What is vitamin D?
Yes, it is definitely a hormone, which sets vitamin D apart from all other vitamins. However, the human body has to change the form of vitamin D that is made in the skin or that is ingested, from a ‘pro-hormone” or a building block to the active hormone calcitriol or activated vitamin D.
What also sets vitamin D apart from all other vitamins is that the human body is capable of making its own vitamin D. This is done in the skin when the skin is exposed to direct sunlight. Luckily, our bodies can make vitamin D because very few foods contain vitamin D, typically only 10 per cent comes from food. The other 90 per cent or so of vitamin D that we need must come from either the sun or dietary supplements.
Vitamin D is technically not a vitamin because it is a pro-hormone. However, with that said, many people do believe that vitamin D is truly a vitamin[i]. By definition, vitamins are essential to life and when they are lacking in the diet certain diseases can develop.
Scientists established long ago that vitamin D deficiency leads to diseases of the bone. Without vitamin D, bones become weak or soft, causing rickets in children and osteomalacia in adults. Vitamin D deficiency may also be related to problems with the immune, cardiovascular, and metabolic systems but the evidence is still unclear about this.
What does seem clear is that when a person doesn’t get enough vitamin D they can develop diseases related to the deficiency. This explains why some people believe that vitamin D is a vitamin despite the fact that it is also a hormone.
More than a vitamin? Why do we need vitamin D?
Vitamin D has traditionally been associated with bone development. It helps the body absorb calcium and phosphorous in the gut, which are both needed to build and maintain strong, healthy bones, and teeth. It is also responsible for preventing bones from being broken down by the body.
However, vitamin D also appears to work as a building block involved in repair and maintenance of the body. Vitamin D receptors have been found in many different types of tissues in the body, leading scientists to believe that it plays a much bigger role in the health of the body [ii]. Vitamin D research continues to bring more light to this subject!
While there may be a bit of a controversy about whether vitamin D is actually a ‘vitamin’, there is no doubt that vitamin D is important for our health. Health authorities consider vitamin D to be essential to maintaining general health and supporting normal growth of bones and teeth. This is why many healthcare practitioners discuss vitamin D intake with their patients and often recommend a vitamin D supplement like Ddrops.
Updates and edits by Carrie Noriega, MD, FACOG.
 Vieth. Why “vitamin D” is not a hormone, and not a synonym for 1,25-dihydroxy-vitamin D, it analogs or deltanoids . J Steriod Biochem & Mol Bio 2004; 89-90: 571-573. https://www.ncbi.nlm.nih.gov/pubmed/15225841
 Boullion R., Okamura, WH, Norman AW. Structure-function relationship in the vitamin D endocrine system. Endocr rev 1995;16:200-57. https://www.ncbi.nlm.nih.gov/pubmed/7781594