'No solid evidence' vitamin D keeps the brain health...

"Vitamin D does not protect against dementia," the Mail Online reports.

This headline was prompted by a review of previous research that investigated the effects of vitamin D on neurological diseases.

Researchers were specifically interested in Alzheimer's disease, motor neurone disease, multiple sclerosis and Parkinson's disease.

They struggled to find any conclusive evidence on either the effects of vitamin D supplements or exposure to sunlight (which helps stimulate the production of vitamin D).

This was largely because very few studies used reliable research methods, such as randomised trials in humans.

In fact, many of the studies were conducted in mice and we don't know if the results would be the same for humans.

We know that vitamin D keeps bones, teeth and muscles healthy. But based on the available research, we can't say whether vitamin D is good for the brain.

Where did the story come from?

The study was carried out by researchers from the University of Adelaide and the University of South Australia.

No funding sources were reported.

It was published in the peer reviewed journal Nutritional Neuroscience.

The Mail Online reporting is quite misleading. It suggests most people think vitamin D is good for the brain. It's debateable whether this is in fact a common belief.

The Mail also focuses its attention on dementia when most of the studies included in this review were related to multiple sclerosis and Parkinson's disease.

The news website also reports that this study disproves claims that vitamin D is good for the brain. This is not the case: unproven is not the same as disproven.

What kind of research was this?

Vitamin D has attracted the attention of neurologists and related specialists in recent years.

This is because some studies suggest lack of vitamin D leads to an increased risk of neurological disease.

And at the same time, other studies suggest there are potential benefits from taking vitamin D supplements for neurological diseases.

What remains uncertain is whether being deficient in vitamin D contributes to the onset of neurological disorders or, conversely, is a symptom of neurological disease.

The researchers carried out a systematic review to investigate this debate, using previously published animal studies and clinical studies investigating the effect of vitamin D on 4 neurodegenerative diseases:

Alzheimer's disease
motor neurone disease
multiple sclerosis
Parkinson's disease

Systematic reviews usually take great care to find all the relevant studies on one topic, and critically assess each study in an unbiased way.

They're a robust way of summarising the research evidence on a particular topic of interest, but the evidence is only as good as the studies they include.

What did the research involve?

The researchers looked for all articles published up to 2016 that had looked at the positive effects of vitamin D on neurodegenerative diseases, or investigated past or present sun exposure in groups of people with neurological diseases.

Studies were included if they looked at:

the link between vitamin D levels or vitamin D supplementation and neurodegenerative disease
the effect of sun exposure on neurodegenerative disease

Studies were excluded if they:

discussed vitamin D alone without neurological outcomes
did not adequately measure vitamin D
used people's opinions or case studies

What were the basic results?

The researchers evaluated 231 studies and included 73 in the review.

Of those:

46 reported on multiple sclerosis, 21 of which reported a protective effect from vitamin D
12 reported on Parkinson's disease, 9 of which reported a positive effect from vitamin D
10 reported on Alzheimer's disease, 8 reported a protective effect from vitamin D
5 reported on motor neurone disease, but only 1 showed a protective effect from vitamin

The researchers also found that sun exposure, independent of vitamin D production, may be protective against multiple sclerosis.

But there was insufficient data to support the use of oral vitamin D supplementation as a substitute for getting vitamin D from sunlight in those with multiple sclerosis.

How did the researchers interpret the results?

The researchers stated that on the basis of this systematic review, it's not possible to make strong recommendations on the benefits of vitamin D in neurodegenerative disease.

They stated: "It's unclear if vitamin D mediates a protective benefit in neurodegenerative disease or whether it is an associative marker of UV exposure, which may contribute to as of yet unidentified neuroprotective factors."

Conclusion

This study finds very little hard evidence supporting the beneficial effect of vitamin D on neurodegenerative diseases.

That said, it's still a useful piece of research as it demonstrates the need for further studies in this area.

The lack of conclusive evidence in this systematic review may be largely due to the quality of the studies included.

Many were conducted on mice, meaning the results are very difficult to generalise to human populations.

There are also other limitations.

Many of the clinical studies in humans used self-reporting questionnaires.

This can lead to inaccuracies as people may forget, underestimate or overestimate their exposure to sun or their vitamin intake.

Definitions of vitamin D sufficiency and insufficiency differed across the studies, meaning there was no agreement on the right supplement levels.

The dose of vitamin D in the capsules given to participants varied between studies, meaning some people may have received more than others.

Finally, there are many confounding factors that may affect vitamin D status, which may have influenced the results of each study.

These include people's physical activity levels, diet and the severity of their disease.

The most important point to note is that neurodegenerative diseases have a variety of different causes, many of which are unknown.

Other factors, such as regular exercise, eating a healthy diet, not smoking and limiting your alcohol consumption, may play a more important role in preventing these diseases than vitamin D.