As most of us continue to long for the warmth of the summer sun, I felt it would be appropriate to add a post about the one vitamin that has a direct connection with sunlight: vitamin D.
Vitamin D is a very unique compound compared to most other nutrients. Not only can our bodies synthesize it from sunlight (giving it the nickname “The Sunshine Vitamin”), but it’s also a fairly stable nutrient in that cooking and extreme temperatures don’t really affect its availability. It also functions more like a hormone (chemical messenger) than a nutrient (something that nourishes), which we’ll talk about more below.
Vitamin D is found in several different forms—depending on whether it comes from a plant- or animal-based source—which can later be converted into active vitamin D. Previtamin D₂ or ergosterol is the plant-based form of vitamin D. When ergosterol is irradiated (exposed to radiant energy, or radiation*) it turns into vitamin D₂, or ergocalciferol—the most commonly sold form of vitamin D in supplements. The animal- and human-based precursor of vitamin D is called 7-dehydrocholesterol (don’t worry about these crazy names). This is the compound which plays a role in synthesizing active vitamin D from sunlight .
Vitamin D belongs to a group of compounds called steroids, which are types of lipids. Being a steroid, it acts more like a hormone than a nutrient (some well-known steroids include the hormones estrogen, progesterone, and androgen). When vitamin D binds to receptors on certain types of cells, it triggers chemical messages which are sent to increase intestinal calcium absorption. It also works closely with the parathyroid hormone to regulate calcium and phosphorous concentrations in the blood as well as balancing the bone-making vitamins and minerals in the body [1,2].
*NOTE: According to the USDA, irradiation does not make foods radioactive. This procedure is commonly used to kill pathogens in certain types of foods (such as raw meat and produce) without affecting the quality of the product . However, there has been much public debate over the past several years about the safety of irradiated foods.
SUNLIGHT AND VITAMIN D PRODUCTION
7-dehydrocholesterol is actually made from cholesterol (which belongs to a closely related group of compounds called sterols) and is produced by the sebaceous glands in our skin. Due to its chemical structure and being stored closely to our skin’s surface, 7-dehydrocholesterol is able to absorb certain wavelengths of light within the UV spectrum. When UVB rays penetrate our skin, this vitamin D precursor absorbs some of these rays and is transformed into previtamin D₃. Over the next 2-3 days, previtamin D₃ (along with any dietary forms obtained from food) slowly turn into vitamin D₃. The kidneys and liver then play a role in activating vitamin D₃ into active vitamin D .
Depending on the type of radiation used, it may be possible to synthesize vitamin D from the use of tanning beds. However, the risks associated with tanning bed use may outweigh the benefits of using this method to obtain vitamin D . Also, sunscreen with an SPF of 8 or more has been shown to effectively block the production of vitamin D from UVB rays .
RECOMMENDATIONS AND SOURCES
Vitamin D is only found in small amounts in non-fortified foods, and only about 50% of the vitamin D₃ form is absorbed (remember, this is the form found primarily in animal-based food sources) . Fish liver oils as well as the flesh of fatty fish are good sources of vitamin D, while small amounts can be found in beef liver, cheese, and egg yolks. According to the National Institutes of Health, mushrooms contain varying levels of vitamin D₂, but can be enhanced if “exposed to ultraviolet light under controlled conditions” .
Fortified foods are the best source of vitamin D in the American diet, with milk typically being fortified with 100 IU per cup (2.5 mcg). Other foods that are usually fortified include orange juice, ready-to-eat breakfast cereals, yoghurt, and margarine . The RDA for vitamin D is 600 IU (15 mcg) per day . A more precise chart of recommendations based upon age groups can be found at http://ods.od.nih.gov/factsheets/VitaminD-HealthProfessional/.
Most people should be able to meet their vitamin D needs through both sun exposure and diet. One suggestion states that “exposing hands, face, and arms on a clear summer day for 5 to 10 minutes two or three times a week should be sufficient to maintain vitamin D nutrition” . However, special attention may need to be placed on dietary sources of vitamin D if you live in a cloudy or polluted area, have a darker complexion, or are currently experiencing winter climates. For those located above 40° north latitude (as well as 40° south latitude), vitamin D production from sun exposure nearly stops for 4 months due to winter climates .
DEFICIENCY AND TOXICITY
Due to vitamin D’s connection with healthy bone-making nutrients, a severe lack of vitamin D can cause problems with our skeletal system. In children, since they are still growing, a lack of vitamin D leads to a condition called rickets which is characterized by inadequate mineralization of bone tissue, causing soft bones. Because of their weakened skeletal system, it becomes very difficult for the child’s bones to bear the weight of their growing body, and the bones in their legs begin to bow. Children with rickets may also develop beaded ribs due to poorly formed cartilage .
The same is true for adults with a vitamin D deficiency, but this disorder is called osteomalacia (“osteo” mean “bones” and “malacia” means softening. Adults may also develop bowed legs; a bending of the spine; and painful, flexible bones .
An excessive intake of vitamin D causes an increase in blood calcium levels which can lead to calcium deposits in soft tissues. The calcium then forms stones (especially in the kidneys in an effort to excrete any excesses). These stones can also form in other tissues such as blood vessels of the heart and lungs. There’s no need to worry about excessive vitamin D formation from sunlight, though—excessive sun exposure causes a breakdown of the vitamin D precursor in our skin, putting a halt on vitamin D synthesis from the sun .
- Gropper, S. S., Smith, J. L., & Groff, J. L. (2009). Advanced nutrition and human metabolism (5th ed.). Belmont, CA: Wadsworth, Cengage Learning.
- Whitney, E., Rolfes, S. R. (2008). Understanding nutrition (11th ed.). Belmont, CA: Wadsworth, Cengage Learning.
- USDA Food Safety and Inspection Service. (23 March 2012). Irradiation and food safety. Retrieved from http://www.fsis.usda.gov/Fact_Sheets/Irradiation_and_Food_Safety/index.asp
- The National Institutes of Health Office of Dietary Supplements (24 June 2011). Dietary supplement fact sheet: Vitamin D. Retrieved from http://ods.od.nih.gov/factsheets/VitaminD-HealthProfessional/