Exploratory serum renal biomarker and eGFR responses to high-protein diets during resistance versus concurrent training in resistance-trained men

BACKGROUND: High protein intake is common among resistance-trained athletes, but its association with renal biomarkers across different exercise modalities remains uncertain. This study investigated whether consuming a high-protein diet (3.2 g·kg⁻¹·d⁻¹) during 16 weeks of resistance training (RT) or concurrent training (CT) affects renal biomarkers and estimated glomerular filtration rate (eGFR) in resistance-trained men. METHODS: Forty-eight resistance-trained men (26 ± 6 years; BMI 25.6 ± 2.9 kg·m⁻²) were randomized to 16 weeks of RT or CT (4 sessions·wk⁻¹) while consuming either 1.6 or 3.2 g·kg⁻¹·d⁻¹ of protein in a 2 × 2 factorial design. Forty-four participants with complete pre- and post-intervention data were analyzed. Fasting blood samples were assessed for blood urea nitrogen (BUN), BUN/creatinine ratio, cystatin C, and eGFR calculated using the CKD-EPI creatinine (2021), cystatin C (2012), and creatinine-cystatin C (2021) equations. Within-group changes were evaluated using paired t-tests, and factorial effects were assessed using linear mixed-effects models (Time × Training × Protein). RESULTS: No statistically significant Time × Protein or Time × Training × Protein interactions were detected for any renal biomarker or eGFR estimate (all p ≥ 0.182). No significant within-group changes occurred in the 1.6 g·kg⁻¹·d⁻¹ groups (all p > 0.05). In both 3.2 g·kg⁻¹·d⁻¹ groups, BUN and cystatin C increased (all p ≤ 0.017), while eGFR estimates decreased (all p ≤ 0.009). The BUN/creatinine ratio remained unchanged (p ≥ 0.614). However, these within-group findings were descriptive because the corresponding interaction effects were not statistically significant. CONCLUSIONS: In resistance-trained men, 16 weeks of high protein intake did not produce statistically detectable differences in renal biomarker or eGFR changes compared with moderate protein intake, irrespective of training modality. Although modest increases in BUN and cystatin C and small decreases in eGFR estimates were observed within the high-protein groups, these findings were not supported by significant factorial interactions. The limited sample size precludes excluding clinically meaningful differences, warranting further investigation in larger studies.