Women who are deficient in vitamin D when diagnosed with breast cancer may have a poorer prognosis compared with those having optimal amounts of the sunshine nutrient in their blood, a Canadian study suggests.

Dr. Pam Goodwin, principal researcher on the study and a clinician-scientist at Toronto's Mount Sinai Hospital, says it's too early to tell women with breast cancer to take vitamin D supplements just to prevent a recurrence. (CBC)

The study by Toronto researchers, released Thursday by the American Society of Clinical Oncology, found that women with low levels of vitamin D were at increased risk for having a more serious grade of breast tumour than those with sufficient vitamin D.

Furthermore, women with too little of the vitamin had a greater chance of recurrence and lower overall survival rates than those with healthier amounts, said principal investigator Dr. Pamela Goodwin, a clinician-scientist at Mount Sinai Hospital.

"Women who had low levels of vitamin D tended to have more aggressive tumours than women who had high levels of vitamin D," Goodwin told CBC News Thursday. "The women who had deficient levels compared to the women who had sufficient levels had an almost doubled risk of metastasis and of dying from breast cancer."

She emphasizes that the association between vitamin D and breast cancer outcome has not been shown to be causal.

"We realize that a lot of women with breast cancer will be wondering what they should do about their vitamin D supplements," Goodwin added.

"For now, we're recommending that they consider taking a supplement that is in the range recommended for bone health, and for many women that's 600, 800 international units a day," she said.

She points out that women of colour may have lower vitamin D levels, as the pigment in their skin prevents them from producing as much of the vitamin when exposed to the sun.

She said that taking high doses of vitamin D isn't a good idea, as there was some preliminary evidence in the study that too much of the vitamin was linked to an increased risk of death during the study period.

Huge contrast in 10-year survival rates

The study involved 512 women, aged 35 to 69, who were diagnosed with breast cancer between 1989 and 1996, then had their health followed until 2007, on average for almost 12 years.

"What we found was that 37.5 per cent of our patients were vitamin D deficient and 38.5 per cent were insufficient," said Goodwin, noting that sufficiency was based on levels considered optimal for good bone health.

"Only 24 per cent had sufficient levels of vitamin D in their blood."

Having too little vitamin D was associated with a younger age, obesity and a lower intake of grains and cereals, which in Canada are fortified with the nutrient. Blood levels were measured in both summer and winter months, but little difference in levels was found.

Vitamin D status at diagnosis was linked to a greater risk of seeing cancer recurrence or spread, with 10-year metastasis-free survival at 69 per cent for women who were deficient versus 83 per cent for those with optimal vitamin D.

There was also a huge contrast in overall 10-year survival between the two groups: 74 per cent among those with too little of the vitamin compared to 85 per cent with enough.

Goodwin said it appeared that having a vitamin D level of 80 to 120 nanomoles per litre of blood is ideal.

"So what we're saying is if you have breast cancer and you're interested in taking vitamin D, consider taking it in the dose level recommended for bone health and also consider asking your physician to do a vitamin D blood level," she said.

"That's a simple blood test and that way you can regulate how much vitamin D you're taking to make sure your blood level is in the range that's safe for bone health as well as for breast cancer outcomes."

Heather Chappell, a breast cancer survivor who lives in Nova Scotia, said that since being diagnosed nine years ago, she's been looking to improve her chances of fending off the disease should it return. She recently began taking a vitamin D supplement.

"Of course in Nova Scotia we don't get a lot of sunlight — especially this winter — because of all the storms. It's great that you can just go to a drugstore and pick up some vitamin D, and at least it makes us think that we're improving our chances."

B.C. oncologist says more study needed

Dr. Stephen Chia, a medical oncologist at the B.C. Cancer Agency, called the study "very interesting."

"We need to better understand as many factors as we can in terms of why some women do better with breast cancer versus others who don't," Chia said from Vancouver.

"And this is looking at something that I think is interesting all round, both to patients and physicians, because vitamin D is essential for many normal body functions."

A number of studies have also suggested that vitamin D may help prevent some cancers, among them breast and colorectal cancers.

But Chia said this study must be taken in the context that much more research is needed to determine whether taking the nutrient could help improve the course of breast cancer and its outcome.

"There's not enough here to say this is a breast cancer treatment today."

Only a few foods contain vitamin D, among them egg yolks and certain types of fish; the best way to obtain the nutrient is through exposing the skin to sunlight.

Last year, the Canadian Cancer Society recommended in general that adults should take a daily supplement containing 1,000 international units of vitamin D, after a growing body of research suggested the nutrient may help prevent some cancers.