LOCAL RISK FACTORS
Affluent, highly educated career women — a
demographic profile that abounds in Marin
— are obviously not at risk for breast cancer
on the basis of those lifestyle aspects themselves. But statistically such women may have
no children, have few children, or delay having
children, and each of those is a clearly established risk factor for the disease.
As Willis explains, breast cancer is essentially a proliferation of breast tissue, and each
menstrual cycle of breast tissue growth and
shrinkage brings a tiny risk of uncontrolled
growth of tissue. The more times a woman
experiences that cycle of growth and shrinkage,
the higher incremental risk she has of contracting cancer. Although a precise mechanism for
reproductive factors altering lifelong cancer
risk has yet to be determined — how long the
body is exposed to cycles of estrogen production
by the ovaries is a prime suspect, along with cellular and genetic factors — most experts believe
natural events, such as pregnancy, that lower
the number of menstrual cycles also reduce the
risk of breast cancer.
By reducing the lifetime number of menstrual cycles, pregnancy lowers a woman’s
cumulative exposure to bodily produced hormones. If she doesn’t get pregnant, she in effect
misses the opportunity to miss having a period,
which is why experts suspect a woman faces
increased risk from never giving birth (
nul-liparity), having a small number of children
(lower parity), and delayed childbearing (
generally defined as having a first child after age 30).
Another theory suggests pregnancy may assist
breast cells in their lifelong maturation from
volatile forms (more prone to genetic mutations) to more stable, “differentiated” forms,
so that when it comes to breast cancer risk, the
sooner pregnancy occurs the better.
According to data compiled by the Marin
County Department of Health and Human
Services in 2012, 57 percent of women in the
M WS either have never given birth or gave birth
after age 30. (Early puberty and later menopause are also being studied as risk factors, but
it’s unlikely those will prove significantly relevant to risk in Marin.) And after specifically
comparing the prevalence of known risk factors in the county with cases of women who got
breast cancer, in order to see if a correlation in
fact exists, the M WS found that the reproductive risk factors coinciding with socioeconomic
profile have played a pivotal role in Marin
breast cancer rates.
Another major breast cancer risk factor
appearing in full force in Marin is excessive alcohol consumption. Even surveys touting Marin
as the healthiest county in the U.S. (such as a
national study conducted by the University of
Wisconsin Population Health Institute) criticize a self-reported binge-drinking rate of three
times the national average. Alcohol can raise levels of estrogen and other hormones associated
with breast cancer. While the exact mechanics
leading cancer cells to grow at a greater rate are
unknown, there is a well-established connection between two alcoholic drinks a day and
higher breast cancer risk. The risk increases
with each additional drink per day. A study by
Dr. Christina Clarke, associate director of the
Cancer Prevention Institute of California and
an adviser to the MWS, estimated that approximately 6.1 percent of breast cancer cases in
Marin were attributable to women having two
or more alcoholic beverages a day.
Excessive drinking is often a response to perceived pressures and experts say high stress and
alcohol may be tied to breast cancer risk as well.
While no well-designed studies have examined
it as a risk factor in Marin specifically, research
has shown that increased levels of cortisol, a
hormone released in response to stress, may
stimulate breast cancer development by varying
the estrogen production in the body, says Janice
Barlow, executive director of Zero Breast Cancer
(formerly Marin Breast Cancer Watch).
MARIN’S
HEALTHY LIFESTYLE
Since the comparative breast cancer incidence
rates were first published, it has been a puzzle,
a paradox — Marin residents simultaneously
existing as outliers on opposite ends of the health
spectrum. Marin is overwhelmingly healthy
— Willis points out that residents are more physically active and have the highest life expectancy
of all the 3, 143 counties in the country — and yet
its breast cancer rates are also high. Bottom line:
the healthy lifestyle here likely offsets the risk
factors in the population, experts say, making
the rate much lower than it would otherwise
be. Some say that Marin women themselves are
thinner than women in other counties (
measured by a lower body mass index) and are fitter,
and most report healthier diets, all associated
with lower breast cancer risk. They also smoke
less, a factor not just relevant to lung cancer but
to breast cancer as well. A large percentage of
Marin women with children also breast-feed,
which delays resumption of menstrual cycles and
thereby may offset reproductively related risk.
“Marin women set the bar for protecting
themselves, with high rates of physical activity
and low obesity rates,” Willis says; the role of
exercise in reducing cancer risk is not theoretical, but “a factual statement.” A study in the
Journal of the American Medical Association
found women ages 35 to 50 cut their risk 18
percent just by taking a brisk half-hour walk
three times a week, a regimen most Marin
women would view as minimal. Asked if
Marin women would have an even higher rate
of breast cancer if they were not as healthy?
“Almost certainly yes,” Halberg says.
ENVIRONMENTAL ROLE
Environment is unproven as a risk factor in
Marin; there’s no reason to believe women are
more likely to get breast cancer simply because
they live here. A major 2003 study led by UCSF
researchers partnered with Marin Breast
Cancer Watch (now Zero Breast Cancer) investigated environmental issues specific to Marin
Bottom line: the healthy lifestyle here likely offsets the
risk factors present in the population, making the rate
much lower than it would otherwise be.
