While the symptoms of low vitamin D are well known and documented, new research shows a more sinister picture.
Vitamin D is an oil-soluble vitamin that has several important functions in the body:
Through these actions, vitamin D helps keep the calcium and phosphate levels in the blood normal, thereby promoting bone health.
Vitamin D may have other benefits, such as improving muscle and immune function, but these areas require more research.
Unlike other vitamins, vitamin D functions like a hormone, and every single cell in your body has a receptor for it.
When your skin is exposed to sunlight, it turns cholesterol into vitamin D. And, even though it's found in many food sources, it's incredibly difficult to get it from diet only.
Here are 3 things you probably didn’t know about having low vitamin D.
A study done in 2008 showed that low vitamin D can be predictive for fatal strokes. 
Low levels of Vitamin D (25(OH)D) and 1,25(OH)2D were both independent predictors for fatal stroke. To note, they were both reduced in people with prior cardiovascular events.
Those suffering from acute stroke are at increased risk for low vitamin D because of reduced sun exposure and not consuming enough vitamin D.
Vitamin D supplementation in stroke patients has already been shown to reduce osteopenia, fractures, and falls while improving muscle strength. [2,3]
The researchers said, “apart from these beneficial effects, our results suggest that vitamin D might also directly protect against stroke. This hypothesis is supported by data indicating that vitamin D may protect against hypertension, diabetes mellitus, and atherosclerosis.”
In addition, vitamin D exerts antithrombotic and neuroprotective effects and was shown to attenuate ischemic cortical injury in rats. We acknowledge that our results may be limited because the z values for vitamin D metabolites were based on data from patients referred to coronary angiography and not from a “healthy” control group.”
The research pointed out that people treated with vitamin D have higher survival rates. In addition, it is a promising preventative or therapeutic approach for those that have had a stroke or are at a high risk of stroke.
Maintaining concentrations of at least 75 nmol/L (30 ng/mL) of Vitamin D have been shown to be most effective in producing favorable health outcomes. 
In 2008 researchers started to debunk the idea that low vitamin D was a cause of muscle and bone pain.
288 people with joint and muscle pain participated in the study. They had vitamin D levels of less than 20 ng/mL. Half received a placebo and the other half received 50,000 IUs of ergocalciferol once a week for 3 months.
Ergocalciferol, also known as vitamin D2, is a type of vitamin D found in food and used as a dietary supplement.
They used two pain scales to measure the outcome.
What were the results?
The vitamin D group was no better off than the placebo group. The researchers came to the conclusion that, “Low vitamin D levels are not associated with diffuse muscle and joint pain. Moreover, treatment with vitamin D does not reduce pain in patients with diffuse pain who have low vitamin D levels.” 
Anti-epileptic drugs may affect vitamin D and bone health according to a 2014 study published in the journal of epilepsy research. 
Vitamin D deficiency is common in patients with epilepsy on antiepileptic drugs. Monitoring of vitamin D should be considered as part of the routine management of patients with epilepsy.
Vitamin D is important for bone health, and vitamin D deficiency may contribute to other disorders (e.g., autoimmune, infections, cancer, degenerative, diabetic, and vascular). However, significantly low vitamin D levels have been observed in those taking antiepileptic drugs.
According to the study, antiepileptic drugs that are enzyme producing are being blamed for osteoporosis risk given their effects on vitamin D.
A low vitamin D level can be diagnosed with a blood test called 25-hydroxyvitamin D or 25(OH)D (OH = hydroxy, D = vitamin D).
Although there is no formal definition of vitamin D deficiency, most groups use the following values in adults:
Vitamin D is an essential nutrient and that low levels of vitamin D can cause a wide range of nasty effects.
However, low levels of vitamin D may cause issues more sinister than we think.
9 factors that can cause you to be at risk for low vitamin D are:
While this is not an exhaustive list, it does provide a framework for determining if you are at risk for developing low levels of vitamin D.
So besides getting sick often, hair loss, extreme fatigue and more - low levels of vitamin D can be a predictor of lowered longevity.
However, increasing your levels of vitamin D aren’t so straightforward as many blogs will have you believe.
Read more about the dosing guidelines according to vitamin D level here.
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