Glomerular filtration rate (GFR) above or below normal: causes in athletes and what to do
A reduced estimated GFR in a biochemical panel is one of the most frequent reasons athletes turn to a doctor or stop taking creatine. Less often the opposite situation occurs — a GFR higher than normal. Both deviations can be either an artifact of the calculation or a reflection of real changes in the kidneys. The editorial team offers a look at the causes and the sequence of checks.
Why the estimated GFR is wrong
The estimated GFR is calculated from the creatinine level, age, and sex. The formula assumes that a person has a typical muscle mass and an ordinary diet. In athletes both assumptions are often violated, so they are a classic example of a group in which the creatinine-based estimate is inaccurate.
Creatinine is formed in the muscles from creatine and creatine phosphate. The more muscle, the more creatinine in the blood at the same filtration rate. As a result, the formula “sees” the kidneys as worse than they actually are.
The opposite situation arises in people with a small muscle mass: after prolonged immobilization, in lean long-distance runners, in elderly people. Low creatinine gives an overestimated eGFR, which can mask a real reduction in kidney function.
That is precisely why the KDIGO guidelines recommend, in case of doubt, using cystatin C, which depends much less on muscle mass, or the combined creatinine–cystatin equation.
Causes of reduced GFR in athletes
Causes of a reduced eGFR should be divided into three groups: false (calculation artifacts), temporary (functional), and true (kidney damage). Whether any action is needed at all depends on this.
| Group | Causes | Comment |
|---|---|---|
| False reduction | Large muscle mass, creatine intake, meat before the test, drugs that block creatinine secretion | Filtration is normal; cystatin C helps confirm this |
| Temporary reduction | Dehydration, intense training the day before, heat, diarrhea, vomiting | Recovers after rehydration and rest |
| True reduction | Rhabdomyolysis, NSAID abuse, hypertension, diabetes, glomerulonephritis, damage from anabolic steroids | Requires examination by a nephrologist |
Among the true causes in athletes, acute kidney injury after rhabdomyolysis is singled out separately. It is provoked by excessive, especially unaccustomed, loads, heat, dehydration, and certain drugs. Dark, cola-colored urine and severe muscle pain are a reason to see a doctor immediately.
Nonsteroidal anti-inflammatory drugs reduce blood flow in the kidneys. Combined with dehydration, for example at a marathon or ultramarathon, they increase the risk of acute kidney injury. Therefore they should not be taken “prophylactically” before competitions.
The use of anabolic steroids is associated with focal segmental glomerulosclerosis, described by Herlitz and colleagues. The risks increase when combined with arterial hypertension, high body weight, and a high-protein diet. Such changes can be persistent.
Causes of elevated GFR
A GFR above 120–130 mL/min/1.73 m² may be a consequence of low muscle mass and low creatinine — that is, again an artifact of the calculation. Such a result occurs in lean people and does not indicate excessive kidney work.
True hyperfiltration is a state in which the glomeruli filter more than usual. It has been described in the early stages of diabetes mellitus, in obesity, during pregnancy, and with very high protein intake.
- low muscle mass (falsely high eGFR);
- pregnancy (physiological hyperfiltration);
- early stages of diabetes mellitus;
- obesity and metabolic syndrome;
- a very high-protein diet.
For healthy athletes, moderate hyperfiltration against the background of a high-protein diet has, according to available data, no proven negative consequences. However, for people with already reduced kidney function or diabetes, large amounts of protein may be undesirable, and the intake limit should be discussed with a doctor.
If a high GFR is combined with albumin in the urine, elevated glucose, or blood pressure, this is already a reason for a thorough examination, not just an interesting figure.
Verification algorithm
The correct tactic for a GFR deviation is not to draw conclusions from a single test. First technical and temporary factors are ruled out, and then the result is confirmed or refuted by another method.
Before a repeat test it is worth refraining from training for 48 hours, not eating much meat the day before, hydrating well, and, if possible, temporarily stopping creatine intake in agreement with a doctor. You should also check whether any drugs that raise creatinine are being taken.
It is advisable to add cystatin C to the repeat draw and calculate the eGFR using the combined equation. In parallel, a urinalysis and the albumin/creatinine ratio in a spot urine sample are taken.
If the reduction is confirmed by cystatin C or albuminuria is detected, a nephrologist consultation is needed. The doctor may order kidney ultrasound, blood pressure monitoring, glucose tests, and other investigations.
Practical recommendations for kidney health
For most healthy athletes the main rules are simple: adequate hydration, especially in the heat, gradual increases in load, and avoiding excessive, unaccustomed eccentric training “to failure” after a break. This reduces the risk of rhabdomyolysis.
Nonsteroidal anti-inflammatory drugs should be used only when necessary, in short courses, and not against the background of dehydration. For chronic pain it is better to look for the cause than to suppress it constantly with pills.
Creatine in recommended doses is considered safe for the kidneys in healthy people. However, people with already reduced kidney function or kidney disease should consult a doctor before taking it.
Regular monitoring of blood pressure, glucose, and weight is important for athletes too. It is precisely hypertension and diabetes that are the most frequent causes of chronic kidney disease in the general population.
Editorial conclusions
Deviations in GFR in athletes are most often explained by features of the calculation: a large muscle mass and creatine lower the value, while a small muscle mass raises it. Temporary changes are caused by dehydration and hard training.
However, real threats exist too: rhabdomyolysis, NSAID abuse, anabolic steroids, hypertension. A reliable answer is possible only after a repeat test under proper conditions with cystatin C and a urine assessment.
The editorial team also suggests reading materials on what the GFR test shows, on the safety of creatine for the kidneys, and on changes in urinalysis after exercise.
References
- Kidney Disease: Improving Global Outcomes (KDIGO) CKD Work Group. KDIGO 2024 Clinical Practice Guideline for the Evaluation and Management of Chronic Kidney Disease. Kidney Int. 2024;105(4S):S117–S314.
- Levey AS, Stevens LA, Schmid CH, et al. A new equation to estimate glomerular filtration rate. Ann Intern Med. 2009;150(9):604–612.
- 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.
- Baxmann AC, Ahmed MS, Marques NC, et al. Influence of muscle mass and physical activity on serum and urinary creatinine and serum cystatin C. Clin J Am Soc Nephrol. 2008;3(2):348–354.
- Poortmans JR, Francaux M. Long-term oral creatine supplementation does not impair renal function in healthy athletes. Med Sci Sports Exerc. 1999;31(8):1108–1110.
- 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.
- Clarkson PM, Kearns AK, Rouzier P, Rubin R, Thompson PD. Serum creatine kinase levels and renal function measures in exertional muscle damage. Med Sci Sports Exerc. 2006;38(4):623–627.
Andriy Melnyk
A strength-sports coach and author of programs for beginner and intermediate levels. Writes about training planning.
