Vitamin D has varied roles in the different systems of our bodies, from our digestive system, to our cardiovascular system, to our immune system, to things like mental health, regulation of our blood sugar, cancer resistance, and fertility, along with being a factor in disease processes.
By Dr. Robert Curtis
(Doctor of Chiropractic, Certified Medical Examiner (U.S. Dept. of Transportation), A.C.E. Certified Health Coach and Fitness Nutrition Specialist)
Published: 11/1/19 (updated 8/4/20)
Of course, Vitamin D's most known role relates to healthy bone consistency.
Below, I’ll first discuss what we know about its link to optimal body systems’ health and then will get into what we know about its role in disease processes.
Side note: click here to see my companion article, Vitamin D – The Basics – What’s Your Status?, where I address its different forms, sources, and what good levels are.
Cells in key areas of the brain have receptors for vitamin D.
Vitamin D has been shown to play a role in circadian rhythms and sleep, to affect the growth of nerve cells, and to have impacts on the developing brain. 
An indication of the importance of vitamin D and heart health is more evident in its lack than presence.
Accordingly, a number of studies report low blood levels of vitamin D are linked with heart disease. 
Again here, as with heart health, vitamin D's role and importance is more revealed in its lacking.
In relation to this, an analysis of a group of studies, showed that in a small subset of study participants, with uncontrolled blood sugar management, benefit was obtained from additional vitamin D. 
This is the most known area of vitamin D activity and impact.
As bones are living tissue and constantly reforming based on the stresses we put on them, their metabolism is dependent on the flow of various factors, including an important role for vitamin D.
In a primary role, vitamin D helps the body absorb calcium more efficiently, with calcium being a key mineral for our bones' strength.
Additionally, vitamin D collaborates with certain hormones to insure calcium remains at optimum levels in the blood, going so far as to prioritize calcium amounts in the blood over bones, as necessary, as calcium is also crucial in things like muscle contraction, including the heart.
When there is enough calcium in the blood, the overage is stored in the bones.
If calcium blood levels fall, the body will respond to raise them by first activating vitamin D stored in the liver to up the absorption of calcium from food and then by pulling calcium out of the bones.
One review of multiple studies showed an association with sufficient levels of vitamin D in our blood (the 25(OH)D form) and a higher degree of bone density though the authors noted that amount levels correlating to this continued to be an unsettled topic. 
Vitamin D seems to help improve the motility of sperm cells imparting a positive effect on male fertility.
There are studies correlating better pregnancies with normal vitamin D levels.
These two points are drawn from a recent scientific review of numerous vitamin D studies. 
Numerous studies of immune-related conditions like atopic dermatitis and rheumatoid arthritis have shown a link to low levels of vitamin D.
In the lab, vitamin D has shown indications of “anti-inflammatory” and “antioxidant” properties with vitamin D reducing an inflammatory molecule in the blood of mice , although not all studies agree.
And in a recent 2019 randomized, double-blind, placebo-controlled study of 1,500 human subjects over the course of a year, no inflammatory reduction benefit was shown, and in the case of those with low vitamin D status, less than 20 ng/mL at the start of the study, they developed an increase in a commonly measured inflammatory marker, hsCRP (high-sensitivity C-reactive protein). 
Though in the case of food allergies, some researchers think vitamin D, due to its effects on the immune system, may play a helpful role.
This ties into a handful of smaller studies pointing out that with low vitamin D levels there is a notable increased risk for allergies to certain foods, but it was also asserted that more research was necessary. [8, 9]
Dietary fat is a needed component for the absorption of vitamin D (one of the fat soluble vitamins) thus, those who don’t absorb sufficient fat run the risk of accordingly having lowered levels of vitamin D, assuming their sun exposure is inadequate, otherwise.
Such persons can be those with celiac disease , inflammatory bowel diseases (IBD) like Crohn’s & colitis, or those who’ve undergone gastric bypass surgery. [11, 12]
This ties in with the flip side of these disease challenges in that good vitamin D levels are observed in those with healthy guts (good gut microbiome status).
Sub-optimal vitamin D has been associated with poorer gut microbiome status, inflammation of the gut, and gut related diseases such as IBD and colon cancer, whereas greater vitamin D amounts have been shown to be helpful in relieving colitis.
In one study a degree of colitis relief was achieved by increasing vitamin D via supplementation. 
When we don’t get enough vitamin D (and calcium) regularly, bones can become weak and brittle.
So, getting sufficient amounts of vitamin D and calcium regularly are necessary to maintain bone health, and severe vitamin D deficiency can lead to weakened bone diseases like rickets in children and osteomalacia in adults.
Osteoporosis (loss of bone mass) can occur over time as well with general insufficiency (as opposed to a severe deficiency) of vitamin D, and calcium, too. 
A recent review of several studies indicated that higher levels of vitamin D in the blood may reduce blood pressure and the risk of heart disease by a small amount.
Overall though, the authors in this assessment concluded that further study was needed as results were not sufficiently clear. 
An association has been shown in people with pre-diabetes having higher levels of insulin resistance along with low vitamin D levels. 
Low levels may also increase the risk of developing diabetes. 
Concerning illnesses, like depression, a growing body of
evidence is demonstrating a connection to low levels of vitamin D. [18, 19]
In regard to an increasing risk for developing either Alzheimer’s or Parkinson’s diseases, a link is shown with Vitamin D at low levels. 
It’s not just colon cancer that’s associated with low levels of vitamin D. Higher levels of vitamin D are associated with lower risk for prostate, and breast cancers.
In the lab, cancer cells don’t seem to do as well when exposed to higher levels of vitamin D. They don’t divide or invade other tissues as well, and they seem to die easier.
It’s unclear whether supplementing with vitamin D would reduce the risks of cancer in people. 
Vitamin D has many health-promoting roles in the body. Most of the evidence is for bone health, but it’s also associated with a healthy immune system, digestive system, heart health, mental health, blood sugar regulation, fertility, and resistance to cancer.
However, the strongest evidence for what vitamin D deficiency causes involves rickets and osteomalacia. The rest of the conditions have some evidence, but it’s not clear to what extent they’re caused by vitamin D deficiency/insufficiency, or what other factors also come into play.
In the realm of a plant-based diet, vitamin D has a crucial role, as with any diet – and with sunlight, is readily produced, and otherwise can be further obtained in fortified foods or supplements if getting enough sun exposure is problematic – best determined in consultation with your primary health care provider.
Overall, testing your vitamin D levels allows you to at least roughly determine what your status is.
When testing, keeping in mind the time of year is important as well, as sun intensity is greatest in late spring on into the beginning of summer, then diminishing gradually, falling to its lowest point late fall on into the beginning of winter.
Accordingly, you would get more vitamin D production in your system during the period of greater sun intensity, and would likely register higher amounts in your test during that time, and typically the lowest levels would result during the late fall into early winter.
Click here to see my companion article, Vitamin D – The Basics – What’s Your Status?, where I address its different forms, sources, and what good levels are.
Looking to try out transitioning to a whole-food, plant-based diet?
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By the way, if you do want to print it, note in full it's fifty-four pages but you're given options to reduce the page count by eliminating sections or not printing pictures for example.
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Oh yeah, and as I just noted above, if you do sign up, I'll become your Certified Meal Garden Professional host – but again; no pressure, no obligation, no credit card – just sharing of my other resources that you can pursue if and when you want : )
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Last but not least, it is advised that for any dietary changes you consult with your primary health care professional and or dietician.
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Thank you, researchers!
1. Vitamin D plays a role in many systems of our bodies. Image by www slon pics from Pixabay (CCO license)
2. Sugar levels are revealed in the blood. Vitamin levels can be, too. Image by Photo Mix from Pixabay (CCO license)
3. Inflammation, a common root of disease processes. Image by Voltordu from Pixabay (CCO license)
Thank you, image artists!