eGFR in muscular men: what the equation gets wrong

The eGFR value estimates how many millilitres of blood your kidneys filter per minute, scaled to a standard body surface area of 1.73 square metres. The e stands for estimated: the laboratory measures your creatinine and puts it with your age and your sex into the CKD-EPI equation, since 2021 without a race coefficient. The equation assumes an average build and has no field for muscle mass, and that is exactly the problem in a trained body.

Creatinine is a waste product of muscle tissue. More muscle means more production per day, but the equation assumes a fixed rate and therefore reads more creatinine as less filtration. At 100 kilos with a low body fat percentage this adds up: a result of 75 to 80 is then not unusual, and the kidney itself noticed nothing.

The newer EKFC equation, which some laboratories report alongside, does not solve this, because it too starts from the same creatinine. Cystatin C does: that protein comes from almost all body cells and not specifically from muscle, so an estimate based on cystatin C settles the argument between muscle mass and real loss of filtration.

Three habits stack up in strength athletes. Creatine supplementation raises creatinine measurably without changing filtration. A protein-rich diet with plenty of meat does the same, because meat itself contains creatinine. And a heavy leg session adds extra muscle turnover in the days after. All three push the calculated value down without touching the kidney, and together they can bring a healthy kidney below 60 on paper.

There is an honest limit to that. Anabolic steroid use is a different story: it genuinely can burden the kidney, partly through raised blood pressure and partly through direct damage to the filter units. The reassurance "it is just my muscle mass" no longer holds there, and a low value should be assessed by a doctor who knows what you take.

The ordinary reading still applies alongside. Above 90 is normal, 60 to 89 is called mildly reduced, and the value falls from about age forty by roughly 1 ml/min per year. Chronic kidney damage only exists once the value stays under 60 for at least three months or albumin in the urine is found. One result under 60 after a heavy week is a reason to repeat rested and on its own not yet a diagnosis.

A high value also deserves attention. Above 90 the equation is imprecise, and sustained high filtration, hyperfiltration, accompanies early diabetes and excess weight. That is a risk signal and no achievement.

Book the draw at least 48 to 72 hours after a heavy session, arrive well hydrated and skip the big meat meal the night before. That way you measure your baseline instead of your training week.

If you use creatine, a two-week pause before a kidney work-up is worth considering; discuss that with your doctor. Also mention the antibiotic trimethoprim if you take it, because it raises creatinine without touching the kidney. Never stop prescribed medication yourself.

If the value stays low under clean conditions, ask for cystatin C. That protein does not come from muscle tissue and settles the argument. If you use anabolic agents, say so at the same time, because then the low value cannot automatically be attributed to your build. Always have a persistently low result assessed by a doctor; only after three months of the same result is there chronic kidney damage.