Three common vitamin D questions answered: Is vitamin D deficiency just for vegans, how is vitamin D made in our skin, and what is the best test for vitamin D?

Is vitamin D deficiency just for vegans?

Many times I have heard people say that they stopped eating a plant-based vegan diet because they tested low in vitamin D. The fact is that vitamin D deficiency is not solely an issue for vegans, vegetarians, and raw foodists. A 2011 study estimated the prevalence of vitamin D deficiency in the US population at 42 percent on average and in some segments of the population this prevalence is as high as 82 percent. The researchers in this study defined vitamin D deficiency as a blood level of vitamin D less than or equal to 20 ng/ml. Opinions on this number differ. For example, various clinicians and the Vitamin D Council define vitamin D deficiency as less than or equal to 30 ng/ml, so these sources may see the estimated average vitamin D deficiency prevalence in the US as higher than 42%.

According to a Vegetarian Times survey, about 3% of the US population follows a vegetarian diet and approximately half of these people identify as vegan. As we can see from the stats mentioned above, vitamin D deficiency is clearly not just a concern for vegans, vegetarians, and raw food enthusiasts. Vitamin D status is an important consideration for everyone regardless of diet.

How is vitamin D made in our skin?

When we are outside and we have bare skin exposed to the sun, ultraviolet B spectrum rays interact with 7-dehydrocholesterol in our skin to produce cholecalciferol, also known as vitamin D₃. After vitamin D₃ is formed, it travels via the bloodstream down to the liver where it is converted to 25-hydroxycholecalciferol, also known as 25-hydroxyvitamin D₃. 25-hydroxycholecalciferol then travels via the bloodstream down to the kidneys, where it is transformed to 1,25-dihydroxycholecalciferol, also known as 1,25-dihydroxyvitamin D₃, which is considered to be the activated form of vitamin D.

Vitamin D₃ is produced in an area of the skin called the epidermis. The epidermis has five layers, the first of which we can see, called the stratum corneum. Vitamin D₃ is produced further down in levels 4 and 5 of the epidermis called the stratum spinosum and the stratum basale, after which vitamin D₃ travels through the bloodstream to the liver for conversion.

Many people spend time in the sun but still find that they test low in vitamin D. There are some factors that can inhibit vitamin D production in the skin, specifically air pollution, skin type, time of day, age, sunscreen usage, latitude, time of year, and other factors. For example, air pollution in significant amounts can potentially obscure the rays of the sun, inhibiting skin production of vitamin D.

Sunscreen use at SPF 8 and above may affect the production of vitamin D in the skin as can latitude and season. For example, the ability of one’s skin to make vitamin D year round is consistent at 35 degrees north latitude and below in the northern hemisphere*, or 35 degrees south latitude and above in the southern hemisphere*. In the United States, 35 degrees north is Los Angeles on the west coast and the border of North and South Carolina on the east coast. The ability of one’s skin to make vitamin D in the winter will be reduced or eliminated for someone living above 35 degrees north and below 35 degrees south. Simply stated, people who live in northern parts of the United States have been found not to produce vitamin D in their skin during the winter months.

How do we determine our vitamin D status?

The best way to know is to get a blood test, and the test currently considered to be the best measurement of vitamin D status is known as the 25-hydroxycholecalciferol, 25-hydroxyvitamin D₃, or 25-OH D₃ test.

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