Monday January 26 2009
Sunshine stimulates the body to produce its own vitamin D
“Taking vitamin D supplements in middle age could cut the risk of Alzheimer's disease in later life,” claims the Daily Mail. The newspaper says new research has showed high levels of vitamin D were “closely linked to staying mentally sharp in old age” and that taking supplements could prove a simple and cheap way of cutting dementia risk.
The study behind this story has found a link between levels of vitamin D in blood and mental awareness. It did this by comparing almost 2,000 elderly people’s blood levels of vitamin D with performance in simple mental tests. However, participants did not receive clinical diagnoses of Alzheimer’s disease or another form of dementia.
While scientists found a relationship between vitamin D and mental awareness, this study is early research and its design means it can’t prove that a lack of vitamin D is a cause of lowered mental ability. Other important factors, including general health and fitness, activity levels, vitamin B12 and blood pressure, may explain the difference in cognitive ability seen in this study.
The findings of this research will need to be confirmed in larger studies, preferably randomised controlled trials, before the value of vitamin D in preventing cognitive decline in old age is known. If further research can confirm that low vitamin D levels can limit cognitive function, then supplements could provide a cheap way to reduce the problematic effects of dementia.
Where did the story come from?
This research was conducted by Dr David Llewellyn, Kenneth Langa and Iain Lang from the University of Cambridge, Peninsula Medical School in Exeter, the University of Michigan and the Veterans Affairs Centre for Practice Management and Outcomes Research in Michigan.
The research used data from the Health Survey England which is funded by the Department of Health. The study was published in the Journal of Geriatric Psychiatry and Neurology, a peer-reviewed medical journal.
What kind of scientific study was this?
This was a cross sectional study exploring the relationship between vitamin D levels and cognitive impairments in the elderly. Previous laboratory and animal studies have suggested that vitamin D may prevent cognitive decline, but the picture in humans is unclear and the results from small, human studies are conflicting.
In this study researchers relied on data collected as part of the Health Survey England (HSE) in 2000. The HSE is a series of surveys about health that are carried out annually. The HSE is designed to provide a nationally representative sample of adults over the age of 16, living in private households in England.
Each year the HSE features a set of core questions plus a changing selection of questions which focus on a particular condition or population group. In 2000, the special focus of the HSE was on older people and social exclusion. In addition to these surveys, physical measures are also taken, including blood samples.
The HSE survey assessed cognition using the Abbreviated Mental Test (AMT). This is a neurocognitive screening tool which includes 10 items assessing attention, orientation in time and space, and memory. People who gave three or more incorrect responses out of 10 were considered ‘cognitively impaired’.
Participants for this publication were people aged over 65 years living in private households plus a sample of people aged over 65 years living in institutions. A total of 4,170 people or their proxies were interviewed. Serum vitamin D levels were attained from blood samples of 1,766 people (from 708 men and 1,058 women).
The researchers then assessed the relationship between the vitamin D in serum (divided into quartiles) and cognitive impairment. They took into account other factors that might be responsible for this link including smoking, alcohol consumption, psychiatric disorders and self-reported medical history.
The researchers also accounted for the season in which serum vitamin D was tested, as sunlight stimulates the body’s own natural production of vitamin D. They took into account impaired mobility as this may result in less time spent outdoors and therefore lower concentrations of vitamin D in the blood.
Those with cognitive impairment were older than those with normal cognition so researchers adjusted for age. In those for whom BMI data was also available, 1,279 participants, the researchers examined whether BMI was contributing to differences in serum vitamin D.
What were the results of the study?
Overall, there were 212 cognitively impaired adults out of 1,766 people aged over 65 years. Those who were cognitively normal, were younger, more likely to have educational qualifications, consumed alcohol, had higher BMI and were less likely to have impaired mobility, stroke or low levels of albumin (a blood protein). Without accounting for these differences, those who were cognitively normal also had higher levels of serum vitamin D.
When researchers took into account the other factors contributing to these outcomes, they found that people with the lowest levels of vitamin D in their blood (8-30 nmol/L) were more than twice as likely to be cognitively impaired than those with the highest levels (66-170 nmol/L).
This was the only significant difference, with individuals falling into the middle two concentration bands (31-44 nmol/L and 45-65 nmol/L) no more likely than those with the highest levels to be cognitively impaired.
When separating participants into men and women, this pattern was only significant for men (i.e. women’s chances of being cognitively impaired were not influenced by their serum vitamin D levels).
When serum concentration of vitamin D was divided into levels of ‘severely deficient (<25 nmol/L)’, ‘deficient (≥25 nmol/L and <50 nmol/L)’ and ‘insufficient (≥50 nmol/L and <75 nmol/L)’, only those with severe deficiency were at increased risk of cognitive impairment, about 2.7 times more likely, than those with sufficient levels of serum vitamin D (>74 nmol/L).
What interpretations did the researchers draw from these results?
The researchers conclude that levels of serum vitamin D are generally lower in the cognitively impaired general population, (i.e. that high levels are associated with lower odds of cognitive impairment).
Importantly, the researchers acknowledge that the cross sectional nature of their study means that they cannot determine whether low levels of serum vitamin D are actually a cause of cognitive impairment. They say that although it is unlikely, it’s possible that a genetic predisposition to both cognitive impairment and vitamin D status would confound the links seen in their study.
What does the NHS Knowledge Service make of this study?
This nationally representative cross-sectional study has shown that there is a relationship between vitamin D levels and cognitive impairment in people aged over 65 years. However, the cross-sectional design of the study means it cannot show causation.
Also, news headlines might imply that there was some link being made in this study with Alzheimer’s disease, a clinical diagnosis of dementia. As dementia is not the same as cognitive impairment, this is not the case.
In their discussion the researchers have raised the most important limitations within the study. These should be kept in mind when interpreting the results and media coverage:
- Diagnoses of cognitive impairment were not made clinically, (i.e. it relied on a screening test that would not have been 100% accurate).
- The researchers acknowledge that their study cannot prove causation. The researchers say a genetic predisposition could be behind the observed relationship, though they say this is unlikely.
- It is possible that other factors may be responsible for reductions in cognitive ability and vitamin D levels. These could include socio-economic status, vascular risk factors, and other aspects of diet or vitamin intake associated with dementia.
- Age is the strongest risk factor for cognitive decline. In their analysis the researchers were able to adjust for the fact that the average age of controls was 77.6 years compared to 83.3 years for those with cognitive impairment.
- While the researchers were able to adjust for the age difference between the two groups, several other measures of general health and fitness could have differed between the younger and older groups. For example, vitamin B12, activity levels or blood pressure might also be different in the older people with lower vitamin D levels. The researchers were unable to assess the effect of these or other potential risk factors. A randomised trial would be needed to eliminate this source of error.
- As the British elderly population is predominantly white, the results of the study may not be generally applicable to more ethnically diverse populations.
- This study has shown that levels of vitamin D are linked to cognitive impairment in men: the results were not significant for women when other factors were taken into account.
The results of this study linking vitamin D levels and cognition should be viewed as early evidence that will need confirmation in future studies. Only a randomised control trial will determine whether the supplement will have a value when used to prevent cognitive decline.
Undoubtedly, more studies that control for other known risk factors will follow. These will be particularly useful if there is any possibility that vitamin D supplements, which are cheap and easy to take, could help prevent dementia in later years. Vitamin D is also important for bone health and when taken with calcium it may protect adults from osteoporosis and reduce the risk of hip and other fractures.