Commentary
Method
This survery is based on a random sample of 5,362 single births in one week of March, 1946. In 1999, at 53 years, the 3,035 remaining participants were asked if they had ever had a diagnosis of CFS/ME with 2,983 giving an answer. This makes it about 1/3 the size of the other 2 cohorts, which might explain the unreliable findings.
CFS prevalence
There were only 37 cases of CFS reported, and of these only 22 (65%) reported the diagnosis had been made by a doctor. Hospital records (primary care records were not checked) ruled out 3 of the 22 cases due to exlusionary medical conditions leaving 34 cases in total, giving a lifetime prevalence of 1.2% (ie people
ever having CFS, no data was given for those with a
current diagnosis). Age of onset was between 41 and 53 years ie the first case started in 1987, which ties in with the time CFS/ME was first being recognised. No doubt many particpants had CFS before this time but didn't know that's what it was. The same pattern of cases not begining until the 80s in seen in the other 2 birth cohorts too.
Ruling out hypotheses
This paper aimed to test a number of hypotheses, most of which were ruled out, including:
- Atopy (allergy)
- Specific childhood symptoms (persistent abdominal pain had been shown to be a risk factor for unexplained hospitalisations and the authors hoped it would be linked to CFS too; it wasn't)
- Childhood Chronic Illness (age 0-15). This contradicts the finding of the much larger 1970 cohort. More to the point, this 1946 study lost a lot of those with childhood chronic illness making it an unrepresentative sample and so these findings are pretty meaningless:
Those followed up to the end of our study were not totally representative of the sample taken at the beginning. In particular, males, those from poorer families, and those who suffered ill health as children were less likely to be followed up. This attrition bias must be considered when assessing our results, especially in regard to our negative findings relating to childhood illness experiences
Frequent exercise a risk factor for CFS... ....or is it?
The biggest finding of this study was that exercising frequently is a significant risk factor for CFS. A later and large study contradicts this finding but here are the results
(full table):
Extreme Energy levels reported by teacher (age 13)
significant, p=0.006
Sports more than weekly (age 31)
NOT significant
Self-rated as very fit (age 31)
NOT significant
Very active at age 36
NOT significant
Sport weekly or more at 41
significant, p=0.006
Vigorous activity at least weekly at 31 and 43
significant, p=0.01, OR=10.8 (2.7-43.8)
Let's just look at that last finding - an odds ratio of 10.8? Woohoo! This must be the answer. But what the authors don't mention is that this figure is based on under half (43%) of all participants ie the data is missing for most of the participants (and the finding is based on just 4 energetic people with self-reported CFS), which makes it rather meaningless. This leaves me wondering if the reviewers of this paper were lazy, useless or biased, but this slightly harsh view may be coloured by Wales going out of the Rugby World Cup today ("it was never a red card, ref!").
Other than that, it's a mixed bag of significant and non-signigicant findings. The extreme energy at age 13 only led to 5 excess cases and would only explain 17% of all CFS cases, suggesting it's a minor factor.
More to the point, these findings were not replicated in the much bigger 1958 cohort, which found that
high levels of activity were not a risk factor for CFS at any age.
Main conclusions
The findings on childhood illness are, according to the authors, questionable because some children with ill health were lost from the adult survey. The findings on exercise were mixed, based on small numbers and were not replicated in the larger 1958 cohort. It doesn't amount to much.