This was the conclusion that researchers in Australia came to, after conducting a systematic review and analysis of more than 70 clinical and pre-clinical studies.
They report their findings in a paper published in Nutritional Neuroscience.
Lead study author Krystal Iacopetta, a doctoral candidate at the University of Adelaide, says, "Past studies had found that patients with a neurodegenerative disease tended to have lower levels of vitamin D compared to healthy members of the population."
But what was not clear from these, she and her colleagues note, is whether low vitamin D contributes to neurodegeneration or merely accompanies it.
Their analysis, says Iacopetta, contradicts "an emerging belief [...] that higher levels of vitamin D can impact positively on brain health."
However, while they found no robust evidence of a "neuroprotective" role for vitamin D, they did not rule out that the "sunshine vitamin" might be a marker for some other protective factor.
They add that further studies are needed to identify a mechanism through which UV exposure might have this effect.
Neurodegenerative diseases are those that damage and kill nerve cells, or neurons, in the brain and other parts of the central nervous system. While they have this feature in common, their causes, symptoms, and how they progress can vary considerably.
Alzheimer's disease, for example, is a neurodegenerative disease that causes dementia and whose hallmarks include buildup of certain toxic proteins in the brain.
Another example is Parkinson's, a disease that kills cells that produce dopamine, a chemical that the brain needs to control movement and other functions.
Multiple sclerosis (MS) is a disease that attacks the protective covering on the fibers that connect neurons to one another, causing breakdown in communication and, eventually, death of cells.
While Alzheimer's and Parkinson's are more common in older people, MS tends to strike earlier in life.
Vitamin D, sunshine, and health
Our bodies make vitamin D when UV rays from the sun fall on exposed skin. It is also present naturally in certain foods and in fortified products.
For many people, these sources of vitamin D may suffice, but some groups may need to take supplements to meet their daily requirement.
Whether vitamin D comes from UV exposure, food, or dietary supplements, it has to undergo two chemical changes before the body can use it. One change takes place in the liver, and the other takes place mostly in the kidneys.
Vitamin D is important for health in several ways. It helps the body to make and maintain bones, regulate cell growth, control muscles, reduce inflammation, and modulate immune function.
In some of these roles, vitamin D interacts directly with genes that instruct cells how to make the proteins that control the various functions.
No 'causal role' in neuroprotection
Iacopetta and colleagues note that an increasing number of studies have been suggesting — on the basis of "associative evidence" — that "vitamin D is neuroprotective."
This has spurred an increase in "clinical and preclinical exploration" of the possibility that the vitamin might be used as a treatment for neurodegenerative diseases.
For their study, they searched well-known databases for reports of clinical and preclinical studies that investigated vitamin D in neurodegenerative disease.
From an initial screening that yielded 231 studies, they reduced the list to 73 by applying "strict criteria." These included the fact that the reports had to describe "original studies" that examined the effect of vitamin D levels or sun exposure on neurodegenerative disease.
Iacopetta says that their analysis took into account methodology, sample size, and effects, both in "treatment and control groups."
But the authors found "no convincing evidence supporting vitamin D as a protective agent for the brain."
They concluded instead that "the link between vitamin D and brain disorders is likely to be associative — as opposed to a directly causal relationship."
UV light may impact brain by other means
The findings, however, do not rule out the possibility that UV exposure might benefit the brain "in ways other than that related to levels of vitamin D," notes senior study author Mark R. Hutchinson, who is a professor at the University of Adelaide.
He explains that "some early studies" have suggested that exposure to UV from the sun might have a "positive impact" on MS and similar neurological disorders.
Their findings allow for the possibility that "UV light may impact molecular processes in the brain in a manner that has absolutely nothing to do with vitamin D," he adds.
A lot more research needs to be done before we can "fully understand what's happening," he concludes.
"We could not establish a clear role for a neuroprotective benefit from vitamin D for any of the diseases we investigated."