Happy Winter Solstice (or the shortest day of the year for people in the northern hemisphere)! Did you know that vitamin D production is influenced by the season, latitude of residence, and skin color?
People make vitamin D after the skin is exposed to the sun. In winter, sunlight exposure is reduced because of shorter days and colder weather, leading to low vitamin D.
Latitude of residence
People living above 35° latitude (or below in the southern hemisphere) are exposed to a “weaker” sunlight in the winter months.
The image below shows how the the sun rays come in on an angle so that it takes longer for the rays to reach the earth’s surface in winter, leading to less energy from the sun.
Skin color is determined by the amount of melanin or pigment in the skin. Melanin shields the skin from the sun, so the more melanin (or darker skin) you have, the less vitamin D you will make.
So what are you doing to get your vitamin D in the winter months? There are 3 ways to make sure you get enough vitamin D. 1) Exposure to the sun. 2) Eat foods rich in vitamin D. 3) Take a vitamin D supplement.
We seldom get enough sun because of environmental conditions, like working indoors, and wearing sunscreen to protect against skin cancer. However, exposing your skin to the sun will increase the amount of vitamin D in the body. The time we need to expose our skin to the sun will differ depending on where you live, what color your skin is, and the amount of skin you expose.
Here is a table from the National Institutes of Health, Office of Dietary Supplements that lists the sources of vitamin D-rich foods.
* IUs = International Units.
** DV = Daily Value. DVs were developed by the U.S. Food and Drug Administration to help consumers compare the nutrient contents among products within the context of a total daily diet. The DV for vitamin D is currently set at 400 IU for adults and children age 4 and older. Food labels, however, are not required to list vitamin D content unless a food has been fortified with this nutrient. Foods providing 20% or more of the DV are considered to be high sources of a nutrient, but foods providing lower percentages of the DV also contribute to a healthful diet.
Vitamin D Fortification and Supplements
Many foods are fortified with vitamin D to help us get the recommended dietary allowance of vitamin D. In the 1920s, vitamin D was first added to milk where it successfully fended off a Rickets epidemic. Rickets is a disease of vitamin D deficiency where the skeleton does not develop normally. It is characterized in children as developing bowed-legs, protruding breastbone, muscle weakness and bone pain. Now, there is vitamin D in many milk products and milk alternatives. If vitamin D-fortified foods aren’t enough, there are also many over the counter supplements available. The supplements can differ in potency and effectiveness, depending on what form it is.
For more information on vitamin D, download our vitamin D tip sheet.