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10 Fascinating Facts About Vitamin D

Updated: Oct 15, 2023

1. Vitamin D is actually a steroid hormone that interacts with our genes.

There are Vitamin D Receptors (VDR) located in almost every tissue in our body and in ALL the major organs. Thousands of VDR binding sites can be found throughout the genome controlling hundreds (possibly thousands) of genes.

2. About 50% to 90% of this nutrient is absorbed through the skin via sunlight.

Very little vitamin D comes from diet and foods like eggs, liver, and oily fish. Plus, optimal absorption from food does require good fat digestion and healthy villi in the intestines of the gut, which many struggle with, unfortunately.

3. We only make vitamin D in the skin when the sun is high enough in the sky that it stimulates a chemical reaction in our skin.

The UV index also needs to be 3 or more. (You can look up the UV index at So, in many countries there is a limited window during the year to make vitamin D. Also, the skin needs to be uncovered and free of sunscreen for this chemical reaction to occur.

Summer is the best time to take advantage of the bright sun to start building up depleted vitamin D levels. Ideally between the hours of 10 and 2 when your shadow is shorter than your height. Just make sure you don’t burn!

The letter D in front of the sun

4. About a billion people worldwide are thought to be deficient in this nutrient.

Fifty percent of the population is thought to have insufficient levels. A recent study of children in Auckland, New Zealand, found one-third didn’t meet the recommended levels!

Vitamin D deficiency is more common in women, dark-skinned people, those who are obese, and the elderly.

5. Vitamin D3 is vital for bone health and is needed to prevent rickets, bone disease, and osteomalacia/osteoporosis.

Man grasping his wrist due to joint pain

It ensures optimal absorption of calcium from the GI tract into the blood. But, vitamin K2 and magnesium are also needed to help lock calcium into bone tissue. Plus, magnesium is needed to convert vitamin D to its final usable active form in the body. When supplementing with Vitamin D3, it is recommended to include K2 and magnesium supplementation.

Daily supplementation of Vitamin D3 (with K2) at levels around 1,000-5,000 IU is recommended over one large weekly or monthly dose that can otherwise overwhelm the kidneys and reduce conversion to its active form.

6. Vitamin D primes and strengthens the immune system.

Image of virus being attacked by immune cells

It is known to reduce the risk and severity of infections, particularly acute respiratory infections. (Studies suggest that complications and death from COVID-19 were more likely to occur in those with low vitamin D levels).

By increasing levels of T regulatory cells, vitamin D prevents overactive immune responses and keeps the immune system balanced.

Therefore, it can reduce the risk of a huge range of immune dysregulation dynamics including autoimmune diseases like diabetes (type 1), multiple sclerosis or SLE, or chronic allergy.

7. Vitamin D even has a role in gut health.

It is critical for keeping good tight junctions that prevent increased intestinal permeability (leaky gut) and all the downstream inflammatory effects.

8. There appears to be a sweet spot like most other nutrients.

Vitamin D toxicity is rare but high levels can deplete other nutrients like magnesium and vitamin A. It can also increase blood calcium levels leading to calcification in soft tissue or even arteries. If vitamin D levels are too high, it can suppress, instead of enhance the innate immune system. This may be helpful for those with chronic inflammatory autoimmune conditions though. At least in the short term as a form of triage.

It is often debated what the optimal vitamin D level is. The medical system largely considers levels above 50 nmol/l to be adequate. However, from a functional medicine perspective, the optimal 25 hydroxy vitamin D3 level is 125 to 200 nmol/L or at least over 100 nmol/L.

A blood test that looks at both inactive 25(OH) and active 1-25(OH) forms of D3 is preferred but is not commonly available. In New Zealand, testing for 25(OH) vitamin D costs around $60, even if requested by your GP, as it is not always subsidized by the NZ government.

"Mortality is the most important clinical outcome. Maintaining optimal vitamin D blood levels of 100-150 nmol/L or 40-60 ng/ml (USA) may increase your life expectancy by 2 years."

Dr. William B. Grant

9. Optimal levels could reduce the risk of cancer by up to 75%.

Women with breast cancer pink ribbon

According to vitamin D researcher Michael F. Holick (Ph.D., M.D.) raising vitamin D levels to at least 125 nmol/L would reduce the risk of cancer, all combined, by 75%!!!

Women with vitamin D blood levels above 150 nmol/L had an 82% lower incidence of breast cancer compared to women with levels <50 nmol/L. This is likely due to vitamin D’s impact on hormonal health and how it regulates and balances other hormones (like oestrogen).

A study in The Journal of Clinical Endocrinology (2010) suggested 85,000 cases of breast cancer could be prevented in North America alone with vitamin D supplementation!!

Colorectal, ovarian, non-Hodgkins, kidney, and endometrial cancer risk could also be reduced with optimal blood vitamin D levels.

10. Vitamin D plays a role in the health of ALL body systems.

An increased risk of cardiovascular disease, obesity, diabetes, and cancer is linked to insufficient vitamin D levels. Low vitamin D levels are also associated with seasonal affective disorder, arthritis, osteoporosis, and autoimmune diseases.

Furthermore, multiple studies in adults and children have demonstrated that vitamin D deficiency is associated with increased risk and greater severity of infection, particularly of the respiratory tract.

The effects of vitamin D on the body are wide-ranging and potentially devastating at suboptimal levels. Please make it a priority to check your vitamin D levels regularly. It could save your life!


Just some of the many!

Vitamin D Metabolism, Mechanism of Action, and Clinical Applications.

Vitamin D: Production, Metabolism, and Mechanisms of Action. (2017).

Wintertime Vitamin D Status and Its Related Risk Factors Among Children Living in Auckland, New Zealand. NZ Med J.2019 Oct 25;132(1504):67-76.

A summary of the health benefits and disease incidence prevention

that could be achieved by raising the public's vitamin D levels to 125 nmol/L.

Holick, Michael F., PhD., M.D., Boston University School of Medicine,

textbook - Physiology, Molecular Biology, and Clinical Applications

(2nd Ed 2010 Humana Press). Page 12.

Vitamin D Supplements and Prevention of Cancer and Cardiovascular Disease. N Engl J Med 2019; 380:33-44

Vitamin D and risk of cause specific death: systematic review and meta-analysis of observational cohort and randomised intervention studies

Meta-analysis of All-Cause Mortality According to Serum 25-hydroxyvitamin D.

Am J Public Health. 2014 Aug;104(8):e43-50.

Avoidance of Sun Exposure Is a Risk Factor for All-Cause Mortality: Results From the Melanoma in Southern Sweden Cohort. J Intern Med.2014 Jul;276(1):77-86.

Serum 25-Hydroxyvitamin D Values and Risk of All-Cause and Cause-Specific Mortality: A Population-Based Cohort Study. Mayo Clin Proc.2018 Jun;93(6):721-730.

Vitamin D and inflammation: Potential Implications for severity of COVID-19.

Vitamin D levels appear to play role in COVID-19 mortality rates: Patients with severe deficiency are twice as likely to experience major complications.

The Possible Role of Vitamin D in Suppressing Cytokine Storm and Associated Mortality in COVID-19 Patients.

The Role of Vitamin D in Prevention and Treatment of Infection

The Implication of Vitamin D and Autoimmunity: a Comprehensive Review

Vitamin D and Breast Cancer. Oncologist. 2012 Jan; 17(1): 36–45.

Effects of Vitamin D Supplementation on Intestinal Permeability, Cathelicidin and Disease Markers in Crohn's Disease: Results From a Randomised Double-Blind Placebo-Controlled Study. United European Gastroenterol J.2015 Jun;3(3):294-302.

Association of intestinal permeability with admission vitamin D deficiency in patients who are critically ill.

Relationships Between Vitamin D, Gut Microbiome, and Systemic Autoimmunity

Vitamin D Deficiency. N Engl J Med 2007; 357:266-281.

Health benefits of vitamin D and sunlight: a D‑bate. Holick, M. F. Nat. Rev. Endocrinol. 7, 73–75 (2011); doi:10.1038/nrendo.2010.234.

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