IntroductionAnabolic androgenic steroids (AAS) and diet supplements (DS) are frequently used by bodybuilders. In this specific group, increased muscle mass, the acute effects of exercise, and the use of creatine may affect the creatinine-based estimated glomerular filtration rate (eGFRcr), potentially leading to an underestimation of the GFR. Cystatin C equations offer a more accurate prediction of GFR that is independent of muscle mass. We aimed to assess the renal functions of bodybuilders who use both AAS+DS, as well as those who only use DS, by calculating the GFR based on cystatin C (eGFRcys) and also using a combination of cystatin C and creatinine (eGFRcys/cr). MethodsThe study included 12 bodybuilders using AAS+DS and 12 bodybuilders using DS. In both groups, serum cystatin C levels, eGFRcys, eGFRcys/cr, urine albumin excretion rates, urine protein excretion rates, and routine tests were examined.ResultsIn AAS+DS users, the average duration of AAS use was 3.08 & PLUSMN;2.02 years, while for DS users, the duration of supplement use was 3.67 & PLUSMN;2.49 years. The spot urine albumin/creatinine and protein/creatinine ratios were higher in AAS+DS users (p<0.001 and p=0.006, respectively). Although eGFRcr was found to be similar in the AAS+DS and DS groups (119.67 & PLUSMN; 24.12 ml/min and 122.08 & PLUSMN; 18.03 ml/min, respectively; p=0.426), eGFRcys and the eGFRcys/cr ratio were significantly lower in the AAS+DS group compared to the DS group (eGFRcys: 120.67 & PLUSMN; 19.48 ml/min vs. 122.08 & PLUSMN; 18.03 ml/min, p=0.039; eGFRcys/cr: 121.83 & PLUSMN; 20.62 ml/min vs. 126.33 & PLUSMN; 21.163 ml/min, p= 0.036, respectively). ConclusionCystatin-based GFR values were found to be significantly lower in AAS+DS users, and urinary albumin and protein excretion were considerably higher compared to DS users. Although these findings suggest a potential link between early kidney damage and the direct use of AAS, the topic requires further investigation.