Over the last three years, Susan Sullivan of the Dept. of Food Science and Human Nutrition has monitored sun exposure, diet and blood levels of vitamin D in 23 girls from ages 10 to 13 years old. All of her subjects live in the Bangor, Maine area. She conducted the study with Dr. Cliff Rosen of the Maine Center for Osteoporosis Research and Education, St. Joseph Hospital in Bangor. In her research and previous experience as a clinical dietitian at Massachusetts General Hospital, Sullivan has focused on the medical consequences of dietary habits. For her 1995 doctoral degree at Boston University, she studied the relationship between fat intake and blood cholesterol levels in kidney transplant recipients.
Vitamin D is an emerging area of medical research, says Sullivan. Medical scientists have yet to understand the whole story about vitamin D and the body. "We've known for a long time that vitamin D has a role in getting calcium into bones. Researchers are now finding evidence that vitamin D could play other roles in health such as cancer prevention and controlling blood pressure. There are vitamin D receptors in lots of tissues in the body that aren't related to bone," she explains.
The largest single source of vitamin D is the skin, which makes the nutrient when it is exposed to sunlight. Diet plays a less important role but, for people at high northern latitudes, helps to supplement the body's vitamin D store during the winter months when sunlight is less intense.
Since having adequate levels of vitamin D supports bone growth, Sullivan monitored bone density in her subjects. She confirmed that as they go through puberty, girls rapidly add calcium to their bones. "Puberty is a very critical time when up to half of a person's adult bone mass is being deposited. If you think about life span, peak bone mass occurs at about the age of 30. This is such an important time when girls are growing their bones."
Sullivan's results were presented in 2003 at the Annual Meeting of the American Society for Bone and Mineral Research. Almost half of the Bangor area girls in her study had insufficient levels of vitamin D in their blood in March, a time of the year when the nutrient usually falls to its lowest level over the course of the year. In September, when the nutrient is usually at its highest level, 17 percent also fell below the standard, currently 20 nanograms per milliliter of blood.
As scientists uncover more details about the role that vitamin D plays in the body, they have begun to suggest that the standard be raised to about 30 nanograms per milliliter, Sullivan adds. "How much vitamin D is necessary for optimal health? We don't really know. There's a real need for more research on that question," she adds.
To generate vitamin D, Sullivan and other nutritionists recommend getting five to ten minutes of sun exposure between roughly 10 a.m. and 2 p.m. daily in the summer. Sunscreen lotions should be used after the first five or ten minutes. Vitamin D fortified foods such as milk, some varieties of orange juice, yogurt, margarine and cereals are helpful. Fatty fish such as salmon also provide a vitamin D boost. Eating three servings per day of dairy products fortified with vitamin D will provide both the vitamin D and calcium to build strong bones. "People who practice sun avoidance, who never go out in the sun without covering up completely, run a real risk of insufficient vitamin D levels," Sullivan adds. Sullivan has received support for her study from the Maine Dairy and Nutrition Council. Dr. Michael F. Holick of the Boston University School of Medicine also contributed to the study by conducting laboratory analyses and assisting with the interpretation of the data.