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 r² = 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 r² = 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.