What are vitamin D2 and D3? Where do human beings get them from? How are they transformed in our body? And, most importantly, what are the practical implications of all this?
Vitamin D is a fat-soluble vitamin that is essential for bone health and for several other functions in the body.
Insufficiency (mildly low levels) or deficiency (significantly low levels) of vitamin D is extremely common. Such a deficiency has been associated with many different health-related problems though in most cases it is not known whether vitamin D is the cause of the health problem.
This is important and corrects common misconceptions, so I will repeat myself:
– For most medical conditions, including depressive disorders, that have been associated with insufficiency or deficiency of vitamin D, it is not known whether vitamin D plays a causative role in those conditions.
– We should NOT assume that vitamin D supplementation will be helpful for the treatment of those medical conditions.
Where do we get vitamin D from?
Though we get small amounts of vitamin D (typically less than 5 mcg/day) from food, the great majority of vitamin D in our bodies is made in the skin exposed to sunlight.
Ultraviolet rays in sunlight act on 7-dehydrocholesterol in the skin to convert it into vitamin D.
Tip: The production of vitamin D in the skin is mostly in the first 30 minutes of exposure to the sun. The significance of this is that 1) even 30 minutes per day of sun exposure to bare skin will markedly improve vitamin D levels, and 2) staying in the sun for a long time on one day will not help much. So, long periods of sun exposure, which increase the risk of skin cancer, are also not helpful in terms of vitamin D production.
Tip: Use of ANY sunscreen, even 15 SPF, will markedly reduce production of vitamin D in the skin.
What are vitamin D2 and vitamin D3?
Vitamin D2 is called ergocalciferol or calciferol. It is found in certain plant foods. It is also sometimes given as a supplement though it may be better to give vitamin D3 instead.
The form of vitamin D made in the skin is vitamin D3, also called cholecalciferol. It is also found in some foods of animal origin, e.g., eggs, cheese, and fish. And, of course, it is available as an over-the-counter supplement.
Biotransformation of vitamin D
The terminology around vitamin D can be confusing, but I will try to make it clear in this short paragraph.
1. Both vitamin D made in the skin (vitamin D3 or cholecalciferol) or that ingested in the diet (vitamin D2 or ergocalciferol or calciferol) are not physiologically active.
2. Vitamin D (both D2 and D3) are converted in the liver to 25-hydroxy vitamin D (or calcidiol; also written as 25-OH vitamin D), but this has low levels of physiological activity as well.
3. The 25-hydroxy vitamin D is further converted in the kidney, and in local tissues throughout the body, to calcitriol (1, 25-dihydroxy vitamin D). It is calcitriol that is the active form of vitamin D in the body.
The calcitriol made in the kidneys then enters the blood while the calcitriol made in local tissues in different parts of the body is used locally.
The conversion of 25-hydroxy vitamin D (calcidiol) to calcitriol in the kidney is closely regulated by parathyroid hormone. This fact becomes important to mental health clinicians in lithium-induced hyperparathyroidism.
When we say, check the vitamin D level, which of these compounds do we measure? The 25-hydroxy vitamin D level is the widely accepted measure of vitamin D status. Why do we not check serum calcitriol since that can be measured and it is almost entirely calcitriol that matters biologically? Because calcitriol has a very short half-life in the blood, only a few hours, while in adults the half-life of 25-hydroxy vitamin D is about two weeks (Bouillon and Carmeliet, 2018).
Next, please read How should vitamin D status be measured?
How should vitamin D status be measured?
How should we define vitamin D insufficiency (deficiency)?
How should vitamin D insufficiency or deficiency be treated?
Vitamin D for depressive disorders?
Does vitamin D supplementation increase the risk of kidney stones?
Bouillon R, Carmeliet G. Vitamin D insufficiency: Definition, diagnosis and management. Best Pract Res Clin Endocrinol Metab. 2018 Oct;32(5):669-684. Review. PubMed PMID: 30449548.
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