Vitamin D and memory: scientists have found that an effect is only observed at doses of 5,000 IU or more
Researchers at Emory University have discovered an interesting link between vitamin D intake and cognitive function in people at increased risk of further decline in memory and thinking. Only those participants who reported taking at least 5,000 IU of vitamin D daily achieved higher test scores. The researchers did not observe this link at lower doses.
It is worth noting that the researchers did not administer 5,000 IU to the participants, nor did they check whether their memory improved as a result. The study was small and observational – involving just 54 people. Therefore, whilst it suggests a link, it does not prove that the high dose of vitamin D was the cause of the improved results. Furthermore, 5,000 IU exceeds the upper limit for daily intake for adults set in the US (as well as in Ukraine) – 4,000 IU.
The study involved people with two risk factors
The study included 54 adults with diagnosed or suspected mild cognitive impairment (MCI) and sleep problems. The average age of the participants was 67.2 years.
Mild cognitive impairment is a condition in which memory, attention or other intellectual abilities are already worse than expected for a person’s age, but the individual usually still retains their independence in everyday life. MCI is associated with an increased risk of developing dementia, although by no means everyone with MCI goes on to develop Alzheimer’s disease or another form of dementia.
The researchers assessed sleep problems using standard questionnaires: the Pittsburgh Sleep Quality Index and the Epworth Sleepiness Scale. Almost all participants met the criteria for poor sleep according to the first scale.
The result only became apparent from 5,000 IU onwards
Cognitive ability was assessed using the Montreal Cognitive Assessment (MoCA). This is a widely used 30-point screening test comprising tasks testing memory, attention, language, orientation and other functions.
The researchers then compared the results with the participants’ self-reported daily vitamin D intake.
After adjusting for age, gender, education, body mass index and the form of vitamin D, participants in the 5,000 IU or more per day category had an average MoCA score approximately 3.78 points higher than those who did not take vitamin D daily. The p-value was 0.039.
At lower doses, the researchers found no statistically significant association with MoCA scores. There was also no noticeable difference between vitamin D₂ and D₃.
However, this ‘effect’ cannot yet be attributed to the vitamin itself
The most significant limitation of the study is its cross-sectional observational design.
The researchers did not randomly assign participants to groups; they did not give one group 5,000 IU of vitamin D and the other a placebo; nor did they measure how their cognitive abilities changed over time. Participants simply reported their usual supplement intake, after which the researchers compared this data with the test results.
Therefore, other explanations are possible. For example, people with better-preserved cognitive function may be better at adhering to a supplement regimen, pay closer attention to their health, or differ from others in other ways that the study did not fully account for.
Reverse causality cannot be ruled out either: it may not be vitamin D that led to higher scores, but rather that people with better cognitive abilities, for whatever reason, were more likely to take high doses.
The authors themselves describe the study as exploratory – that is, preliminary – and point to the need to verify the findings in larger samples.
A sample size of 54 people calls for particular caution
Another limitation is the size of the study.
A total of 54 participants is rather small even for a general analysis. After dividing the participants according to their vitamin D doses, the individual groups become even smaller.
Furthermore, the statistical association for doses of 5,000 IU was close to the conventional significance threshold: p=0.039. In the article’s abstract, the authors describe this result as ‘marginally significant’, i.e. borderline statistically significant.
Interestingly, in the same model, higher education and being overweight were also statistically associated with higher MoCA scores. This further illustrates how cautiously one should interpret a single association found in a small group.
5,000 IU — this is not a recommendation to take this dose
This is particularly important given the potential appeal of a specific figure from the study.
A dose of 5,000 IU exceeds the upper tolerable level for long-term daily vitamin D intake for adults in Ukraine, which is 4,000 IU. This does not mean that 5,000 IU is necessarily toxic: in cases of confirmed deficiency, a doctor may temporarily prescribe even higher doses. However, such a dose should not be regarded as a safe preventive standard for long-term self-administration.
Moreover, the new study does not prove that taking 5,000 IU specifically improves memory or slows the progression of dementia. It merely found a statistical association between this dose and higher scores on a cognitive test in a small group of participants.
Long-term excessive intake of vitamin D can lead to elevated blood calcium levels and associated complications, including kidney problems. Therefore, the study’s findings should not be interpreted as a recommendation to take 5,000 IU for the sake of improving memory.
Other studies present a mixed picture
The idea of a possible link between vitamin D and cognitive function has long been debated. Low blood levels of the vitamin have repeatedly been linked to cognitive decline; however, this does not necessarily mean that taking supplements can reverse the process.
A systematic review of 20 randomised controlled trials previously found mixed results and did not identify convincing evidence that vitamin D generally improves cognitive function in adults.
There are, however, some individual studies with positive results. For example, in a randomised trial involving 183 older adults with MCI, the group receiving 800 IU of vitamin D per day for one year showed improvements in certain cognitive measures compared with the placebo group. However, differences between studies in terms of participant composition, baseline vitamin D levels, dosages and the tests used do not yet allow for a simple, universal conclusion.
Furthermore, a large observational study from 2026 involving 5,065 older Americans did not support the idea that vitamin D supplements are a universal means of slowing cognitive decline. In particular, the authors saw no grounds for recommending them to people with normal baseline vitamin D levels solely for the purpose of preventing cognitive decline.
What scientists need to investigate next
The new findings are of particular interest to a very specific group: people who had both sleep problems and mild cognitive impairment.
The authors suggest that the early stage of cognitive changes may be a period when potentially modifiable factors are still capable of playing a significant role. However, a much more rigorous trial will be required to test vitamin D specifically.
Ideally, participants with comparable baseline vitamin D levels should be randomly assigned to groups receiving different doses or a placebo, their blood vitamin D concentrations measured, and changes in cognitive function monitored over the long term.
Until such data becomes available, the correct conclusion is considerably more modest: in a small group of people with sleep disorders and MCI, taking 5,000 IU of vitamin D was associated with higher scores on a cognitive test, but the study did not establish that it was vitamin D specifically that caused this difference.
Source
The researchers studied 54 people with diagnosed or suspected mild cognitive impairment and sleep disorders. The average age of the participants was 67.2 years. Vitamin D intake was assessed via self-reports, and cognitive function was evaluated using the Montreal Cognitive Assessment. The study was a cross-sectional observational study, not a randomised trial of supplements.
Authors: Sirui Zhou, Sudeshna Paul, Lynn Marie Trotti, Donald Bliwise, Victoria Pak.
Journal: Sleep Medicine, volume 146, article 108960. Published online on 10 April 2026; print issue: October 2026.