A new meta-analysis published in the journal Nutrition confirms that people with Alzheimer’s and Parkinson’s have lower vitamin D levels than healthy controls.
Both Alzheimer’s and Parkinson’s have become of greater interest in public health in recent years, due to increasing life expectancy, an overall larger elder population and subsequent increase in incidence of both diseases. Vitamin D has been proposed to play a role in both. Many studies have looked at the vitamin D levels of people with these diseases and compared them to healthy controls and have found low levels. Furthermore in a recent high quality study, a randomized controlled trial showed that vitamin D may be of some help in Parkinson’s. Similar studies are underway for Alzheimer’s.
In this study, the researchers led by Yan Zhao wanted to pool together all the studies that have looked at vitamin D levels in Alzheimer’s or Parkinson’s patients and compared them to healthy controls, to see what all the studies combined had to say. They searched the MEDLINE database and found eleven studies that met inclusion criteria; six studies on Alzheimer’s and five on Parkinson’s.
Here is what they found:
- Vitamin D levels were lower in people with Alzheimer’s compared to controls in five of six studies.
- When the six Alzheimer’s studies were pooled together, vitamin D levels were significantly lower than controls. The standard mean difference was -1.39.
- Vitamin D levels were lower in people with Parkinson’s compared to controls in all five of five studies.
- When the five Parkinson’s studies were pooled together, vitamin D levels were significantly lower than controls. The standard mean difference was -1.33.
There have been several proposed reasons for why people with Alzheimer’s and Parkinson’s have lower levels, or why people with low levels may be at greater risk of getting Alzheimer’s or Parkinson’s. Some of these proposals include:
- People with Alzheimer’s and Parkinson’s get less sun exposure than most people.
- Low vitamin D levels may prevent the immune system from clearing what are called b-amyloid plaques and lewy bodies, both of which play central roles in Parkinson’s and Alzhiemer’s.
- Vitamin D receptors are widely expressed in human brains, and further research has suggested that vitamin D likely plays a key role in brain development.
The reason why people with Alzheimer’s and Parkinson’s have low vitamin D levels could be a combination of the reasons listed above.
Lastly, while this current meta-analysis only highlights an association between vitamin D and Alzheimer’s and Parkinson’s, not a causative relationship, it does highlight that patients with these diseases are more likely to be deficient in vitamin D compared to healthy controls. Therefore, patients with Alzheimer’s and Parkinson’s should be screened for vitamin D deficiency and treated as needed.