Creatinine Alone Can't Tell You How Your Kidneys Are Doing

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Serum creatinine is the kidney number almost every lifter sees first. It sits on a standard metabolic panel, and the estimated glomerular filtration rate (eGFR) printed next to it is calculated from it. For most people that works well enough. For people carrying a lot of muscle, it can go wrong in two directions. It can make healthy kidneys look damaged, and it can stay unremarkable while other markers are already moving. A 2023 pilot study from the nephrology department at Eskisehir Osmangazi University in Turkey, led by Sultan Ozkurt, measured kidney function in bodybuilders using markers that are less tied to muscle, and it shows why creatinine should never be read on its own.

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1. Why creatinine misreads a muscular body

Creatinine is a byproduct of muscle. The authors point out that in bodybuilders, high muscle mass can push serum creatinine higher than kidney function alone would explain. That falsely high creatinine produces a falsely low eGFR. The equations that turn creatinine into an eGFR have proven validity for most patients, but their accuracy has not been confirmed in people with an extremely muscular body composition.