Vitamin D

What is it?

Vitamin D is a fat-soluble vitamin that occurs naturally in some foods, is added to others, found in dietary supplements, and is naturally produced when skin is exposed to UV rays of the sun.

What other names is it known by?

Other names for vitamin D include:

• the sunshine vitamin

• cholecalciferol (D3)

• calciferol

• calcidiol

• calcitriol

• calcifediol

• 25-Hydroxy vitamin D

• and ergocalciferol (D2).

The forms of vitamin D in foods and supplements are usually D3 and D2.

What does it do?

It is required for regulating calcium and phosphate in the body, and therefore plays a role in keeping the bones, muscles, and teeth healthy. It promotes calcium absorption in the gut and helps to maintain normal calcium and phosphate concentrations in the blood in order to support bone mineralization and muscle health.

In addition, it plays a role in:

• helping to reduce inflammation in the body

• supporting cellular growth

• neuromuscular and immune function

• neurotransmitter signaling

• muscle contraction

• regulation of heart beats

• reduction of blood clotting

• and the metabolism of glucose.

Vitamin D has several functions in the health of the skin and gut. In the skin it functions to regulate the process by which skin is replenished, it supports normal cell growth, plays a role in maintaining the integrity of the skin's function as a barrier, and aids in wound healing. It also is thought to protect against certain cancers of the skin. In addition, it plays a role in regulating hair growth and vitamin D deficiency is thought to play a role in alopecia.

In gut health, vitamin D plays a role in not only maintaining the integrity of the gut epithelial barrier, therefore protecting against leaky gut syndrome, but it helps to increase gut microbe diversity, improving the types of good gut bacteria present. In doing so, it can help support immunity. Many studies have identified a correlation between vitamin D deficiency and imbalances in the health of the gut microbiome, often associating the vitamin D deficiency and microbe imbalance with autoimmune and gut diseases.

Where do I get it?

Vitamin D obtained from foods, supplements and sun exposure needs to undergo 2 transformations in the body in order to be active. The first one occurs in the liver where Vitamin D is converted to 25-hydroxyvitamin D [25(OH)D], or calcidiol. The second occurs in the kidneys which forms the physiologically active 1,25-dihydroxyvitamin D [1,25(OH)2D], or calcitriol.

A study published in 2019 investigated how much vitamin D could be produced based on a certain amount of sun exposure, while also avoiding sunburn, and comparing that with dietary intake. The study was conducted in Switzerland. What the authors found was that in the spring and summer months, with 22% of the skin exposed, 1000 IU of vitamin D was synthesized in 10-15 minutes of sun exposure. In fall and winter, the same level of vitamin D production was often unachievable before sunburn due to the amount of reasonable skin exposure during these cold months, and the time it would take to get to that level of vitamin D. The authors also found that the vitamin D intake from foods represented only 10% of the vitamin D production and remained fairly consistent throughout the year. It was concluded that during the fall and winter months, it was very difficult to get the levels of vitamin D recommended for health.

It’s a cultural practice in several countries in Europe and Scandinavia to supplement with vitamin D and omega-3 during the winter months, and this study may explain why. Nevertheless, talk with a doctor, dietitian, and dermatologist about appropriate sun exposure and if that’s something that would be a healthy practice for you. Some individuals may be more prone to skin cancers than others, and it’s best to get a professional opinion before going without protection for your skin. Also ask whether a vitamin D supplement is right for you, when, and which one might be the best option.

Foods and dietary supplements are in the form of D2 (ergocalciferol) and D3 (cholecalciferol) which are well absorbed in the small intestine. Because vitamin D is a fat-soluble vitamin, the presence of fat can help enhance absorption.

When the blood is tested for Vitamin D, it is usually looking at the serum concentrations of 25(OH)D and reflects the vitamin D produced via skin from exposure to UV rays, and that obtained from foods and dietary supplements.

It is challenging to meet daily vitamin D requirements from foods alone because, so few foods actually contain adequate levels of vitamin D. This is why, in many parts of the world, foods are fortified with vitamin D (such as milk), and supplements are recommended to have adequate levels. The chart below provides some examples of foods which contain vitamin D. It’s important to note that an animal’s diet will also affect the amount of vitamin D in its tissues.

Vitamin D is measured in both mass (micrograms or mcg) and international units (IUs). International units are a measure of the biological effect of a substance. Both the micrograms and IUs are provided below.

How much do I need?

In the US, the RDA is as follows:

*Adequate Intake; infants are often not able to get enough vitamin D from breast milk alone and may require supplementation.

How to get it in my diet?

It’s challenging to get enough vitamin D in the diet; therefore you may want to talk to your doctor or dietitian about whether vitamin D supplementation is right for you. Some ideas on how to include foods rich in vitamin D into your meals are below.

Breakfast: 2 large, scrambled eggs with ½ cup white mushrooms = 52 IU

Lunch: Sockeye salmon salad with ½ cup white mushrooms = 117 IU

Dinner: 3 oz trout = 81 IU

Total: 250 IU (350 IU deficit)

References

DSM Nutritional Products. (2020). Vitamins Basics. Retrieved from https://www.dsm.com/content/dam/dsm/human-nutrition/pdfs/vitamin-basics.pdf

Drake, V. J. (2011). Vitamin D and Skin Health. Oregon State University, Linus Pauling Institute. Retrieved from https://lpi.oregonstate.edu/mic/health-disease/skin-health/vitamin-D

Religi, A., Backes, C., Chatelan, A., Bulliard, J-L., Vuilleumier, L., Moccozet, L.,…& Verne, D. (2019). Estimation of exposure durations for vitamin D production and sunburn risk in Switzerland. J Expo Sci Environ Epidemiol, 29(6), 742-752. Retrieved from https://pubmed.ncbi.nlm.nih.gov/30992519/

U.S. Department of Health and Human Services, National Institutes of Health, Office of Dietary Supplements. (2021). Vitamin D: Fact Sheet for Health Professionals. Retrieved from https://ods.od.nih.gov/factsheets/VitaminD-HealthProfessional/

Yamamoto, E. A., & Jorgensen, T. N. (2020). Relationships Between Vitamin D, Gut Microbiome, and Systemic Autoimmunity. Font. Immunol, 10:3141. Retrieved from https://www.frontiersin.org/articles/10.3389/fimmu.2019.03141/full