Vitamin D supplements are extremely important during the winter months because reduced skin exposure to sunlight during the winter, as well as a change in the angle at which the sun’s rays hit earth’s atmosphere, result in limited stimulation of our skin to make vitamin D. If you live north of the 37^thparallel (which runs from Richmond, Virginia to San Francisco, California), your skin will have very little opportunity to synthesize vitamin D from sunshine between November and February.[fn value=3][/fn] In fact, one study monitoring vitamin D levels in the blood of 7,473 adults found that vitamin D levels dropped by half (from 30 nanograms per milliliter of blood down to 14 nanograms per milliliter of blood).[fn value=4][/fn] Other factors affecting the ability of the skin to synthesize vitamin D include:[fn value=1][/fn] [fn value=5][/fn]

• Aging – Aged skin has fewer vitamin D precursors available for the synthesis of vitamin D
• Air pollution – Despite being at the proper latitude, San Diego and Los Angeles have so much air pollution, it blocks the type of sunlight required for vitamin D synthesis
• Skin color – Darker skin has more melanin, which blocks some of the sunlight required to make vitamin D

Research over a decade old has alerted us to a very important fact – well over 50% of Americans have low levels of vitamin D.[5] The commonality of low vitamin D, combined with the knowledge that vitamin D is critical for just about every tissue in the human body, has led to an increase in the consumption of vitamin D supplements. Indeed, vitamin D supplements consistently rank as one of the most consumed dietary supplements.[fn value=6][/fn] However, some people are still hesitant to take supplementary vitamin D because of old and inaccurate fears dating back to the 1950s that fortification and supplementation will cause toxicity.[fn value=7][/fn]

Thankfully, a new study published in Mayo Clinic Proceedings clears up a lot of this misconception and shows how uncommon it is to observe vitamin D toxicity.[fn value=8][/fn] This study followed over 20,000 people for 10 years and tracked their vitamin D status. The governmental organization, The Institute of Medicine, considers a vitamin D level greater than 50 nanograms per milliliter to be high. The threshold for what most clinicians consider toxic levels of vitamin D is 140 nanograms per milliliter.[fn value=8][/fn] During this 10-year study there were 233 people whose vitamin D levels ever tested above 50 nanograms per milliliter, and only 37 whose blood levels exceeded 100 nanograms per milliliter (36 of these people never developed a single symptom of vitamin D toxicity). Even more importantly, only one case of vitamin D toxicity occurred in the 20,000 study participants, which equates to 0.005% of participants developing symptoms of vitamin D toxicity. This case of toxicity occurred in a person who was supplementing with 50,000 IU of vitamin D daily (most experts recommend around 2,000 to 5,000 IU per day and no more than 10,000 IU per day) and whose blood level was 364 nanograms per milliliter.

“The evidence is clear that vitamin D toxicity is one of the rarest medical conditions and is typically due to intentional or inadvertent intake of extremely high doses,”[fn value=7][/fn] writes Michael Holick MD PhD, a professor of medicine, physiology and biophysics at Boston University School of Medicine and one of the world’s leading experts on vitamin D.

The term “vitamin D” refers to several different forms of this vitamin. Two forms are functional in humans: vitamin D2, which is made by plants, and vitamin D3, which can be made by human skin when exposed to sunlight. Vitamin D2 supplements are largely made from irradiated plant sterols while vitamin D3 supplements are mostly derived from lanolin. For many years it was thought that vitamin D2 is just as effective as vitamin D3 at raising blood levels of vitamin D. However, it has become more and more accepted within the scientific community that vitamin D3 is a more effective supplemental form.[9] For more information about dosing and testing vitamin D please click here. (There are few vitamin D3 supplements that are vegan; therefore if you are looking for a vegan option you might prefer vitamin D2).

Learn more about vitamin D and immune function in the winter

Do you take a vitamin D supplement? Has taking a vitamin D supplement supported your mood through the winter? Feel free to share your success story in the comments section below.