Sunlight is the primary source of vitamin D for most people, and while dark skin offers better protection from ultraviolet light, it also means lower levels of the vitamin in darker-skinned people.

University of South Carolina researchers examined more than 100 women who had been diagnosed with breast cancer in the past five years.

In the study, participants donated blood samples, and their vitamin D serum levels were evaluated by doctors.

Sixty percent of African-American women were found to be vitamin D deficient, compared to 15 percent of white women.

The levels were lowest in patients with triple-negative breast cancer, the most aggressive and least responsive type of breast cancer, suggesting vitamin D may play a role in the development and aggression of the disease.

Doctors said additional research could lead to a more advanced breast cancer prevention strategy for women of all color.