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Vitamin D


Until relatively recently, Vitamin D was appreciated mostly for the crucial role it plays in bone development and health. Now, there are numerous studies that demonstrate a link between vitamin D deficiency and many diverse conditions: cancer, infection, insulin resistance, muscle weakness, depression, autoimmune and cardiovascular diseases. However, it is currently still unknown whether vitamin D is a cause or a consequence of these conditions. In addition, it is unclear whether supplementation can improve these diseases and what are the optimal levels needed to reduce the risk of developing them. Despite these uncertainties, one thing is clear: vitamin D deficiency affects great many people around the globe.


Only few foods are naturally abundant in vitamin D. These include oily fish, such as salmon, mackerel and tuna, and fish 

Vitamin D levels can imporve throgh sun exposure and supplementation.

oil, such as cod liver oil. Some foods, for example, dairy products and cereals, are fortified with vitamin D. The most efficient way to obtain vitamin D is through sun exposure. Vitamin D3 (cholecalciferol), one of the inactive forms of vitamin D, is synthesized in the skin upon the action of ultraviolet light. Liver and kidneys transform vitamin D3 into the active form – calcitriol (1, 25-dihydroxy vitamin D3). Understandably, many factors that affect sunlight intensity, such as geographical position, time of the year, or weather, also alter vitamin D production. In addition, elderly people and those with darker skin innately synthesize less vitamin D, while overweight people store most of it and thus have lower bioavailability. It is this variability in vitamin D production, coupled with indoor-oriented lifestyles and disproportionate fear of the sun, that puts many people at risk of vitamin D deficiency.

Risk Factors


Majority of people with mild to moderate vitamin D deficiency show no symptoms. Bone pain, muscle weakness, and fractures are some of the signs of severe deficiency. Screening for vitamin D deficiency is not recommended for the generally healthy population. However, if you have any of the following risk factors, determining your vitamin D levels would be prudent.


  • Limited sun exposure (indoor lifestyle, hospitalization, protective clothing, sunscreens)

  • Dark skin

  • Being overweight

  • Older age

  • Osteoporosis

  • Malabsorption (diminished ability to absorb nutrients)

  • Medications that affect vitamin D metabolism (some weight loss, cholesterol lowering, and anti-seizure drugs)



Vitamin D deficiency is determined by a blood test. The test that most accurately evaluates vitamin D status is the one that assesses levels of an inactive form - 25-hydroxy D.  This form is converted by the kidneys and some other tissues to the bioactive form (1,25-dihydroxy D). Results of the test are most often expressed in nanograms per milliliter (ng/ml), although nanomoles per milliliter are also used (nmol/mL).  There is disagreement in the field about the optimal levels of vitamin D.  While the Institute of Medicine recommends levels of 20 – 50 ng/mL ( 50 - 125 nmol/ml) as adequate for most people,  other authorities,  such as the Endocrine Society,  recommend raising the minimum level to 30 ng/mL (75 nmol/mL). These values refer to levels needed for bone health. Optimal vitamin D levels required for extraskeletal health have not yet been determined.

 Vitamin D deficiency can be evaluated by a blood test.

Sun to the Rescue?


The duration of sun exposure needed to provide sufficient levels of vitamin D depends on skin pigmentation, area of the skin exposed, latitude, time of day and time of year. For example, studies have shown that in the summer a person with darker skin would need to spend approximately twice as much time in the sun to synthesize the same amount of vitamin D as a person with fairer skin. Likewise, 15 minutes of sun in Miami produces similar vitamin D levels as one sunny hour in Boston. Because of this variability, general recommendations for adequate sun exposure cannot be made. However, each person can still find their own optimal exposure that minimizes risks and maximizes sunlight’s health benefits. If you know that 30 minutes in the sun gives you a mild sunburn (pinkish skin several hours later), then spending 50 % of that time, i.e. 15 minutes, will 

Regular non-burning sun exposure has many helath benefits.

not lead to skin burn but will still provide you with a good amount of vitamin D.

It is estimated that by being in the sun with about 20% of the body exposed (slightly more than both arms) for half the time needed for a mild sunburn is equivalent to approximately 1400–2000 IUs of vitamin D3 supplementation. The face should be protected with a hat as it is most prone to skin damage.


Skin burns must always be avoided as they increase the risk of melanoma and other skin cancers. However, regular non-burning sun exposure is important not only because of vitamin D but because it brings other health benefits as well. Sunlight increases the production of serotonin and endorphins that improve our mood, as well as melatonin that promotes healthy sleep. Even if safe sun exposure cannot prevent your vitamin D deficiency (in winter for example), regularly spending some time in the sun will bring wide-ranging health benefits.

The Vitamin D and Omega-3 Trail (VITAL) study


The VITAL study is an ongoing clinical trial that aims to determine the role of high dose Vitamin D (2 000 IU) and Omega-3 (1g) supplementation in the prevention of cancer and cardiovascular disease. It is a large study with close to 26,000 participants who are being followed over time for instances of cancer of any type and major cardiovascular events (stroke and heart attack).


The first set of results was published at the end of 2018, at a 5.3 year follow up.  At that follow up mark, the study participants who were taking 2 000 IU of vitamin D daily had a similar rate of cancer and cardiovascular disease as the participants that were not given the supplement. The study did suggest a reduction in cancer incidence in African Americans, as well as lower cancer mortality in the general population that received supplementation.


Because of its large and diverse study population as well as the importance of the results, the VITAL study received quite a bit of media attention. Unfortunately, many headlines rushed to declare vitamin D as ineffective without taking a closer look at the study and the factors that may be affecting the interpretation of the results.


Contrary to how it was most often presented in the media, the focus of the study was not on the preventive value of vitamin D sufficiency. At the start of the study, the majority of participants were already in the sufficiency range (>30 ng/mL) and all participants (even the control arm) were allowed to use additional vitamin D supplementation (as long as the amount did not exceed 800 IU). Instead of looking at the differences between people with sufficient and deficient levels of vitamin D (it would be unethical to not treat deficiency), the study only aimed to address high dose supplementation.


Other studies that have looked at the association between vitamin D levels and colorectal cancer as well as cardiovascular risk, have found that once sufficiency levels are met, additional supplementation, above the sufficiency threshold, does not impart extra health benefit. Although low vitamin D levels were associated with increased risk of colorectal cancer and cardiovascular disease, levels higher than sufficiency (30 ng/mL for colorectal cancer and ~25 ng/mL for cardiovascular disease) did not lead to further lowering of the risk. Considering high baseline levels in the VITAL study, it is possible that the study participants may have already met vitamin D requirements for extraskeletal health.


It is also important to note that cancer is a slowly developing disease and it may take more than 5 years for any effect of high vitamin D dose to become apparent. The already observed lower cancer mortality with high dose vitamin D indicates an association between vitamin D and cancer aggressiveness and progression.


The study did not find any adverse effects associated with 2000 IU vitamin D supplements.

Effects on other conditions such as diabetes, high blood pressure, and infections are expected to be published later.



Vitamin D supplements come in two forms: D2 (found in plants, mainly mushrooms) and D3 (found in animals, mainly fatty fish). Foods, such as milk and cereal, are most often fortified with D2 form. Supplementation is routinely done with D3 as it more efficiently raises levels of vitamin D in the blood.




The Institute of Medicine recommends 600 IU of vitamin D daily for a healthy population, aged 1-70 years. The recommended daily dose for people over 70 is higher, 800 IU. Other authorities argue for higher intakes, 1000-2000 IU, needed to achieve levels above 30 ng/mL. The maximum safe dose is currently set by the Institute of Medicine at 4,000 IU.


The best way to determine the dose you need is to evaluate your baseline vitamin D levels. As approximation, 400 IU (10 mcg) of vitamin D3 will raise your vitamin D levels over time by 4 ng/ml (10 nmol/L).


USP (United States Pharmacopeia) certified supplements are available from Nature made.

Last updated: February 25, 2019

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