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Background/Objectives: Prospective cohort studies are useful for studying how biomolecular status affects risk of adverse health outcomes. Less well known is that the longer the follow-up time, the lower the association (or “apparent effect”) due to “regression dilution”. Here, we evaluate how follow-up interval from baseline to “event” affects the relationship between baseline serum 25-hydroxyvitamin D 25(OH)D concentration and the later incidence of stroke and major cardiovascular events (MACEs). Methods: Findings for the relative risk (RR) of stroke and MACEs with respect to serum 25(OH)D concentrations at baseline from prospective cohort studies were plotted against mean follow-up time. Fifteen studies from mainly European countries and the United States were used for stroke and nine studies for MACEs. Linear regression analyses were used to study data for follow-up periods of up to 10 years and for more than 10 years. Results: For stroke, the linear regression fit for 1–10 years is RR = 0.34 + (0.065 × follow-up years), r = 0.84, adjusted r2 = 0.67, p 20 ng/mL should, therefore, be recommended for everyone likely to be at risk for stroke or MACEs and indeed in the general population.
Grant et al. (Fri,) studied this question.