How follow-up period in prospective cohort studies affects the relationship between baseline fish consumption and risk of Alzheimer's disease and dementia

Background Diet plays an important role in the risk of Alzheimer's disease (AD) and dementia. Adherence to the Mediterranean diet is regarded as one way to reduce risk. Fish is a component of the Mediterranean diet. Participant variables change with time, resulting in “regression dilution”. Objective The objective is to investigate the role of follow-up period after measurement of dietary fish intake with risk of AD and dementia. Methods A recent meta-analysis of prospective cohort studies is used to examine this effect for dietary fish and incidence of dementia and AD. Results For seven dementia studies with mean follow-up periods from one to ten years, the regression fit to the data is relative risk (RR) = 0.19 + (0.087 × Follow up [years], r = 0.84, adjusted r 2 = 0.66, p = 0.009). However, when mean age of the participants was added to the analysis, the effect of follow-up period became non-significant. For five AD studies with follow-up periods from 3.9 to 9.8 years, the regression fit to the data is RR = 0.11 + (0.095 × follow up [years]), r = 0.93, adjusted r 2 = 0.87, p = 0.02). The RR for the regression fit to the shortest follow-up periods are 40% greater for dementia than the standard meta-analysis suggests and 70% greater for AD. Conclusions Prospective cohort studies of diet and health outcomes should reassess variables every few years. Meta-analyses should consider the effect of mean follow-up periods and mean age of the participants during follow up.