After lung cancer, prostate cancer is the
biggest cancer killer. No one knows what causes it, but
like breast cancer, prostate cancer is influenced by sex
hormones, and risk increases with age. Almost all men
develop silentor latent prostate cancer cells if they
live long enough. The big question is why a small percentage
of them develop the kind of prostate cancer that kills.
The highest rates of prostate
cancer occur in northern Europe and North America. The
lowest rates are found in Asia. But when, for example,
men move from Japan, where rates are low, to Canada or
the U.S., their risk eventually goes up. Thus, many researchers
think diet is one culprit. Other cultural, as well as
genetic factors, are undoubtedly involved, too. African
American men have the highest rate of prostate cancer
in the world; Native Americans, Mexican Americans, and
Chinese Americans have lower rates than other Americans.
However, age remains the major risk factor.
Nothing is certain yet, but
scientists have been looking at the following possible
preventers and promoters of prostate cancer.
Possible
protectors
•
A diet rich in fruits, vegetables,
and whole grains. For one thing, a plant-based
diet tends to be low in fat (see below). Moreover, plant
foods are rich in vitamins, minerals, and various phytochemicals
that may be beneficial. Many studies suggest that tomatoes
are protective, possibly because they are rich in the
carotenoid lycopene; cooked and processed tomatoes have
more absorbable lycopene than raw ones. And cruciferous
vegetables such as broccoli, kale, and cabbage contain
compounds called indoles, which may inhibit the growth
of prostate cancer cells. Still, it’s not clear
which substances are most important, so it’s best
to vary your intake of fruits and vegetables.
•
Selenium. Two major
studies found that a high intake of selenium (200 micrograms
daily) substantially lowered the risk of prostate cancer.
But overall the evidence has been mixed, and there are
many questions (see Wellness Letter, November 2005). Important
research on selenium is underway. Too much selenium can
be harmful, so get it from food—such as fish, whole
grains, and Brazil nuts—and possibly from a multivitamin/mineral
pill. If you take a selenium supplement, don’t take
more than 200 micrograms a day.
•
Fatty fish. A 2001
Swedish study found that men who ate little or no fish
were more than twice as likely to develop prostate cancer
as those who ate moderate to high amounts. In a 2003 Harvard
study, men who ate the most fish were least likely to
develop advanced prostate cancer (fish oil supplements
did not decrease the risk). Prostate cancer is very rare
among Inuit men in Greenland, and researchers attribute
this in large part to their high fish intake. Besides
its healthful fats, fish is also a good source of vitamin
D, which may be protective.
•
Vitamin D. People
who get little or no sun exposure tend to have higher
rates of prostate cancer, and mortality rates for this
cancer tend to be higher in northern regions, where there’s
less sun. Many researchers believe that lack of vitamin
D may be the explanation, since the skin makes this vitamin
when exposed to sunlight. Similarly, dark-skinned people
may be at higher risk for prostate cancer because their
skin makes less D. Fortified milk is the major dietary
source; many people over 60 should take a supplement containing
D (see Wellness Letter, October 2005).
•
Vitamin E. The evidence is weak. Though
some early studies suggested a protective effect, more
recent research has not. One large study found a benefit
only in smokers.
•
Soy (primarily because of its isoflavones)
and green tea are also under
investigation. These are staples in most Asian diets,
though no one knows if they are partly responsible for
the lower rates of prostate cancer in Asia.
Possible
bad guys
•
Dietary fat. This has been linked to increased
prostate cancer risk since the 1970s, though not in all
studies. If fat does play a role, it’s unclear whether
the amount, type, or source is the key. Some studies point
to red meat, and specifically to its saturated fat. But
the problem could also be the carcinogens formed when
meat is grilled. Or it could be that people who eat lots
of meat tend to eat less produce.
•
Zinc. The findings about zinc have been contradictory.
But in 2003 a study at the National Cancer Institute found
that men who took more than 100 milligrams of zinc a day
from supplements were twice as likely to develop advanced
prostate cancer as those who took no zinc. Zinc increases
blood levels of testosterone, which may promote prostate
cancer. This is one reason to avoid high doses of zinc.
•
Calcium or dairy products. Some studies,
including the Physicians’ Health Study, have linked
high intakes of calcium and/or dairy to increased risk.
But a few have found no connection, and some studies have
even found a reduced risk. One complication: dairy products
contain many nutrients besides calcium, some of which
may decrease the risk of prostate cancer (such as vitamin
D) or increase it (such as saturated fat). Don’t
stop consuming dairy products for fear of cancer. For
one thing, there’s fairly good evidence that dairy
products may reduce the risk of colon cancer and hypertension,
among other benefits.
•
Flaxseeds. Some doctors
warn men to avoid flaxseeds because of worries about prostate
cancer, but there actually has been very little research
on this. One study found that flaxseeds seemed to increase
tumor growth. But another showed that men with prostate
cancer who ate flaxseeds daily were able to slow the growth
of their cancer. There have been more studies on alpha-linolenic
acid (ALA), a heart-healthy fat found in flaxseeds, but
they, too, have yielded conflicting results. Flaxseeds
also contain compounds called lignans, which may have
an anti-cancer effect. Flaxseeds can be part of a healthy
diet, but don’t go overboard. For the potential
heart benefits, you need only a little ALA—the amount
in less than 1 tablespoon of ground flaxseed, 1 tablespoon
of canola oil, or half an ounce of walnuts a day.
Besides
diet
Smoking
and alcohol do not seem
to affect the risk of prostate cancer. Vasectomy,
despite some scary media reports a while back, does not
increase the risk, nor does an enlarged
prostate (benign prostatic hyperplasia). There’s
some evidence that obesity increases the chances of developing
aggressive prostate cancer or having it recur. Exercise
may well reduce the risk, though researchers have had
a hard time proving this. (An upcoming article will discuss
exercise and various types of cancer.)
You may have heard that your
sex life—that is, having too few, or too many, orgasms—affects
the risk of prostate cancer. Studies on this have yielded
contradictory findings. Don’t worry that your sex
life is harming your prostate.
