Hidden Burden after Mild COVID-19: Impaired Autonomic Function and Increased Adiposity in Young Adults – A Cross-Sectional Observational Study

Background: Persistent symptoms after COVID-19, known as long COVID, may involve cardiac and autonomic dysfunction. However, there is limited data on cardiovascular alterations, body composition changes, and lingering symptoms in young adults recovering from mild COVID-19, as well as how these factors affect heart rate variability (HRV). Considering these gaps, this study examines HRV parameters, investigates alterations in body composition, and characterizes the persistence of symptoms approximately one month after mild Severe Acute Respiratory Syndrome Coronavirus 2 (SARS-CoV-2) infection in young adults, in comparison with a group of healthy controls. Methods: This cross-sectional observational study assessed participants diagnosed with mild COVID-19 one-month post-infection at the University Hospital of Federal University of São Carlos. Assessments included: (I) clinical evaluation; (II) RR interval (iRR) recording at rest and during the respiratory sinus arrhythmia maneuver (M-RSA) using a heart rate monitor with an elastic transmitter band (Polar H10, Polar, Finland); and (III) body composition analysis via bioelectrical impedance (InBody® 720, Biospace Co. Ltd., South Korea). HRV indices were derived using Kubios software. Results: A total of 62 participants were assessed – 31 in the COVID-19 group and 31 in the control group. Participants with prior mild COVID-19 had significantly higher body fat percentages and a higher prevalence of persistent symptoms, such as fatigue and dysgeusia ( p < 0.05). HRV analysis showed lower root mean square of successive differences (rMSSD) values in the COVID-19 group both at rest (25.49 ± 15.45 ms) and during M-RSA (15.88 ± 0.10 ms) ( p < 0.0001), indicating reduced parasympathetic activity. In addition, significant negative correlations were found between total body fat mass and mean iRR ( p = 0.009; r = –0.32), as well as between fat mass, body fat percentage, and rMSSD at rest ( p = 0.02; r = –0.28). Conclusion: Young adults recovering from mild COVID-19 demonstrate impaired autonomic heart rate control, persistent post-infection symptoms, and increased body fat. These findings suggest subtle but detectable cardiovascular and metabolic alterations, even in mild cases of COVID-19.