Participants had an excellent grasp of causation, risk groups and rates of anal cancer.
Also, it is not possible to establish causality with a cross-sectional study design, and there is a risk of reverse causation.
A criticism of previous studies has been that BMI often is assessed up to several decades after diabetes diagnosis, thereby increasing the risk of reverse causation (12).
To minimize the risk of reverse causation as an explanation for our findings, we tested the association between these metabolites with all-cause mortality in the discovery cohort.
Epidemiological studies also carry a risk of reverse causation, which could ensue if persons with undiagnosed RA in HUNT2 were erroneously included as incident cases of RA, because RA in itself increases the risk of CV disease and events [ 10].
To establish whether antibiotic use truly preceded CD, i.e. to evaluate the risk of reverse causation, we performed a sub-analysis of individuals who were exposed to antibiotics before the age of six months.
The long sleep diabetes association is also subject to reverse causation, as individuals at higher risk of developing diabetes like those with depression or chronic disease may sleep longer because of the underlying condition.
Although the examples above illustrate that epigenetic changes occur in asthma and COPD, they bear the risk of reverse causation as epigenetic modifications are inducible and may represent a response to the pathology rather than being its root.
The present pooled analysis has a number of strengths, including size (high statistical power even after risk factor stratification), prospective design (reducing the risk of reverse causation bias), and inclusion of well-characterized cohort studies (facilitating an assessment of the independent effects of stress).
To reduce the risk of reverse causation and the effect of comorbidity, which may act as a confounder by increasing the possibility of receiving antibiotic prescriptions, we performed two post-hoc analyses restricted to individuals exposed to antibiotics in the first six months of life or individuals without hospital admission.
