SAN FRANCISCO -- For decades, researchers have puzzled over a paradox: why does Marin Country, consistently ranked as healthiest county in all of California, have such a shockingly high incidence of breast cancer?

In a study published online earlier this week and scheduled to appear in the November issue of the Journal of the American College of Surgeons, researchers working at University of California San Francisco and Oklahoma City's InterGenetics, Inc. examined the San Francisco suburb's elevated rate of breast cancer. They found it can likely be attributed to a genetic variation in a vitamin D receptor that may be more widespread in Marin women than in the greater population as a whole.

The retrospective pilot study employed DNA genotyping on 164 Caucasian Marin women diagnosed with primary breast cancer between 1997 and 1999, along with suitable control subjects matched for age and ethnicity. Inside the former group, researchers found a high frequency of a particular genetic variation.

"While the findings must be validated in a much larger, prospective study," author Dr. Kathie Dalessandri told Medical Xpress, "we found that women who were at high risk for breast cancer were 1.9 times more likely to have a specific vitamin D receptor variation than the general population."

Marin's rate of breast cancer is at least 15 percent higher than in all other counties in California, a number that stays relatively constant across all parts of the county.

"You know, you can't be a middle-aged woman in Marin County and not be aware of the fact that the incidence of breast cancer is much higher here than elsewhere in the United States," said Marin resident Kim Wright-Violic told ABC News. "In a community that's this small, inevitably you know somebody who either has had cancer and is in remission or has died of cancer."

Previously, the most recent comprehensive study on Marin's breast cancer phenomenon was conducted in 2003 and concluded that, because time spent in Marin had no relationship with the likelihood of developing breast cancer, environmental factors present in the region itself didn't play a role in the high rate of the disease's occurrence.

Additionally, while breast cancer screenings occur at a slightly higher rate in Marin than in the rest of the state, four separate studies have shown that more frequent mammograms only account for a couple of additional cases per year--not nearly enough to explain why the disease is so widespread.

The UCSF report suggests that supplementing vitamin D intake could mitigate some of the risk factors involved, although the study's authors admit it is currently unknown exactly what level of vitamin D is necessary to prevent breast cancer within this population.

However, not everyone is entirely convinced that vitamin D alone is the solution. The San Francisco Chronicle reports:

Jeanne Rizzo president of the Breast Cancer Fund, an advocacy group that focuses on the environmental causes of breast cancer, said it's too soon to stock up on vitamin D.

"But this tells us we should be looking at things that are not just traditional risk factors and it's important to continue this investigation," said Rizzo, who is also a Marin County resident.

In coordination with the Marin Health and Human Services agency, researchers are in the process of conducting a follow-up larger-scale study called the Marin Women's Study, involving a much bigger sample.

Related on HuffPost: