The purpose of this study is to evaluate whether a 3-month period of quercetin supplementation (500 mg of Quercetin Phytosome®) is useful in the treatment of chronic fatigue, as assessed by the Fatigue Impact Scale (FIS-40). Secondary end points are sleep assessment through Pittsburgh Sleep Quality Index (PSQI) and muscle performance assessment through the Short Physical Performance Battery (SPPB) and body composition assessment using DXA.

Inclusion Criteria:

• age ≥ 18 years old
• Chronic fatigue symptoms recorded using a self-reported fatigue score assessed using the validated Fatigue Impact Scale (FIS- 40) that comprised chronic tiredness impacts related to physical, cognitive and psychosocial functions.

Exclusion Criteria:

• Patients with any active medical condition that explains chronic fatigue (untreated hypothyroidism, sleep apnea, narcolepsy, drug side effects, and iron deficiency anemia)
• Previous diagnosis not unequivocally resolved (chronic hepatitis, malignant tumors)
• Past neuropsychiatric disorders or current severe depressive disorder with psychotic or melancholy features, bipolar disorder, schizophrenia, delusional disorder, dementia, anorexia nervosa, bulimia nervosa
• Participation in another clinical trial of the same or different nature within 30 days prior to inclusion in the study
• Failure to provide signed informed consent
• Consumption of certain drugs/supplements that could affect outcome measures in the past 90 days or whose discontinuation could be a significant problem
• Anticoagulant treatment
• Pregnancy or breastfeeding
• Smoking, alcohol or substance abuse
• Obesity (BMI > 30 kg/m²)