APOE and diabetes mellitus: Although several cross-sectional and case–control studies have suggested an inverse association between serum 25-hydroxyvitamin D (25[OH]D) and cognitive function decline assessed using neuropsychological tests among patients with T2D [17–19], these studies had limitations of small sample size, low quality of study design, and insufficient adjustment for critical confounders such as apolipoprotein E (APOE) ε4 genotype, severity of diabetes, and lifestyle factors.