Body Composition and Obesity in Nephrology Patients: Intersecting Effects of Sex, Age, and COVID-19

Background: Excess body weight is a major global health problem and an established independent risk factor for chronic kidney disease (CKD). This study aimed to determine the prevalence of overweight and obesity and to evaluate sex-, age-, and time-related trends—including the COVID-19 period—among patients treated at the Outpatient Clinic of the Division of Nephrology, Dialysis and Arterial Hypertension, University Hospital of Split, from 2016 to 2024. Methods: This study included 3033 subjects over 18 years of age, 44.8% men and 55.2% women with a mean age of 60 years. Body composition was assessed using the Tanita MC-780 bioelectrical impedance analyzer and body mass index (BMI, kg/m2) was measured. Results: The study population had median BMI of 28.0 kg/m2, with 33.1% overweight and 37% obese participants, including 6.6% with class III obesity. Men showed greater muscle and bone mass (p < 0.001), whereas women had higher fat mass and obesity prevalence (38.2% vs. 35.6%, p < 0.001). Participants under 65 years had higher absolute fat and muscle mass but similar fat percentage compared to older adults. Overweight and obesity prevalence increased with age, peaking at 75–78% in the 55–74-year group. BMI and fat mass rise significantly during and after the COVID-19 period, while phase angle values declined. Conclusions: Excess body weight is highly prevalent in nephrology patients, particularly in middle-aged adults. The COVID-19 pandemic further worsened body composition indicators, reinforcing the need for preventive strategies.