Sullivan, P F, Kovalenko, P, York, T P et al. · Psychological medicine · 2003 · DOI
This study looked at fatigue that interferes with daily life in nearly 8,000 twins to understand what causes it. The researchers found that fatigue is connected to many different things—including depression, anxiety, stress, and how people view their health—but not always in the same way for everyone. The study suggests that fatigue is not one simple condition but rather several different types with different underlying causes.
This study provides evidence that fatigue—and by extension, conditions like ME/CFS—is not a single disorder but rather a heterogeneous symptom with multiple distinct causes and subtypes. Understanding this complexity is crucial for developing more targeted treatments and explaining why patients respond differently to the same interventions. The finding of different genetic and environmental contributions in men versus women has important implications for precision medicine approaches in fatigue-related disorders.
This study examines general fatigue in the community, not chronic fatigue syndrome specifically, so findings may not directly apply to ME/CFS patients. The study is correlational and cannot establish causation—for example, while depression correlates with fatigue, it does not prove depression causes fatigue in all cases. Additionally, fatigue lasting only ≥5 days in the prior year is substantially milder than ME/CFS diagnostic criteria, limiting direct generalizability.
About the PEM badge: “PEM required” means post-exertional malaise was an explicit required diagnostic criterion for participant inclusion in this study — not that PEM was studied, observed, or discussed. Studies using criteria that do not require PEM (e.g. Fukuda, Oxford) are tagged “PEM not required”. How the atlas works →
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