Vitamin D
Vitamin D is a fat soluble vitamin that’s stored in the body’s fat tissues. An important source of vitamin D is sunlight. When sunlight hits the skin, the UV rays are synthesized into vitamin D. Small amounts of vitamin D are found naturally in fatty fish, some fish oils, liver and egg yolks of chickens given vitamin D supplements. Some foods are fortified with vitamin D, like cereals and milk.

The main function of vitamin D is to help the body absorb calcium for building strong bones. However, recent research suggests inadequate levels of vitamin D may be associated with an increased risk for high blood pressure, heart disease, some types of cancer, diabetes, rheumatoid arthritis and multiple sclerosis.

Measuring Vitamin D
Last year, the American Academy of Pediatrics increased the vitamin D recommendations to 400 IU (international units) per day for infants, children, and adolescents. Despite the recommendations, many children in the U.S. aren’t getting enough vitamin D. A study recently published in the journal, Pediatrics, found 61 percent of children and adolescents in the U.S. (roughly 50.8 million) had insufficient levels of vitamin D. About 9 percent (about 7.6 million) were found to be vitamin D deficient.

Inadequate intake of vitamin D means many children are not building strong bones and may be at future risk for osteoporosis and fractures. But the researchers had other concerns as well. Children who were vitamin D deficient were more likely to have high systolic blood pressure and elevated levels of high-density lipoprotein (HDL, the bad cholesterol) compared to children with normal “D” levels. Another study early last year found teens with low levels of vitamin D were roughly two-and-a-half times more likely to have high blood pressure and/or high blood sugar and nearly four times more likely to have metabolic syndrome.

Vitamin D, Diet and Fat
The sun is a very important source of vitamin D. Researchers say 10 to 15 minutes of sunlight on unprotected skin is generally sufficient for adequate D levels. People with darker skin don’t absorb the sunlight as well and need an extra 5 to 10 minutes of sunlight. But across much of the U.S., there is an insufficient amount of sunlight between November and March for the skin to absorb enough UV light. And even in the summer, many people don’t spend enough time outside to get enough vitamin D from sunlight.

Yanbin Dong, M.D., Ph.D., Cardiologist with the Medical College of Georgia, says teens may be especially at risk for inadequate sunlight exposure because many of them spend a considerable amount of time indoors. Thus, these teens need to ensure they get enough vitamin D in their diets.

Recently, the Georgia researchers looked at blood samples to measure levels of vitamin D in teens. The results were compared with dietary recalls to determine how much vitamin D was coming from their food. The investigators found most of the teens were not getting enough vitamin D in their diets. Inadequate intake was especially a problem in African-American teens.

The researchers took the study a step further, using a DEXA scan (dual energy x-ray absorptiometry, a tool normally used to assess bone health), to get measurements of body fat. Researcher Inger Stallmann-Jorgensen, R.D., says body composition data was compared with the information from the dietary assessments. According to Stallmann-Jorgensen, the study found that low levels of vitamin D correlated with higher levels body fat, particularly visceral fat, the bad type of fat that wraps around internal organs.

Researchers don’t yet understand the cause and effect relationship between vitamin D and body fat. In other words, does low vitamin D cause the body to store fat (and thus gain weight), or does the excess fat prevent the body from using the vitamin D?

The investigators are now looking at data from another study, in which teens were given daily vitamin D supplements to see if the treatment changes blood levels of vitamin D and fat storage.

AUDIENCE INQUIRY
For general information on vitamin D and nutrition:
American Dietetic Association, http://www.eatright.org
International Food Information Council, http://www.ific.org

BIBLIOGRAPHY
Ginde, Adit, M.D., et al., “Demographic Differences and Trends of Vitamin D Insufficiency in the U.S. Population, 1988-2004,” Archives of Internal Medicine, March 23, 2009, Vol. 169, No. 6, pp. 626-632.

Kremer, R., et al., “Vitamin D Status and Its Relationship to Body Fat, Final Height, and Peak Bone Mass in Young Women,” Journal of Clinical Endocrinology, January 2009, Vol. 94, No. 1, pp. 67-73.

Kumar, Juhi, M.D., et al., “Prevalence and Associations of 25-Hydroxyvitamin D Deficiency in US Children,” Pediatrics, published online August 3, 2009.

Looker, A., et al., “Serum 25-hydroxyvitamin D Status of the US Population,” American Journal of Clinical Nutrition, December 2008, Vol. 88, No. 6, pp. 1519-1527,

Wagner, Carol, M.D., et al., “Prevention of Rickets and Vitamin D Deficiency in Infants, Children and Adolescents,” Pediatrics, November 2008, Vol. 122, No. 5, pp. 1142-1152.

Yetley, E., “Assessing the Vitamin D Status of the US Population,” American Journal of Clinical Nutrition, August 2008, Vol. 88, No. 2, pp. 558S-564S.