Tuesday October 14 2008
The skin uses sunlight in the production of vitamin D
“People with lower levels of vitamin D are more likely to have Parkinson's disease,” says The Daily Mail today, following a US study into the links between the disorder and insufficient blood levels of what it terms the “sunshine vitamin”. The newspaper said that the area of the brain most affected by Parkinson's is highly sensitive to vitamin D, but it is not clear whether lack of the vitamin is a cause or consequence of having the disease.
This study measured vitamin D levels in almost 300 people aged around 65, who either had Parkinson’s disease, Alzheimer’s disease or were generally healthy More than half of patients with Parkinson's had low levels of vitamin D, as did 41% of those with Alzheimer's disease.
However this study is unable, on its own, to confirm whether a lack of vitamin D is a cause of Parkinson’s disease, as it does not establish whether a lack of vitamin D preceded the onset of the disease. The results of this study should be interpreted as preliminary without further research to corroborate its findings.
Where did the story come from?
Dr Marian Evatt and colleagues from Emory University School of Medicine in the US carried out this research. It was supported by the National Institutes of Health and other funding sources in the US, and published in the peer-reviewed
medical journal, Archives of Neurology
What kind of scientific study was this?This was a cross-sectional study comparing the frequency of vitamin D deficiency among in people with Parkinson’s and Alzheimer’s disease with healthy “control” participants of similar ages. Three groups of 100 people with an average age of around 65 took part in the study.
The researchers selected participants from an existing research database of volunteers compiled between 1992 and 2007. In it, volunteers with Parkinson’s or Alzheimer’s disease had been found through memory and movement disorder clinics, while healthy “control” participants had come from general medical clinics and community events. Over 90% of the volunteers were white.
All participants had a thorough examination by cognitive or movement disorder neurologists, with classification of participants as having either Parkinson’s or Alzheimer’s disease, or healthy control status (no previous neurological disease or cognitive impairment), based on standard criteria set by the researchers. People who had restless leg syndrome, essential tremor, or a history of stroke or transient ischaemic attack were excluded.
The researchers selected every fifth volunteer in the database for their study (by date of enrolment), until they had 100 people with Parkinson’s disease, with an average age of 65 years.
They then matched 100 randomly selected people with Alzheimer’s disease (average age 66 years) and 100 healthy controls (average age 66 years) after matching them with the Parkinson’s disease group for age, race, gender, region of residence, and variants of the APOE gene they carried. The APOE gene is known to affect risk of developing Alzheimer's disease, and may affect risk of dementia in Parkinson's disease.
The researchers obtained blood samples for the 300 participants, and tested them for levels of vitamin D [a particular form called 25 (OH) D]. Those testing the blood samples did not know which group each individual was from.
The researchers defined vitamin D insufficiency as having 30 nanograms per mL or less and vitamin D deficiency as having 20 nanograms per mL or less. Four people were excluded for having unusually high vitamin D levels.
The proportion of people with vitamin D insufficiency or deficiency was compared between the three groups. They looked at whether the month or season when the blood sample was taken affected these results, as vitamin D is known to be produced by sunlight acting on the skin, and levels of sunlight vary across the year.
What were the results of the study?
The researchers found that over half of the people with Parkinson’s disease (55%) had vitamin D insufficiency, and this was higher than the proportion of people with vitamin D insufficiency in the Alzheimer’s group (41%) or the healthy controls (36%).
The proportion of people with vitamin D deficiency was also higher in the Parkinson’s disease group (23%) than in the Alzheimer’s disease group (16%) or the healthy control group (10%), although only the difference from the healthy controls reached statistical significance.
Other than more samples in the Parkinson’s disease group being taken in the summer/autumn than in the healthy control group, but there were no significant differences between any other groups.
What interpretations did the researchers draw from these results?
The researchers concluded that low levels of vitamin D are more common (prevalent) among people with Parkinson’s disease than among those with Alzheimer’s disease or healthy people of a similar age. They say that their data could “support a possible role of vitamin D insufficiency in [Parkinson’s disease]”.
They call for further research to determine why vitamin D levels vary in these groups, and to study the role of vitamin D in the development of Parkinson’s disease.
What does the NHS Knowledge Service make of this study?This study takes a snapshot of vitamin D levels in different groups of older adults who had either Alzheimer’s disease and Parkinson’s disease, or were generally healthy. There are some limitations to note:
- An association between two factors (in this case vitamin D levels and Parkinson’s disease) at one point in time cannot prove that one factor caused the other.
- This type of study cannot establish whether people in this study had low vitamin D levels before they developed Parkinson’s disease, or whether their levels of vitamin D dropped after they developed Parkinson’s disease. The authors acknowledge that the latter may be possible, as patients with Parkinson’s may have decreased activity levels and less sun exposure.
- Patients with neurodegenerative diseases such as Parkinson’s and Alzheimer’s disease may have other risk factors for vitamin D deficiency, such as a life indoors, lack of dietary sources of the vitamin, renal impairment, several social and economic variables, or taking medications that affect vitamin D absorption or metabolism. The researchers were unable to take these into account in their analysis as the original database did not record this type of detail.
- The majority of people in this study were white, and lived in southern latitudes of the USA (all participants lived south of 39°N). The results may not be representative of people from different ethnic backgrounds or living in different regions.
- The proportion of blood samples taken in different seasons differed across the groups. This may have affected results. However, if this was a problem it should have reduced the difference seen between the Parkinson’s and healthy groups.
Although this type of study cannot prove causality on its own, it can point the way towards areas that need future research.
Elderly people are known to be at risk of vitamin D deficiency, and anyone who is concerned that they or an elderly relative are not getting enough should seek their doctor’s advice on whether increasing their vitamin D intake through diet or supplements would be appropriate.