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Bob. I am betting that the evidence base for CFS primarily involves research which was retrospective and/or did not medically evaluate patients, two obvious major flaws in such research. I would like to know how the association for CFS can be "robust" if comprised from poor quality research:
Results: Individuals who reported exposure to trauma were 2.7 (95% confidence interval = 2.27–3.10) times more likely to have a functional somatic syndrome. This association was robust against both publication bias and the generally low quality of the literature. The magnitude of the association with PTSD was significantly larger than that with sexual or physical abuse. Chronic fatigue syndrome had a larger association with reported trauma than did either irritable bowel syndrome or fibromyalgia. Studies using nonvalidated questionnaires or self-report of trauma reported larger associations than did those using validated questionnaires.
Retrospective research is notorious for producing suspicious positives, particularly when people are suffering now, which may distort memories of previous suffering. In chronic pain, there is a tendency for the research to show associations when retrospective but not prospective. One study had both measures on the same people and found a retrospective but not prospective association, which questions the presumed association, although there may be limitations with that study IIRC.
I think the association between trauma and CFS is a combination of poor research artifact, misdiagnoses, real effects of stress on physiology (often exaggerated and overgeneralized but still plausible), and perhaps an early warning system that something is going wrong biologically e.g. as an example for this I often draw a parallel to the increased rates of pre-morbid depression in those who later develop Parkinson's disease.
Chronic fatigue syndrome had a larger association with reported trauma than did either irritable bowel syndrome or fibromyalgia.
I bet that part comes from the CDC studies using the notorious Reeves et al operationalization of CDC criteria which saw a sudden 10 fold increase in prevalence figures and an unhelpful overlap with depression.
Well-conducted prospective research on medically diagnosed ME/CFS or GTFO!?