Multiple factors influence vitamin D levels

Vitamin D is produced in the body via sunlight. It is also obtained from foods such as oily fish (salmon, mackerel, sardines), fortified milk, cereals, and from supplements. Low vitamin D levels may be caused by a lack of sun exposure, lack of dietary vitamin D, malabsorption, side effects from medications or supplements, chronic diseases such as kidney or liver disease, and other causes. In addition, seniors, infants and toddlers, dark-skinned people, and pregnant or breastfeeding women may be particularly at risk for low vitamin D levels. Now, we are discovering there are other factors that may affect vitamin D levels.

The new study explored the importance of skin pigmentation, total cholesterol, and baseline blood levels of vitamin D (measured as 25- hydroxy vitamin D) on vitamin D production after ultraviolet B (UVB) exposure. Participants in this study had four UVB exposures several days apart to the chest and back, and each exposure was equivalent to about 30 minutes of sun exposure in the middle of a clear summer day in Denmark. Results showed:

• Of the 182 participants screened for baseline vitamin D, 67% were considered vitamin D insufficient and 18% were vitamin D deficient

• Baseline levels of vitamin D were higher in people who ate fish at least once a week

• Among a group of 50 participants, researchers found significantly higher vitamin D production after UVB exposure in participants with a low baseline vitamin D level compared with those with a higher baseline level

• Fair-skinned and dark-skinned people had similar increases in vitamin D levels after UVB exposure

• There is an association between total cholesterol and vitamin D production, and a low cholesterol level might hinder vitamin D production

Checking vitamin D levels

Here are some tips regarding vitamin D levels:

Talk with a doctor. A healthcare professional can help you decide whether or not it is important to check your vitamin D level. People who live in areas with little sunlight or who are at risk or suffering from chronic diseases such as cardiovascular disease or cancer may especially want to discuss the topic of vitamin D with a doctor.

Be careful with sun exposure. Many physicians recommend brief amounts of time (less than 15 minutes a day) of sun exposure for general health, but studies have shown that sun exposure may not be enough to raise vitamin D levels that are low. Further, excess sun can increase the risk of skin cancer, and the authors of this study do not recommend UVB treatment for low vitamin D levels for that reason. Instead they recommend treating low vitamin D levels with vitamin D supplements. Talk with your doctor about treatment strategies for low vitamin D levels.

(J Invest Dermatol 2010;130: 546-53)