Higher skeletal muscle mass associates with higher measured glomerular filtration rate in healthy individuals

To the Editor, Low muscle mass is a prevalent issue among patients with chronic kidney disease [1 ] and has been associated with adverse outcomes, including increased risk of mortality and end-stage kidney disease [2 ].Improving muscle mass and function shows beneficial effects on kidney function [3 -6 ], suggesting that muscle mass and performance are associated with kidney function.Many aspects of this association remain unexplored.This study aimed to investigate whether an association between skeletal muscle and measured glomerular filtration rate (mGFR, 125 I-Iothalamate) exists in healthy individuals, investigating computed tomography (CT) -derived muscle mass measurements, muscle quality, and 24hour urinary creatinine excretion rate (24-h CER).We hypothesized that higher muscle mass, quality, and 24-h CER levels are associated with higher mGFR.In this cross-sectional study, skeletal muscle area (SMA) and density (skeletal muscle radiation attenuation, SMRA) were based on CT-scans at vertebral level L3.Muscle area was indexed for height (skeletal muscle index, SMI), and SMA, SMI, and SMRA were stratified by age.The 24-h CER was assessed as a biochemical marker for muscle mass.Participants underwent GFR measurements.An extensive description of the methodology is provided in Annex I of the Online Supplementary Material.In total, 888 healthy participants were included, 49% of whom were male (Table 1 ).Age was 53 11 years, body mass index (BMI) was 26.2 3.41 kg/m 2 , and body surface area (BSA) was 1.96 0.19 m 2 .The 24-h CER was 13.6 4.64 mmol/24 hours, SMA was 147.8 34.6 cm 2 , SMI was 47.6 8.38 cm 2 /m 2 , and SMRA was 48.6 7.52 Hounsfield units (HU).Non-standardized mGFR was 112.5 21.1 ml/min, and BSA-standardized mGFR was 99.3 15.5 ml/min per 1.73 m 2 .A significant interaction was found for age with SMA, SMI, and SMRA.Therefore, the analyses were stratified for age ( or > mean age).SMA showed a statistically significant positive correlation with 24-h CER ( r value = 0.66, P < .001).In univariable linear regression analyses, SMA, SMI, SMRA, and 24-h CER were significantly associated with non-standardized mGFR, with higher levels being associated with higher mGFR ( Supplemental Table S1).Other determinants of mGFR in the total population were fe-