TORONTO — Vitamin D may not have the cancer preventive powers proponents
ascribe to it, at least in so far as breast cancer is concerned, a new study
suggests.
The work, published Tuesday in the Journal of the National Cancer Institute,
showed women who took 400 international units of vitamin D along with 1,000
milligrams of calcium for seven years were no less likely than women who took
placebo pills to develop breast cancer.
The lead author, medical oncologist Dr. Rowan Chlebowski, said the results of
the study may put "a little bit of the brakes on the bandwagon" promoting
vitamin D as a cancer prevention tool.
"What this does, it says you've got to step back and look at it in a different
light," Chlebowski, who is with the Los Angeles Biomedical Research Institute,
said in an interview Tuesday.
"It isn't like we're on the cusp of having this wonderful advance. It looks like
now we have to go back and answer some basic questions."
The findings were reported Wednesday in the Journal of the National Cancer
Institute.
Dr. Michael Pollak, a breast cancer researcher at McGill University in Montreal,
called the study important and the results disappointing. But he said more
questions need to be answered.
"I do not regard this study as closing the book on this question," Pollak said
in an email from Tokyo, where he was attending a scientific meeting.
For the study, 36,282 post-menopausal women were randomly assigned to take
calcium and vitamin D daily or a placebo. The women were followed for seven
years.
The first goal of the study was to see if the women taking the supplements had
lower rates of hip fractures. But as is often the case with studies, the trial
was designed to try to answer another question as well: Did taking the
supplements lower the risk of breast cancer?
Over the course of the seven years, 528 women taking the supplements developed
breast cancer, as did 546 women taking the placebos. The difference between
those two numbers was not statistically significant.
When the findings were presented at a scientific conference 1 1/2 years ago,
vitamin D proponents challenged the results, saying the dose of 400
international units daily was too low to have an effect.
So Chlebowski and his colleagues went back and analyzed blood samples the women
had given at the start of the study, comparing vitamin D levels in the women who
got cancer to an equal number of women who did not.
They saw no pattern between the women's blood levels of vitamin D and their risk
of developing cancer. Furthermore, when the researchers estimated the women's
daily intake of vitamin D - based on information provided by the women - the
minimal intake difference couldn't account for the large differences of vitamin
D in the women's blood.
Chlebowski said women who were more physically active and who were leaner had
higher vitamin D readings - which could suggest studies that pointed to a cancer
prevention role for vitamin D were ascribing to the supplements an effect that
was caused by other factors. It is known that women who exercise and have a
healthy body weight are at lower risk of developing breast cancer.
Julia Knight, a cancer epidemiologist at the Samuel Lunenfeld Research Institute
at Toronto's Mount Sinai Hospital, was an author of an earlier study that
suggested women with low levels of vitamin D had higher rates of breast cancer.
Like Pollak, Knight said the new study doesn't close the door on a role for
vitamin D in cancer prevention. "For me, it's not enough to stop looking at the
question."
But Knight acknowledged she thinks it is unwise to overestimate the protective
role vitamin D might play. "I think it's a big mistake to sort of sell it as
some kind of magic pill. I don't think that's fair to tell people."
Knight suggested further study may show that taking vitamin D after menopause
may be too late to influence a woman's risk of developing breast cancer. There
is a growing school of thought, she said, that ensuring adequate levels of
vitamin D during adolescence - when breasts are developing - may be key.
Pollak suggested vitamin D supplementation may be more important for some women
than for others, for instance those who live in climates where lack of sunlight
in winter lead to vitamin D deficiencies.
"Especially for women in countries like Canada or Sweden, we need more data to
be sure if vitamin D supplementation may have a role for some women for breast
cancer risk reduction and-or other health benefits," he said.
