E2 ModerateModerate confidencePEM not requiredLongitudinalPeer-reviewedReviewed
Standard · 3 min
Fatigue in a community sample of twins.
Sullivan, P F, Kovalenko, P, York, T P et al. · Psychological medicine · 2003 · DOI
Quick Summary
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.
Why It Matters
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.
Observed Findings
Interfering fatigue affected 9.9% of the twin sample in the previous year
Fatigue showed significant associations with 42 of 52 assessed correlates
Major depression, generalized anxiety disorder, reported major health problems, and neuroticism were the strongest correlates
Two distinct clusters of correlates emerged: (a) mood/anxiety disorders and personality, and (b) health beliefs combined with alcoholism and life stress
Genetic and environmental contributions to fatigue differed by sex, with genetic effects more important in women and shared environmental effects in men
Inferred Conclusions
Fatigue is a heterogeneous construct with multiple distinct etiological pathways rather than a unitary symptom
Different subtypes of fatigue may require different treatment approaches based on their primary correlates
Sex differences in genetic versus environmental contributions suggest personalized intervention strategies may be needed
Understanding the complexity of fatigue may provide insights into more severe conditions like chronic fatigue syndrome
Remaining Questions
Do the identified subtypes of fatigue in this community sample apply to patients with diagnosed ME/CFS?
What This Study Does Not Prove
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.
Tags
Symptom:Fatigue
Method Flag:PEM Not DefinedWeak Case DefinitionMixed CohortExploratory OnlySex-Stratified
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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What are the causal mechanisms linking these correlates to fatigue—does depression cause fatigue or do they share a common biological origin?
Why do genetic factors predominate in women while environmental factors predominate in men, and what biological or social mechanisms explain this sex difference?
How do the subtypes of fatigue identified here relate to different symptom presentations or clinical outcomes in more severe fatigue disorders?