Recently I wrote a blog post about some of the reasons why vitamin D is not useless. I mostly covered cardiovascular health and some reduced mortality-related research, here I am getting more into the neurological side of things.
But first, mortality reduction! Some of the most interesting research on vitamin D reducing mortality rates come from ICU patients. The New England Journal of Medicine reported a terribly high 45% mortality rate in vitamin D-deficient patients, but a considerably less horrible 16% death rate in patients with sufficient vitamin D. That is, even sufficient vitamin D slashes risk of death in the ICU by two thirds. Six years later, a study on 135 ICU patients revealed a 32.2% risk of death when vitamin D was below 12ng/mL, but a 13.2% risk when levels were higher. This is about a 2.4 higher risk (Clinics (Sao Paulo)). However, studies attempting to rescue already-deficient ICU patients have not always been successful, underscoring the importance of prevention.
Now for something that will land you in hospital...stroke! After a stroke, around 40% of victims are moderately or severely debilitated, while 15% die soon after (NIH). Vitamin D deficiency is common among stroke victims, with poorer functional outcomes associated with lower levels. For example, each 10ng/mL decrement in vitamin D levels were linked in one study with a doubling in the risk of a poor functional outcome after 90 days (J Stroke Cerebrovasc Dis). In another, vitamin D levels of 30ng/mL or above was associated with a 90% better functional improvement compared to levels under 10ng/mL (Cerebrovasc Dis). While many recommend levels of 50-80ng/mL in order to prevent many age-related diseases, you can see that even sufficient levels are enough to save and improve many lives.
It is well known these days that multiple sclerosis, an autoimmune disease where the immune system attacks the myelin sheaths of neurons (these enable efficient communication), has a higher incidence in colder climates. It is estimated that while the Southern states of the USA have a 57-78/100,000 incidence rate, the northern states have 110-140 cases per 100,000 residents, but in Canada the incidence is 291/100,000 (Healthline)! Genetically low levels of vitamin D are linked with a higher risk of multiple sclerosis too, which most likely calls for supplementation instead of relying on sun exposure (PLoS Med). In fact, more than 90% of people with MS have deficient vitamin D, defined as less than 20ng/mL (clinicaltrials.gov). Interestingly when the immune cells of patients with MS were exposed to vitamin D, their division slowed, which may be one of multiple (sorry) mechanisms behind its effects (Brain).
As for age-related cognitive decline, vitamin D has been linked to a higher risk and faster rate of memory loss and loss of executive function (JAMA). In a study of 76 participants, those with dementia had an average vitamin D level of 16.2ng/mL, while those with mild or no cognitive impairment had an average level of 20ng/mL. For all participants, lower levels of vitamin D sped up cognitive decline, not only those who were already impaired.
So, why is vitamin D levels overlooked in standard blood tests, unlike iron? And if they aren't already doing it, why are people suffering from dementia not given time in the sunshine or supplements? Lack of concern or bias against "mere" vitamins? The "not enough evidence" line looks increasingly like an excuse, especially if someone you love has dementia or any of these issues.