Vitamin D was linked to sharper cognition, but only at 5000 IU and up

Nearly half of people with mild cognitive impairment also have disturbed sleep, and that combination marks out a group at elevated risk of progressing to Alzheimer's disease. Vitamin D has long been discussed as a possible influence on brain health, yet its relationship with cognition in people carrying both MCI and poor sleep has remained unclear. If a cheap, widely available supplement genuinely tracked with better cognitive function in this high-risk group, that would be worth knowing. This study set out to explore exactly that question, in what its authors describe as the first attempt to do so in this specific population.

Researchers assessed 54 adults with diagnosed or suspected mild cognitive impairment plus disturbed sleep, defined by validated questionnaires: a Pittsburgh Sleep Quality Index score of 5 or more, or an Epworth Sleepiness Scale score of 10 or more. Participants averaged 67.2 years of age, 46% were male, and average BMI was 29.5. Cognition was measured with the Montreal Cognitive Assessment, known as the MoCA, while vitamin D supplement intake was captured by self-reported questionnaire. The analysis adjusted for age, sex, body mass index, the type of vitamin D taken and education, and everyone was assessed at a single point in time.

People taking 5000 IU or more of vitamin D daily showed a marginally significant association with higher total MoCA scores, at 3.78 points, compared with those taking no daily vitamin D. Lower doses showed no significant relationship with cognition at all, and there was no difference between people taking vitamin D3 and those taking D2. Two other factors also tracked with better scores: holding a college degree was associated with 3.14 higher points, and being overweight with 3.59 higher points. Those parallel associations are a useful reminder that many things separate supplement takers from non-takers in a study of this kind.

The authors present this as the first study to examine vitamin D supplementation and global cognition specifically in people with both sleep disturbance and MCI, so it opens a question rather than settling one. Because everyone was measured once, the design cannot distinguish whether vitamin D supports cognition or whether cognitively sharper people are simply more likely to take high-dose supplements. Self-reported intake adds further uncertainty, and 54 people is a small base for comparing dose groups. A trial that assigns doses at random and follows people over time would be needed to test whether this association means anything causal.

For a reader taking a typical daily vitamin D dose, this study offers no evidence of a cognitive benefit: only the highest doses showed any association, and even that was described as marginal. It is a reason to read headlines about vitamin D protecting the ageing brain with scepticism, since the evidence here is a one-time association in a small and quite specific group. Notably, education tracked with cognitive scores at least as strongly as the supplement did. Nothing in these data supports reaching for high doses in the hope of sharper thinking.