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Unveiling the Power of Vitamin D: Mechanisms of Action and Functions of Vitamin D

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Unveiling the Power of Vitamin D: Mechanisms of Action and Functions of Vitamin D

Discover the essential functions of Vitamin D as we unravel its impact on bone health, immune function, and disease prevention. From clinical applications to practical tips, this concise guide simplifies the complex world of Vitamin D, emphasizing the importance of balance for overall well-being. Join us on this insightful journey into the vital pathways of Vitamin D.

Understanding the Target for Vitamin D

In a 2018 article, authors explain how active Vitamin D3, called calcitriol, works and what it does in the body.1 Here is what they had to say:

  • Intake of vitamin D3 increases absorption of calcium and phosphorus in the small intestine.
  • Vitamin D increases the elimination of calcium in the kidneys (increasing calcium retention and reabsorption).
  • It increases bone mineralization.
  • It boosts the development of osteoclasts (the osteoclast is the cell that resorbs bone) and bone reabsorption. Bone reabsorption is the process by which osteoclasts break down the tissue in bones and release the minerals. This results in the transfer of calcium and phosphorus into the blood.
  • Decreases parathyroid hormone (PTH) synthesis and release from the parathyroid gland. PTH controls the level of calcium in the blood, and it also controls phosphorus and vitamin D levels.
  • Vitamin D influences over 200 genes. Vitamin D plays a role in cell growth, specialization, cell death, and the formation of new blood vessels.
  • It regulates cell cycle, controlling cell growth and differentiation.
  • High levels of vitamin D hinder blood vessel growth around a tumor and encourage cell transformation.
  • Boosts the body’s natural defense system and helps kill harmful microbes, controlling their ability to cause disease.
  • Vitamin D is an excellent immunomodulator (regulates immune response), by inducing differentiation in immune cells. Prevents autoimmune diseases.
  • It increases and regulates insulin secretion.
  • It reduces renin synthesis (renin is a central hormone in the control of blood pressure and various other physiological functions). Aids in blood pressure regulation.
  • Vitamin D improves cardiovascular health. Increases myocardial contractility and smooth muscle cell proliferation.
  • It reduces inflammation.
  • It reduces left ventricular hypertrophy (LVH). LVH is the thickening of the heart muscle of the left ventricle of the heart.
  • Vitamin D improves hematopoiesis, the process by which blood cells are formed.
  • Vitamin D aids in brain development and improves brain function.

Clinical applications of vitamin D

Vitamin D targets a wide range of disorders in clinical applications, including issues related to the storage of vitamin D in the body. A deficiency in Vitamin D can contribute to various health problems. Some of these issues caused by a lack of Vitamin D include: 1,2,3:

  • Bone-related problems affecting bone health and muscles, such as rickets, soft bones disease (osteomalacia), fragile bones (osteoporosis), and bone breaks (fractures).
  • Multiple sclerosis, osteoarthritis, rheumatoid and psoriatic arthritis
  • Obesity and metabolic syndrome
  • Type 1 and type 2 diabetes mellitus
  • Hypertension
  • Cardiovascular disorders
  • Autoimmune disorders
  • Neuropsychiatric disorders
  • Infectious diseases, such as upper respiratory tract infections, and influenza
  • Chronically fermenting cells. Vitamin D has been researched especially for cancers of the breast, prostate, and colon
  • Some leukemias and myeloproliferative disorders
  • Skin disorders like psoriasis and ichthyosis.

How do we measure vitamin D and what do those values mean?

Vitamin D is usually measured through serum levels of 25-hydroxy-vitamin D (25(OH)D), also known as calcidiol, the circulating form of vitamin D3.

A normal level of Vitamin D varies from person to person, and according to their health condition. Although a consensus regarding the ideal levels of Vitamin D is hard to achieve, most researchers and studies demonstrate that vitamin D insufficiency is a worldwide health problem4.

Maximizing Vitamin D Levels: Understanding Its Longevity and Boosting Production

Vitamin D, a fat-soluble vitamin, lingers in the body for an extended period, with a half-life of approximately two to three weeks. This longevity allows the body to maintain consistent levels, influenced by factors such as the type of vitamin D (D2 or D3), individual variations, and overall health.

Adequate sun exposure, consumption of vitamin D-rich foods, including plant-based vitamin D sources, and supplements if necessary, contribute to sustaining optimal vitamin D levels. Regular monitoring through blood tests ensures informed decisions on maintaining vitamin D status, reducing the risk of deficiencies, and understanding how to raise vitamin D levels effectively.

In terms of vitamin D status in the body in what relates to vitamin D levels (serum 25(OH)-D levels) most experts agree5,6:

  • Severe deficiency < 5 ng/ml (< 12.5 nmol/L)
  • Deficiency < 20 ng/ml (< 50 nmol/L)
  • Insufficiency 20-32 ng/ml (50-80 nmol/L)
  • Sufficiency 32-100 ng/ml (80-250 nmol/L)
  • Excess >100 ng/ml (> 250 nmol/L)
  • Intoxication >150 ng/ml (>325 nmol/L)

What is the recommended daily intake of vitamin D?

For optimal health benefits of vitamin D, it’s recommended to have a circulating level of calcidiol above 30 ng/mL (75 nmol/L). Additionally, maintaining blood concentrations above 32 ng/mL (80 nmol/L) is crucial not only for maximizing intestinal calcium but also for supporting the production of active vitamin D in various tissues.

In summary, keeping the circulating form of vitamin D3, calcidiol, at levels above 75 nmol/L or 30 ng/mL is advisable.

Is it possible to produce too much Vitamin D and get intoxicated?

Vitamin D intoxication is among the rarest medical conditions in all medical literature. The registered cases of intoxication are often caused by negligent or intentional ingestion of extremely high doses of vitamin D for prolonged periods of time 8.

Vitamin D intoxication is defined as a 25(OH)D > 150 ng/ml. It can cause problems like too much calcium in the blood and urine, too much phosphorus, and a decrease in PTH. This can lead to the buildup of calcium salts in the kidneys (nephrocalcinosis) and the hardening of soft tissues, especially blood vessels.

There has never been a reported case of vitamin D intoxication from sun exposure. Have you ever heard of a lifeguard with vitamin D intoxication? No matter how much sun exposure a person has, this will never cause vitamin D intoxication because sunlight itself destroys any excess vitamin D and previtamin D5,10.

The Role of Melanin in Sunlight Exposure and Vitamin D Production

One factor is the presence of melanin (the pigment that produces skin and hair color in humans) in the skin. The pigment melanin in the skin absorbs UV radiation, competing with provitamin D3 and limiting its conversion to previtamin D3. This can reduce the effectiveness of sunlight in producing vitamin D3 in the skin. People with darker skin, usually more exposed to sunlight, produce less vitamin D3 because they have more melanin11.

However, this is not the primary factor preventing excess production of vitamin D in the skin of people constantly exposed to sunlight. During the initial exposure to sunlight, provitamin D3 is efficiently converted to previtamin D3, but with continued exposure, no more than 10-20% of the initial provitamin D3 will end up as previtamin D39.

Vitamin D deficiency – a Worldwide Health Concern

Subclinical vitamin D deficiency and insufficiency may affect the majority of men and women in all age groups in many geographical regions. A systematic review covering 168,000 people from 44 countries reported vitamin D deficiency (serum 25(OH)D <50 nmol/L or 20 ng/mL) in 37% of studies12.

Factors such as low sunlight exposure, age-related decreases in cutaneous synthesis, and diets low in vitamin D contribute to the high prevalence of vitamin D inadequacy13,14. Children, young or middle-aged adults, are equally at increased risk of vitamin D deficiency/insufficiency13,14, however, newborns and institutionalized elderly appear to be at higher risk of exhibiting lower vitamin D levels12.

Conclusion: Navigating Vitamin D’s Vital Impact

Our exploration of Vitamin D reveals its key roles in bone health, immune function, and disease prevention. The message is clear: maintaining optimal levels is crucial for overall well-being. Whether through sunlight, diet, or supplements, finding the right balance requires informed choices and regular checks. As we continue this journey, the profound benefits of Vitamin D underscore the importance of a comprehensive approach. Stay tuned for our next post, simplifying deficiency factors and offering practical prevention tips.

In our next post, we will describe the factors that contribute to vitamin D deficiency and how to prevent it!


  1. Türkmen AS, Kalkan I, Chapter 15 – Vitamin D Deficiency in Children: Health Consequences and Prevention, In Handbook of Food Bioengineering, Food Quality: Balancing Health and Disease, Academic Press, 2018, Pages 471-492, ISBN 9780128114421,
  2. Wang H, Chen W, Li D, Yin X, Zhang X, Olsen N, Zheng SG. Vitamin D and Chronic Diseases. Aging Dis. 2017 May 2;8(3):346-353. doi: 10.14336/AD.2016.1021. PMID: 28580189; PMCID: PMC5440113.
  3. Bikle DD. Vitamin D metabolism, mechanism of action, and clinical applications. Chem Biol. 2014 Mar 20;21(3):319-29. doi: 10.1016/j.chembiol.2013.12.016. Epub 2014 Feb 13. PMID: 24529992; PMCID: PMC3968073.
  4. van Schoor NM, Lips P. Worldwide vitamin D status. Best Pract Res Clin Endocrinol Metab. 2011 Aug;25(4):671-80. doi: 10.1016/j.beem.2011.06.007. PMID: 21872807.
  5. Holick MF. Vitamin D status: measurement, interpretation, and clinical application. Ann Epidemiol. 2009 Feb;19(2):73-8. doi: 10.1016/j.annepidem.2007.12.001. Epub 2008 Mar 10. PMID: 18329892; PMCID: PMC2665033.
  6. Grant WB, Holick MF. Benefits and requirements of vitamin D for optimal health: a review. Alternative Medicine Review: a Journal of Clinical Therapeutic. 2005 Jun;10(2):94-111. PMID: 15989379.
  7. Holick MF, Chen TC. Vitamin D deficiency: a worldwide problem with health consequences. Am J Clin Nutr. 2008 Apr;87(4):1080S-6S. doi: 10.1093/ajcn/87.4.1080S. PMID: 18400738.
  8. Koutkia P, Chen TC, Holick MF. Vitamin D intoxication associated with an over-the-counter supplement. N Engl J Med. 2001 Jul 5;345(1):66-7. doi: 10.1056/NEJM200107053450115. PMID: 11439958.
  9. Holick MF. Photosynthesis of vitamin D in the skin: effect of environmental and life-style variables. Fed Proc. 1987 Apr;46(5):1876-82. PMID: 3030826.
  10. Webb AR, DeCosta BR, Holick MF. Sunlight regulates the cutaneous production of vitamin D3 by causing its photodegradation. J Clin Endocrinol Metab 1989; 68:882 – 7;; PMID: 2541158
  11. Bikle DD. Vitamin D: Production, Metabolism and Mechanisms of Action. [Updated 2021 Dec 31]. In: Feingold KR, Anawalt B, Blackman MR, et al., editors. Endotext [Internet]. South Dartmouth (MA):, Inc.; 2000-. Available from:
  12. Hilger J, Friedel A, Herr R, Rausch T, Roos F, Wahl DA, Pierroz DD, Weber P, Hoffmann K. A systematic review of vitamin D status in populations worldwide. Br J Nutr. 2014 Jan 14;111(1):23-45. doi: 10.1017/S0007114513001840. Epub 2013 Aug 9. PMID: 23930771.
  13. Holick MF. Vitamin D deficiency. N Engl J Med. 2007 Jul 19;357(3):266-81. doi: 10.1056/NEJMra070553. PMID: 17634462.
  14. Holick MF. High prevalence of vitamin D inadequacy and implications for health. Mayo Clin Proc. 2006 Mar;81(3):353-73. doi: 10.4065/81.3.353. PMID: 16529140.

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