Are low vitamin D levels associated with poor cognition in the elderly?

This is a recurring story: low levels of vitamin D are associated with poorer cognition (and many other outcomes), but interventions to replace vitamin D have no effect on improving anything other than vitamin D levels. (LOE = 1a-)

Goodwill AM, Szoeke C. A systematic review and meta-analysis of the effect of low vitamin D on cognition. J Am Geriatr Soc 2017;65(10):2161-2168.  [PMID:28758188]

Meta-analysis (other)

Other

Various (meta-analysis)

These authors systematically reviewed several databases and registries to find observational and interventional studies of vitamin D and cognition. The authors don't report their process for including and excluding individual studies. Overall, they identified 41 studies for systematic review. For meta-analysis they identified 26 observational studies with between 19 to 9556 patients and 3 interventional studies with between 63 and 128 patients. The study duration ranged from 4 months to 10 years, with only 5 studies that lasted more than 5 years. Two studies only recruited women and 5 studies only recruited men. Overall, the studies were of low to moderate risk of bias. In the observational studies, low vitamin D levels were associated with poorer cognition (odds ratio = 1.24; 95% CI 1.14 - 1.35), but the interventions showed no effect on cognition.