There are a number of environmental factors that affect vitamin D3 synthesis:
Time of Day
The latitude a person is at affects that person’s ability to synthesize vitamin D3. There is an inverse relationship between distance from the equator and UV light exposure. Thus, with increased distance from the equator (increased latitude), there is decreased UV light exposure and vitamin D3 synthesis. The link below shows the latitude and longitude lines of the United States.
Seasons also make a difference in vitamin D3 synthesis. In Boston (42० N), vitamin D synthesis only occurs from March-October, because during late fall and winter not enough UV-B reaches the earth’s surface to synthesize vitamin D3. However, in Los Angeles (34० N), vitamin D3 synthesis occurs year round2. The difference is the angle of the sun relative to latitude and how many UV-B photons are absorbed before they reach the earth’s surface1.
Time of day is also an important factor in affecting vitamin D3 synthesis. Vitamin D3 synthesis increases in the morning before peaking at noon, then declines the rest of the day1.
Another factor that plays an important role in vitamin D3 synthesis is skin pigmentation. As shown in the figure below, skin pigmentation tends to be darker around the equator to help protect inhabitants from the harmful effects of sun exposure.
Skin color is the result of increased production of the pigment melanin, as shown in the link below.
Very dark skin color can provide a sun protection factor (SPF) 8-30 for those individuals who never burn3. These individuals will require approximately 5- to 10-times greater sunlight exposure than a light-skinned, white person to synthesize the same amount of vitamin D33,4.
Age also plays a factor in vitamin D3 synthesis. Aging results in decreased 7-dehydrocholesterol concentrations in the skin, resulting in an approximately 75% reduction in the vitamin D3 synthesis capability by age 704.
Clothing is another factor that influences vitamin D3 synthesis. More clothing means that less sun reaches your skin, and thus less vitamin D3 synthesis.
Sunscreen, “Sensible Sun Exposure”, and Tanning
There is quite a spirited debate on sunscreen, sun exposure, skin cancer, and vitamin D synthesis. On one side are the vitamin D researchers, on the other side are dermatologists. Vitamin D research found that SPF 8 sunscreen almost totally blocked vitamin D3 synthesis5. However, the SPF value equals 1/(# photons that reaches your skin) meaning that SPF 30 means 1/30 UV photons reach your skin. Thus, vitamin D3 synthesis shouldn’t be totally blocked. In addition, studies indicate that consumers apply 1/2 or less of the amount required to get the listed SPF protection5. Researchers recommend sun exposure on the face, arms, and hands for 10-15 minutes 2-3 times per week between 10 AM-3 PM3,6. However, dermatologists do not like “sensible sun exposure” because this is also the peak time for harmful sun exposure. Dermatologists say that “sensible sun exposure” appeals to those who are looking for a reasons to support tanning and are at highest risk (primarily young, fair-skinned females) of sun damage. They argue that vitamin D can be provided through supplementation5.
What about tanning beds? Not all tanning beds provide UV-B rays that are needed for vitamin D3 synthesis. In fact, some advertise that they only use UV-A rays that are safer, even though this is not the case7. Virtually every health organization advises against using tanning beds, because the risks are far greater than the potential benefits7,8.
References & Links
1. Shils ME, Shike M, Ross AC, Caballero B, Cousins RJ, editors. (2006) Modern nutrition in health and disease. Baltimore, MD: Lippincott Williams & Wilkins.
3. Holick M. (2008) Vitamin D: A D-lightful health perspective. Nutr Rev 66(10 Suppl 2): S182.
4. Stipanuk MH. (2006) Biochemical, physiological, & molecular aspects of human nutrition. St. Louis, MO: Saunders Elsevier.
5. Gilchrest B. (2008) Sun exposure and vitamin D sufficiency. Am J Clin Nutr 88(2): 570S.
6. Byrd-Bredbenner C, Moe G, Beshgetoor D, Berning J. (2009) Wardlaw’s perspectives in nutrition. New York, NY: McGraw-Hill.
7. Whitney E, Rolfes SR. (2011) Understanding nutrition. Belmont, CA: Wadsworth Cengage Learning.
8. McGuire M, Beerman KA. (2011) Nutritional sciences: From fundamentals to food. Belmont, CA: Wadsworth Cengage Learning.
United States Latitude and Longitude Lines – http://modernsurvivalblog.com/survival-skills/basic-map-reading-latitude-longitude/
Melanin – http://www.nytimes.com/imagepages/2007/08/01/health/adam/9620Melanin.html