine blood test shows an unexpected creatinine value. Hard training, high-protein diets, dehydration, painkillers and performance-enhancing drugs all place demands on renal function, but they do so in very different ways. Our editorial team sorted out what is a normal adaptation, what is a lab artifact and what is a genuine warning sign.
How the kidneys respond to training
The kidneys filter roughly 180 liters of plasma per day and fine-tune fluid, electrolyte and acid-base balance. During intense exercise, blood is redirected toward working muscles and skin, so renal blood flow can drop substantially for the duration of the session. For a healthy, well-hydrated person this is a temporary and fully reversible shift.
After long or very strenuous efforts, markers of kidney filtration often move in the "wrong" direction. Studies of marathon runners have documented transient rises in serum creatinine that meet the formal definition of acute kidney injury, yet values typically return to baseline within a day or two. Researchers still debate whether this reflects real tubular stress or simply muscle breakdown and fluid shifts.
Exercise can also produce temporary protein or even blood in the urine. Post-exercise proteinuria is common after hard intervals and usually clears within 24–48 hours. A single abnormal dipstick taken right after training therefore says little about long-term kidney health.
The picture changes when several stressors stack together: extreme heat, inadequate fluids, a large volume of eccentric work, anti-inflammatory painkillers and drugs that independently burden the kidneys. That combination, rather than training alone, is behind most serious cases seen in sports medicine.
Lab markers and how athletes misread them
Creatinine is a breakdown product of creatine phosphate in muscle. Because muscular athletes have more muscle mass, they produce more creatinine and often show values above the laboratory reference range even with perfectly normal kidneys. Creatine supplementation and large servings of cooked meat can raise the number further.
The estimated glomerular filtration rate (eGFR) that US labs report is calculated from creatinine, age and sex. The current CKD-EPI 2021 equation was not designed for people with unusually high muscle mass, so it can underestimate kidney function in bodybuilders and strength athletes. This is one of the most frequent reasons for unnecessary alarm.
| Marker | What it reflects | Common athlete-specific pitfalls |
|---|---|---|
| Serum creatinine / eGFR | Filtration estimate | Raised by muscle mass, creatine use, recent heavy training |
| Cystatin C | Filtration, less muscle-dependent | Can be affected by thyroid status and corticosteroids |
| Urine albumin-to-creatinine ratio | Glomerular damage | Transiently elevated after exercise |
| BUN (urea) | Protein intake, hydration | Rises with high-protein diets and dehydration |
| Creatine kinase (CK) | Muscle damage | Very high after novel or eccentric training |
When creatinine looks high, many nephrologists add cystatin C, which is far less dependent on muscle mass. Guidance from KDIGO supports combined creatinine–cystatin C equations when a more accurate estimate is needed.
Timing matters too. Blood drawn the morning after a heavy leg session or a long run can show elevated creatinine and CK. A more reliable picture comes from testing after 48–72 hours of light activity, with normal hydration and no unusual protein load the evening before.
Everyday risk factors: heat, NSAIDs, diet
Dehydration is the most common amplifier of renal stress. Losing fluid through sweat reduces blood volume, and the kidneys respond by conserving water and concentrating urine. Training in summer heat, cutting weight for a weigh-in or using diuretics pushes the system closer to its limits.
Non-steroidal anti-inflammatory drugs (ibuprofen, naproxen) are widely used by athletes for aches. They block prostaglandins that help maintain kidney blood flow when circulation is under strain. Taking NSAIDs before endurance events, especially in the heat, has been linked to an increased risk of acute kidney injury.
Exertional rhabdomyolysis is the most dramatic scenario. Massive muscle breakdown releases myoglobin, which can clog renal tubules. Warning signs include severe muscle pain and swelling and dark, cola-colored urine. This is a medical emergency, not something to "sleep off".
High-protein diets are often blamed, but in people with healthy kidneys, current evidence does not show that protein intakes typical for athletes cause kidney disease. The situation is different for anyone with existing chronic kidney disease, where protein intake should be set with a physician.
- Hot-weather training without adequate fluid and electrolytes
- Regular NSAID use, especially around long events
- Rapid weight cutting and diuretic use
- Unaccustomed high-volume eccentric training
- Pre-existing high blood pressure or diabetes
Performance drugs and kidney injury
Anabolic-androgenic steroids (AAS) have been associated with several kidney problems in case series and cohort data. The most concerning is focal segmental glomerulosclerosis (FSGS), described in bodybuilders with long-term AAS use. Researchers suggest a combination of increased body mass, raised filtration pressure and possible direct toxic effects of androgens on the kidney.
AAS also tend to raise blood pressure and worsen lipid profiles, both of which accelerate kidney damage over time. Some oral 17-alpha-alkylated compounds are additionally associated with liver cholestasis, and severe jaundice can be followed by bile cast nephropathy, a recognized cause of kidney injury.
Diuretics, used to cut water before competitions or to mask other substances, can cause electrolyte disturbances and acute kidney injury. They are prohibited by the World Anti-Doping Agency (WADA) at all times. Stimulant-based fat burners may raise blood pressure and, combined with heat and dehydration, contribute to rhabdomyolysis.
Unregulated products add another layer of uncertainty. The FDA has repeatedly warned about "dietary supplements" containing undeclared drugs, including steroid-like compounds and SARMs. Users may not even know what they are taking, which complicates both risk assessment and diagnosis.
Monitoring and prevention
For a healthy recreational athlete, an annual metabolic panel and urinalysis during a routine check-up is usually enough. People with high blood pressure, diabetes, a family history of kidney disease or regular use of medications that affect the kidneys need closer follow-up as agreed with their doctor.
Tell the physician about creatine, high-protein diets and recent training before interpreting results. That context helps avoid both false alarms and missed problems. If creatinine is borderline, a repeat test after rest and the addition of cystatin C are reasonable next steps.
- Drink to thirst and monitor urine color, especially in hot conditions.
- Avoid NSAIDs before and during prolonged exercise unless prescribed.
- Increase training volume gradually to limit muscle breakdown.
- Check blood pressure regularly.
- Disclose all supplements and medications to your doctor.
Seek urgent care for dark urine with severe muscle pain, markedly reduced urine output, swelling of the legs or face, or confusion after exertion in the heat. These symptoms can signal acute kidney injury or rhabdomyolysis.
Editorial conclusions
Short-term changes in kidney markers after hard training are common and usually harmless. Muscle mass and creatine intake can make creatinine look worse than kidney function really is, so results need context and sometimes cystatin C.
Real danger arises when stressors combine: heat, dehydration, painkillers, rhabdomyolysis and drugs such as anabolic steroids or diuretics. Long-term AAS use has been linked to structural kidney disease in published case series.
Regular check-ups, honest disclosure to your physician and sensible hydration remain the most effective tools for protecting the kidneys.
Related reading from our editorial team: how to interpret creatinine on a creatine supplement, the effects of anabolic steroids on blood pressure, and early warning signs of rhabdomyolysis.
References
- 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.
- Mansour SG, Verma G, Pata RW, et al. Kidney injury and repair biomarkers in marathon runners. Am J Kidney Dis. 2017;70(2):252–261.
- Inker LA, Eneanya ND, Coresh J, et al. New creatinine- and cystatin C–based equations to estimate GFR without race. N Engl J Med. 2021;385(19):1737–1749.
- Pope HG Jr, Wood RI, Rogol A, et al. Adverse health consequences of performance-enhancing drugs: an Endocrine Society scientific statement. Endocr Rev. 2014;35(3):341–375.
- 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.
- Bosch X, Poch E, Grau JM. Rhabdomyolysis and acute kidney injury. N Engl J Med. 2009;361(1):62–72.
