Vitamin D is so important to prevent and also to help overcome so many diseases. Its importance has been underestimated for many years. Nowadays, finally, scientific research is warning us of the impact of vitamin D to our health. Since it is summer now in the northern hemisphere, I thought that I should dedicate an article to the importance of vitamin D. Hopefully, after reading this and the next article (Part 1 and Part 2), you will be more motivated and less afraid to consciously sunbath :). And I will explain later what I mean with being “conscious” in Part 2.
Vitamin D deficiency is a pandemic . When you look at the estimated vitamin D deficiency in many countries , you see that in the majority of them, it accounts for more than 70% of the population. Even in countries with a lot of sunlight, such as Brazil, you see that 77% of the population has vitamin D deficiency! This really is a major global health concern. And why is it such a big problem? Well, because vitamin D is crucial to all systems in our body and vitamin D deficiency has been linked to many diseases: musculoskeletal disorders (muscle weakness, falls, fractures), cancer, autoimmune diseases, infectious diseases, neurologic and psychiatric diseases, infertility and adverse pregnancy and birth outcomes, among many other conditions, including all-cause mortality [1, 3]. We will go in more detail through this, but first:
What is vitamin D anyway?
Vitamin D is the vitamin produced in our skin when we are exposed to sunlight. The ultraviolet radiation from the sun is divided in ultraviolet A (UVA) and ultraviolet B (UVB). The UVB radiation is the one that stimulates a precholesterol molecule (7-dehydrocholesterol) present in the human skin to be converted to vitamin D3 (cholecalciferol). Vitamin D3 is then metabolized in the liver to 25-hydroxyvitamin D (25(OH)D) and then in the kidneys to 1,25-dihydroxyvitamin D (1,25(OH)2D or calcitriol) and then travels to the target tissues that contain vitamin D receptors. It is also known that many other tissues in the body, including the brain, prostate, breast, and others, can also locally produce 1,25(OH)2D .
Vitamin D receptors are found in most tissues in the body, including the brain. It is estimated that more than 2000 genes are directly or indirectly regulated by vitamin D (specifically, the 1,25(OH)2D) . Interestingly, within 24h of vitamin D exposure, we can change the expression of more than 1000 genes !
Vitamin D is also present in very small amounts in a few food items (e.g., mushrooms, fish oil), but this is often not sufficient to meet vitamin D requirements. The largest source of vitamin D is therefore sun exposure.
And, as you can imagine, the main cause of vitamin D deficiency is thus lack of exposure to the UVB from the sun.
There is however, a seasonal variation of UVB. The angle between the sun and the Earth impacts the number of UVB rays that reach the Earth’s surface . These rays are the strongest in the summer at 12h-14h. In the winter and in the early morning and late afternoon, the angle is too low and therefore, the UVB rays don’t reach the Earth and consequently, almost no vitamin D3 can be synthesized .
And did you know that vitamin D is so important that actually nonsmokers who avoid sun exposure have a life expectancy similar to smokers who are exposed to the sun?  The researchers even state “avoidance of sun exposure is a risk factor for death of a similar magnitude as smoking” .
Why is vitamin D so important?
Bone and muscle function
The lack of sunlight and its association with the bone-deforming disease called rickets in children was first described in 1822 . Vitamin D deficiency in children can indeed cause growth retardation and classic signs and symptoms of rickets [1, 10]. In adults, vitamin D deficiency can lead to osteoporosis (low bone mineral density – weak bones, which are more susceptible to have a fracture) and increase the risk of fracture [1, 11, 12]. You probably have heard that you need calcium for your bones, but you need appropriate levels of vitamin D as well since it is needed for intestinal calcium absorption.
Vitamin D is also extremely important for the muscles . In fact, vitamin D deficiency has long been associated with muscle weakness . Vitamin D receptors are present in the skeletal muscle . Also, vitamin D deficiency has been associated with an increased risk of falling  and therefore, its effects in fall prevention have been recognized .
However, vitamin D deficiency affects our body in many other ways beyond bone and muscle health. Vitamin D can be considered a hormone, because we don’t need to eat it but it can be formed by our body when exposed to sunlight. It is so important for so many tissues in our body that, as the authors in  write, it is a :
“hormone of reproduction and fertility
hormone of growth and development
hormone of immune and stress response
hormone of the digestive system
hormone of endocrine regulation
hormone of central nervous functions“
The correlation between vitamin D deficiency and several types of cancer has long been recognized. In fact, UVB exposure and vitamin D have been inversely associated with 15 types of cancer .
Almost 80 years ago, a correlation was already observed between living in higher latitudes (i.e., more northern) and an increased risk of dying of common cancers . Because living at higher latitudes diminishes vitamin D3 production (UVB rays are weaker), it has been suggested that an association may exist between vitamin D deficiency and cancer mortality. Both men and women exposed to the most sunlight throughout their lives were less likely to die of cancer [18, 20, 21].
Nowadays, the role of vitamin D in cancer prevention and also treatment has been acknowledged and proved by many scientific studies (e.g. [4, 21-24]). Indeed, several studies showed that vitamin D deficiency increases the risk of both developing and dying from cancer [21, 25]. Adults with vitamin D levels below 25 ng/ml (50 nmol/L), who were then followed for up to 19 years, had an up to 50% increased risk of developing colorectal, breast, prostate, and many other types of cancers [25-27]. A meta-analysis showed that increasing intake to 1000 International Units (IU) of vitamin D3 per day is associated with a decreased risk of colorectal and breast cancer of 50% . Men who took >400 IU vitamin D per day had a reduced risk of developing several cancers, including pancreas, esophagus and non-Hodgkin lymphoma .
In , the authors state:
” Both prospective and retrospective studies help support the concept that it is vitamin D deficiency that is the driving force for increased risk of common cancers in people living at higher latitudes.“
I would rather say “it is one of the main driving forces”.
Vitamin D sufficiency does not only prevent cancers, but it can also improve the survival of cancer patients: in a study in colorectal cancer patients, the ones with highest concentration of vitamin D (i.e., 25(OH)D) had almost 50% (half!) reduced mortality when compared to patients with the lowest blood concentrations . And also, the higher the vitamin D level, the lower the death rate . The same was found for breast cancer [30, 31], ovarian cancer patients  and lymphoma .This is highly relevant!! Interestingly, a study showed that from all the colorectal cancer patients participating in the study, only 3% had sufficient vitamin D levels, meaning that the big majority (97%!) of these patients are deficient in vitamin D !
This was shown in observational studies, meaning that they observed patients’ levels in the beginning of the study but did not actually do an intervention. Do you know what is a randomized controlled trial? It is a study in which people are divided in 2 groups, 1 group receives the intervention (called the experimental group) and the other one doesn’t (called the control group); it is the best type of study to prove if a treatment actually works and it is often the standard used in clinical trials. So, in one of these types of studies, it was found that vitamin D supplements also reduce cancer mortality significantly .
And what about melanoma? Did you know that actually, melanoma is more common in indoor than in outdoor workers?  The author of this paper published in Lancet states:
“outdoor workers have a decreased risk of melanoma compared with indoor workers, suggesting that chronic sunlight exposure can have a protective effect.”
Actually, some authors argue that sun exposure may have an antimelanoma effect through activation of the vitamin D system . You might be saying “What?! Doesn’t sun exposure cause skin cancer?” Well, it’s not black and white. Sun exposure in moderation is essential for you, including to prevent skin cancer, or many other types of cancer as I mentioned above. But if you keep catching sunburns, then you are clearly overdoing it and yes, your skin cancer risk does increase. Well, so shouldn’t you catch sunlight but be “protected” with sunscreen? There are many reasons why I do not recommend “normal” sunscreens (I will discuss this in my next article), but one of them is that sunscreens block the UVB – even a sunscreen with solar protection factor 15 blocks 99% of the UVB . Therefore you will not synthesize any vitamin D, and thus, you might actually be increasing your risk of cancer.
Sunlight exposure, either before or after diagnosis, has been associated with higher survival in melanoma patients [38, 39]. Also, higher vitamin D levels at diagnosis are associated with both better survival and thinner melanomas [40, 41]. Conversely, vitamin D deficiency is associated with a worse prognosis . In addition, melanoma rates do increase with decreased levels of vitamin D . In fact, in a very recent study published in Science Reports, the authors concluded that vitamin D should be used as an adjuvant in melanoma therapy .
And why does vitamin D have a such a profound role in cancer prevention and treatment? It is known that the active form of vitamin D (1,25(OH)2D) inhibits malignant cell proliferation, stimulates apoptosis (programmed cell death), suppresses inflammation and inhibits tumor angiogenesis (formation of new blood vessels which is crucial for a tumor to survive), proliferation (growth), invasion, and metastasis .
Researchers have shown that living more northern is also associated with an increased risk of autoimmune diseases, including type 1 diabetes and multiple sclerosis [45, 46]. Children who received 2000 IU of vitamin D per day during the first year of life and who were followed for 31 years had a 78% reduced risk of developing type 1 diabetes compared to children who did not receive vitamin D supplementation .
The association between vitamin D deficiency and multiple sclerosis (MS) is well known. Higher vitamin D levels are associated with a lower incidence of multiple sclerosis in women . Also, most patients with this pathology have very low levels of serum vitamin D, which has led to the recommendation of screening always MS patients for vitamin D deficiency and give supplementation . It has even been suggested that due to its immunosuppressive actions, the active form of vitamin D may prevent the induction of MS . Indeed, in an article entitled “Vitamin D: a natural inhibitor of multiple sclerosis”, they highlight that MS prevalence increases with decreasing solar radiation, suggesting that sunlight might be protective in MS. They also showed that vitamin D completely inhibited MS induction and progression . Women who received >400 IU vitamin D/day had a >40% reduced risk of developing multiple sclerosis .
There is also an increasing evidence that vitamin D can have an important impact on our mental health. For example, many studies have linked vitamin D deficiency to depression and cognitive performance, in all age groups [53, 54]. Depressive symptoms seem to improve when the individuals are supplemented with vitamin D . Not only vitamin D deficiency and depression seem to co-occur (e.g., ), but also vitamin D deficiency may be a risk factor for late-life depression . Brain serotonin is synthesized from the amino acid tryptophan by an enzyme (tryptophan hydroxylase 2), which is activated by vitamin D .
People who live more northern are at risk of vitamin D deficiency and are also more likely to develop schizophrenia .
It was shown that vitamin D supplementation can prevent respiratory tract infections, such as colds, flu, bronchitis and pneumonia .
Also, it has been shown that exposure to sunlight helps in the treatment of tuberculosis [61-63]. In a study published in Science, it was shown that a probable mechanism for this is that when a macrophage is infected with tuberculosis, it stimulates the cell to increase the production of 1,25(OH)2D and increase the expression of vitamin D receptor . This will enhance the gene expression of the bactericidal protein cathelicidin, which is known to kill tuberculosis and other infective agents.
Inflammatory Bowel Disease
Vitamin D is also a powerful anti-inflammatory. Restoring normal levels can help patients with inflammatory bowel disease. It has been shown that people living in the south have lower incidence of inflammatory bowel disease than people living in the north. A plausible explanation for this is that people in the south are more exposed to sunlight and UVB radiation, and therefore have likely higher levels of vitamin D . In a study, after Crohn’s disease patients were given vitamin D supplements for 24 weeks, the majority of them achieved remission , demonstrating the potential of vitamin D to help in the management of this inflammatory bowel disease.
Vitamin D is very important for reducing hypertension and the risk of atherosclerotic heart disease, heart attack, and stroke. A meta-analysis of prospective studies found an association between lower vitamin D levels and higher risk of cardiovascular disease . The same has been found for ischemic stroke . Interestingly, in patients with stroke, it was found that the lower the blood vitamin D levels, the higher the severity of stroke, and also, vitamin D levels were a prognostic marker of favorable outcome and survival .
Several studies have shown an association between vitamin D deficiency and a higher risk of Alzheimer’s. It has been suggested that levels of vitamin D (i.e, 25(OH)D) below 20 ng/ml (50 nmol/L) are associated with cognitive impairment and the development of dementia . In a systematic review and meta-analysis, it was found that lower vitamin D concentrations were associated with poorer cognitive function and a higher risk of Alzheimer’s disease . In a large prospective study, researchers found a strong association between vitamin D and risk of dementia and Alzheimer’s disease: participants who were severely vitamin D deficient had more than twice the chance to develop Alzheimer’s, concluding in their own words that “vitamin D deficiency is associated with a substantially increased risk of all-cause dementia and Alzheimer disease” . On the other hand, people who have high levels of vitamin D have a low risk of developing this neurodegenerative disease . A possible mechanism for this is that vitamin D helps clearing amyloid-β plaques that develop in the brain of Alzheimer’s patients .
Increase of lifespan
Vitamin D3 supplementation has been shown to prevent overall mortality in adults, meaning that it may increase the lifespan, as shown in systematic reviews and meta-analysis articles [75-77]. These were quite big studies: the study  took into account the results in 56 randomized clinical trials and 95,286 participants and the study  considered a total of 95 studies with 880,128 participants. However, this does not mean that you can do whatever you want and take a vitamin D supplement and this will be the magic bullet to extend your life. You still have to eat healthy and have a healthy lifestyle if you want to effectively prevent chronic diseases and improve your health .
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