Family Circle
THE RARE TRUTH ABOUT
DIET AND CANCER
By Michael Castleman
What’s going on? For years, health authorities have touted
fruits and vegetables as the dietary keys to reducing
cancer risk. Then last February [2001], a Harvard study
showed that fruit and vegetable consumption has nothing to
do with breast cancer risk. For just as long, the experts
have maintained that a low-fat diet is critical to
prevention of many malignancies, notably breast cancer. But
some influential studies have shown that dietary fat plays
no role in breast cancer risk. For ages, the experts have
promoted bran cereals and other high-fiber foods for
prevention of colon cancer. Then several highly publicized
studies suggested that fiber does little, if anything, to
prevent the disease.
Meanwhile, over the past few years, various studies have
crowned several grocery items as “anti-cancer foods,” among
them: tomatoes for prevention of prostate cancer, tofu and
other soy foods for breast cancer, and cruciferous
vegetables--broccoli, cauliflower, cabbage, and brussels
sprouts for cancers of the bladder and breast. Almost as
quickly as these reports have appeared, the supplement
companies have introduced pills containing the constituents
the researchers have identified as the key anti-cancer
nutrients: lycopene from tomatoes, isoflavones from soy,
and indoles from cruciferous vegetables. But cancer
prevention experts generally oppose taking these
supplements.
So what’s going on here? How does diet really affect cancer
risk? Family Circle asked leading authorities--including
experts from the National Cancer Institute (NCI), the
American Cancer Society (ACS) , Memorial-Sloan Kettering
Cancer Center in New York, and MD Anderson Cancer Center in
Houston--to separate fact from fiction, help from hype.
“It’s no surprise that people are confused about diet and
cancer,” says Marion Nestle, Ph.D., professor and chair of
the department of nutrition and food sciences at New York
University and chair of the committee that developed--and
is currently revising--the ACS nutrition guidelines. “The
studies that get publicized in the news media are often
contradictory. This subject is complicated.”
One reason for the complexity is that cancer risk depends
on more than just diet, according to Tim Byers, M.D.,
M.P.H., a professor of preventive medicine at the
University of Colorado Health Sciences Center and a member
of the ACS nutrition-guidelines committee: “Family history
is very important. So is smoking. And recently, obesity and
lack of exercise have emerged as increasingly important
risk factors. Sometimes it’s hard to separate the effects
of diet from all the other risk factors. But as part of an
overall healthy lifestyle, diet can clearly help reduce
cancer risk.”
EAT MORE FRUITS AND VEGETABLES
For years, the NCI’s “5 A Day” program has urged Americans
to reduce their risk of every major cancer by eating at
least five daily servings of fruits and vegetables--and
preferably more, eight or nine. (What’s a serving? See
sidebar) “Fruits and vegetables are the best dietary
sources--often the only sources--of cancer-preventive
antioxidant nutrients, among them: vitamins A, C, E, folic
acid, and selenium,” explains Colleen Doyle, M.S., R.D.,
director of nutrition and physical activity programs for
the ACS. Scientists generally agree that a specific type of
cell damage--oxidative damage--is the underlying cause of
cancer. Antioxidants help prevent and repair this damage.
But in a headline-grabbing study published last February
[2001], Harvard researchers concluded: “Fruit and vegetable
consumption...is not significantly associated with reduced
breast cancer risk.” They analyzed eight previous studies
involving 352,000 women who developed 7,400 cases of breast
cancer. Compared with the women who ate the fewest fruits
and vegetables, those who consumed the most gained no
significant protection from the disease.
Based on this study, should the NCI junk it’s “5 A Day”
program? “Absolutely not,” insists Peter Greenwald, M.D.,
Dr.P.H., director of the NCI’s division of cancer
prevention. “No single study reveals the truth. You have to
look at the totality of the evidence. The vast majority of
studies--more than 100--show that as fruit and vegetable
consumption increases, cancer risk decreases.”
The landmark study in favor of fruits and vegetables for
cancer prevention was published almost a decade ago by
Gladys Block, Ph.D., professor of epidemiology and
nutrition at the University of California, Berkeley, School
of Public Health. She analyzed 156 studies of diet and
cancer. The vast majority of them (82 percent) showed that
as fruits and vegetable intake increased, risk of every
major cancer decreased. Many of the others showed a trend
toward a protective effect, but did not reach statistical
significance. “That’s about as convincing as epidemiology
ever gets,” she says. “The evidence is overwhelming in
favor of plant foods for reduction of cancer risk.”
Since Dr. Block’s report, many other studies have shown
that fruits and/or vegetables help prevent cancers of the
colon, lung, prostate, bladder, and non-Hodgkins lymphoma.
Fruits and vegetables can even help smokers avoid lung
cancer (see sidebar). And other research has shown that
antioxidant nutrients in plant foods help prevent the
genetic damage at the root of cancer.
Even the Harvard report that showed no protective effect
for fruits and vegetables was not as negative as its media
coverage implied. In that study, as plant food consumption
increased, breast cancer risk decreased--but not quite
enough to reach statistical significance. Why didn’t this
study produce statistically significant results? “It’s not
clear,” Dr. Greenwald explains. “Epidemiology isn’t
perfect, and diet is only one factor among many in breast
cancer risk. But I’m sticking by fruits and vegetables. So
is every other scientist I know in cancer prevention.”
“The news media want good stories,” Dr. Block observes.
“Findings contrary to accepted beliefs make better stories
than findings that confirm the same old thing. But it
disturbs me how the media often fasten on just one study.
Overall, the evidence clearly shows that as fruit and
vegetable consumption goes up, cancer risk goes down.”
“We’re standing firm by our recommendation to eat a diet
based on plant foods,” the ACS’s Doyle says. “A plant-based
diet helps prevent not only cancer, but also heart disease,
high blood pressure, diabetes, and other chronic
conditions. Fruits and vegetables are the basis of a
healthy diet.”
Do they have to be fresh? “No,” Dr. Byers says. “I think
nutritionists have created the misimpression that you have
to shop at a farmer’s market for just-picked produce to get
enough nutrients. But frozen vegetables are just about as
nutritious as fresh. And even canned vegetables retain many
cancer-preventive nutrients. It’s much better to eat canned
peas or corn than not to eat them at all. I tell people: It
doesn’t matter how you eat your fruits and
vegetables--fresh, frozen, canned, or dried. Just eat them.
Eat more of them.”
Unfortunately, according to Linda Nebeling, Ph.D, M.P.H.,
R.D., only 20 percent of American men and 31 percent of
women eat the healthy minimum of five daily servings of
fruits and vegetables. “It's not difficult to get five,”
she explains. “Just have some fruit with breakfast, a
salad, and vegetable at lunch and dinner, and a few fruit
or vegetable snacks each day. Instead of reaching for a bag
of chips, reach for an apple.”
YOU CAN EAT FAT, JUST DON’T BE FAT
For years, cancer experts have advised eating a low-fat
diet based on international studies showing that countries
with such diets have lower cancer rates--particularly less
breast cancer--than countries with high-fat diets, for
example, the U.S. In addition, many studies have shown that
as dietary fat intake increases--especially the saturated
fat in meats--so does risk of many cancers, among them:
breast, colon, ovarian, lung, bladder, prostate, and
non-Hodgkins lymphoma.
But starting about a decade ago, influential studies began
showing no association between dietary fat and cancer risk,
notably risk of breast cancer. One was a report from the
Nurses Health Study, a Harvard project that has followed
the diet, lifestyle, and health of more than 80,000 women
nurses for many years. Nurses who consumed the most fat
were no more likely to develop breast cancer than those who
ate the least. Another study by NCI researchers followed
40,000 women for more than five years. Again, fat
consumption had nothing to do with breast cancer risk. And
an Italian study even showed that as total dietary fat
rose, breast cancer risk actually fell.
“The fat issue has become complicated--and for most people,
very confusing,” says Sally Scroggs, M.S., R.D., senior
coordinator of health education at the Cancer Prevention
Center at MD Anderson Cancer Center in Houston.“The type of
fat you eat now appears to be more important than the total
amount of fat you eat. Fats differ chemically and the
various types have different effects on cancer risk. Many
studies show the strongest statististical association
between cancer and the saturated fat in meats. Other
studies show no statistical associations between cancer
risk and the monounsaturated fat in olive oil and the
omega-3 fatty acids in salmon and other cold-water fish.”
The ACS nutrition guidelines recommend limiting fatty
meats. “That’s still good advice,” Dr. Greenwald explains.
“You don’t have to become a vegetarian. Just eat more plant
foods and less meat, and when you eat meat, choose lean
cuts--skinned chicken breasts instead of sausage.”
All the experts consulted for this report endorsed a
low-fat diet because it helps prevent heart disease, the
nation’s leading killer. But for cancer prevention, the
experts are changing their emphasis, moving away from
recommending a low-fat diet toward a focus on preventing
obesity. It may seem odd to distinguish eating fat from
being fat. After all, the former leads to the latter. But
in addition to dietary fat, other factors contribute to
obesity, notably a high-calorie diet and lack of exercise.
Over the past decade, Americans have switched in droves to
low-fat and fat-free foods. But our lower-fat diet doesn’t
mean we’re eating fewer calories. “Low-fat and fat-free
foods substitute sugar for fat,” Doyle explains. “They have
almost as many calories as their high-fat counterparts.
That’s why we have record sales of reduced-fat foods and
record levels of obesity. You can’t focus just on fat. You
have to consider calories, and control your caloric
intake.”
“For cancer risk,” Dr. Byers says, “being fat is more
important than eating fat. Americans are eating more
calories than ever, getting fatter, and many are becoming
obese. Obesity is strongly associated with many cancers.”
Many cancers is right: Recent studies have linked obesity
to increased risk of cancers of the breast, colon, cervix,
uterus, ovary, kidney, bladder, brain, gallbladder,
pancreas, larynx, prostate, and small intestine.
“If I could make one change in the American diet,” says
Moshe Shike, M.D., director of cancer prevention and
wellness programs at Memorial Sloan-Kettering Cancer Center
in New York City, “it would be to eat fewer calories. After
smoking, obesity is the nation’s biggest health problem.
Obesity increases risk not only of cancer, but also of
heart disease, high blood pressure, diabetes, arthritis,
and many other serious conditions.” Many large population
studies have shown a statistical association between
obesity and cancer risk. For some cancers, for example,
breast cancer, we know why. Excess fat tissue increases
women’s blood levels of estrogen, and the hormone, in turn,
promotes the growth of breast tumors.
“Americans’ calorie intake is out of control,” Dr.
Greenwald says. “Fast-food meals are ‘super-sized.’ May
restaurant portions are now large enough for two. Combine
that with most Americans’ lack of exercise, and what you
get is our current epidemic of obesity.”
“Even dishes have gotten bigger,” Doyle says. “I bought new
plates recently after 13 years. All the dishes I saw were
much bigger than the ones I was replacing.”
That’s why the new ACS nutrition guidelines, scheduled to
be released early next year [2002] stress obesity
prevention through calorie control and exercise. “It’s
pitiful how little the average American exercises,” the
ACS’s Doyle says. “The emerging consensus is that being fat
is more of a cancer risk than eating fat, which is why the
ACS guidelines are placing new emphasis on calorie control,
weight control, and exercise.”
Is your weight increasing your risk of cancer? If your Body
Mass Index (BMI) is greater than 25, you risk
weight-related health problems. To calculate your BMI, see
the sidebar.
KEEP EATING FIBER
Fiber, which your mother may have called “roughage,” is the
undigestible portion of plant foods. Insoluble fiber gives
celery its crispness, and whole wheat bread its heartiness.
soluble fiber gives oatmeal its gummy texture.
Conventional wisdom has long held that a high-fiber diet
helps prevent colon cancer. But recently three noted
studies have called that belief into question. The first
was a 1999 report from the Nurses Health Study that tracked
the women for 16 years. The researchers found no
association between their fiber intake and their risk of
colon cancer. Then earlier this year [2001], two studies
showed that dietary fiber--a wheat-bran supplement in one
case, and fruits and vegetables in the other--did not
prevent recurrence of colon polyps, the precursors of colon
cancer.
Have these studies spurred the experts to rethink the
age-old recommendation to eat a high fiber diet? “No way,”
Dr. Byers says. “I continue to eat a high-fiber diet.
Everyone should. Even if it doesn’t help prevent colon
cancer, there’s no question that a high-fiber diet helps
prevent constipation and hemorrhoids. If you follow the
cancer-prevention recommendation to eat a plant-based diet,
you’ll automatically get lots of fiber.” In addition, fiber
helps prevent obesity because it’s bulky. It fills you up
while only providing a small number of calories. Obesity is
a major risk factor for heart disease, so eating a
high-fiber diet helps prevent heart attack as well as
cancer.
“With all due respect to the researchers who performed the
recent studies,” Doyle explains, “the subjects taking the
fiber supplements still didn’t get as much fiber as we
recommend--just 13.5 grams compared with the 25 to 35 grams
a day we recommend--and those studies lasted only a few
years. Colon cancer takes decades to develop. It’s possible
that those studies did not provide enough fiber for long
enough to show any protective effect.”
“The typical American consumes only 10 to 12 grams of fiber
a day,” Scroggs explains. “That’s just one-third to
one-half of what the ACS recommends. The food sources most
people are familiar with--bran cereals and whole-grain
breads--are great, but they’re not the only good ones. A
half-cup of beans provides 7 to 8 g of fiber. A bag of
microwave popcorn contains about 12 grams. And all fruits
and vegetables are high in fiber.”
“The ACS still recommends a high-fiber diet,” Doyle
explains, “but maybe it’s not the fiber in plant foods
that’s so important for cancer prevention. Maybe what
counts is all the antioxidant nutrients that come along
with the fiber in fruits, vegetables, beans, and whole
grains. If that’s the case, it just reinforces our
recommendation to eat a diet based on plant foods.”
Meanwhile, a study still in progress has renewed experts’
faith in fiber for prevention of colon cancer. The European
Prospective Investigation of Cancer (EPIC), launched in
1992, is the largest study of diet and cancer ever
undertaken, involving more than 500,000 people in 10
European countries. It’s results have not been published,
but Dr. Greenwald recently learned of preliminary results
showing that the participants whose diets contained the
most fiber had the lowest risk of colon cancer. “I believe
that a high-fiber diet helps prevent colon cancer,” he
says, “And I’m betting that soon the EPIC trial will show
it.”
HIGHLY TOUTED “ANTI-CANCER FOODS:”
GOOD TO EAT, BUT NOT MAGIC BULLETS AGAINST CANCER
Over the past several years, various studies have singled
out certain foods as particularly potent cancer
preventers--and news reports have crowned them “miracle
anti-cancer foods.” For example, cruciferous vegetables
have been shown to reduce risk of cancers of the bladder
and breast. Soy foods have been associated with decreased
risk of breast and edometrial cancer. Tomatoes have been
shown to help prevent cancers of the prostate, lung, and
stomach. Fish consumption has been linked with decreased
risk of breast and colon cancer. Olive oil appears to
reduce risk of breast and colon cancer. Garlic has been
shown to help prevent cancers of the colon and stomach. And
drinking green tea (five or more cups a day) appears to
reduce risk of cancers of the breast, esophagus, colon, and
pancreas.
Every expert consulted for this report agreed that for most
people, all these foods can be part of a healthy diet. But
they also agreed that there is no such thing as a “miracle
anti-cancer food.”
“For some reason,” Dr. Nestle says. “Americans are obsessed
with the single food or single nutrient that’s somehow
‘best.’ There are no dietary magic bullets against cancer.
I wish people would stop looking for them. All the
so-called anti-cancer foods--crucifers, soy, tomatoes,
fish, olive oil, garlic, and green tea--are good foods. But
people should look at their overall diet and not become
fixated on these or any other foods.”
“Researchers who perform diet-and-cancer studies tend to
break out the foods with the greatest statistical effect,”
Dr. Nebeling explains, “and yes, in some studies,
cruciferous vegetables or these other foods have been
associated with the greatest reduction in cancer risk. But
if you look at the totality of the evidence, the so-called
anti-cancer foods are not head and shoulders above other
fruits, vegetables, beans, and whole grains.”
“If you don’t like or can’t eat some of these foods, it
doesn’t matter,” Scroggs explains. “I had a patient
recently who’d heard that fish and green tea are good for
preventing cancer--only she despised fish and was very
sensitive to caffeine, which green tea contains. In fish,
it’s the omega-3 fatty acids that appear to be involved in
reducing cancer risk. I told her she can get omega-3s from
walnuts and sesame seeds. In green tea, it’s the
antioxidants. I told her she can get them from any fruits
and vegetables.”
“Variety is what’s important,” Dr. Byers says, “a little of
everything and not too much of any one food. A little tofu
in a stir fry dish is fine. It may well have an
anti-estrogen effect that might help prevent breast cancer.
But it frightens me to see so many women eating large
amounts of soy foods. If you eat a lot of soy, it may have
a pro-estrogen effect that might actually promote breast
cancer. We just don’t know yet. The same goes for fish. A
little tuna is fine. But tuna can be contaminated with
mercury. If you eat tuna every day, you risk harm from
mercury toxicity.”
LIMIT ALCOHOL CONSUMPTION
The new ACS nutrition guidelines recommend limiting alcohol
because it is associated with cancers of the mouth, larynx
(voice box), esophagus, breast, liver and colon.
Most alcohol-related cancer develop largely in alcoholics.
But social drinking increases risk of breast and colon
cancer, Dr. Greenwald says, with as little as a few drinks
a week possibly raising breast cancer risk. Like obesity,
alcohol increases women’s blood levels of estrogen, which
in turn promotes the growth of breast tumors.
“If you have a personal or family history of breast
cancer,” Dr. Byers says, “it’s prudent not to drink. And
for premenopausal women in general, it’s prudent to limit
alcohol consumption. But after about age 60, women’s risk
picture changes. They become more likely to die of heart
disease than breast cancer. A little alcohol, on the order
of one drink a day, can help prevent heart attack.
Alcohol’s health effects are complicated. Women should
discuss their risk of breast cancer and heart disease with
their physician and get individual counseling on this
issue.”
SUPPLEMENTS FOR CANCER PREVENTION?
STILL VERY CONTROVERSIAL
Many studies have shown that certain vitamin and mineral
supplements help prevent cancer: At the National Cancer
Institute, researchers discovered that in smokers, vitamin
E supplementation significantly reduces risk of lung
cancer. Harvard researchers have linked selenium intake
from food and supplements to reduced risk of advanced
prostate cancer. Supplemental folic acid, a B vitamin, if
taken for more than 15 years, helps prevent colon cancer,
according to other Harvard researchers. Researchers at the
University of West Virginia School of Medicine have shown
that large doses of vitamins A, B6, C, and E, plus zinc,
reduce risk of bladder cancer recurrence. Vitamins A, B, C,
and E help prevent mouth and throat cancer, according to
National Cancer Institute researchers. And European
researchers found that calcium supplementation helps
prevent the recurrence of the colon polyps that can
eventually become cancerous.
But the experts consulted for this report were sharply
divided on the value of vitamin supplements for cancer
prevention. None endorse exotic regimens using large doses
of single-nutrient supplements based on a highly publicized
1996 study showing that beta-carotene, a form of vitamin A,
actually INCREASED lung cancer risk among longtime smokers.
And none support taking nutrients in pill form isolated
from the so-called anti-cancer foods, for example, lycopene
from tomatoes. The experts agree it’s not yet clear which
nutrients have the most cancer-preventive value, and that
in general, nutrients work best in groups--as they appear
in foods--not in isolation. But several take a “multi,” a
multivitamin-mineral supplement.
Dr. Block takes a multi and believes everyone should: “It’s
prudent, and not just for cancer prevention, but also for
prevention of heart disease and other serious conditions.
Those opposed to supplements say that everyone should
simply eat well, then they wouldn’t need vitamins. I wish
everyone ate better, but the fact is, people don’t.
Multivitamins are a cheap, convenient way to gain a little
nutritional insurance. It’s a shame there’s so much medical
resistance to vitamins.”
Doyle takes a multi occasionally: “I prefer to get my
nutrients from food, but when I travel, it’s often
difficult to eat as nutritiously as I’d like. That’s when I
take a multivitamin with iron.”
Dr. Shike shuns vitamins and says they have no place in
cancer prevention. “There is little scientific basis to
recommend them.”
Dr. Byers has “nothing against people taking
multivitamins,” but he does not take vitamins himself. “The
more research we do, the more it appears that vitamins and
minerals work best in the complex combinations we find in
foods. So why not just eat vitamin-rich foods? Put your
energy into eating fruits and vegetables, and then if you
like, take a multivitamin.”
IT’S IN YOUR HANDS
Cancer develops because of a complicated combination of
heredity and other factors. “You can’t do anything about
your heredity,” Dr. Shike says, “but most of the other
factors are things we can control.” He offers the following
recommendations: Don’t smoke. Maintain your recommended
weight. Eat a low-fat, high-fiber diet with lots of fruits,
vegetables, beans, and whole grains, and a minimum of meat.
Drink only a little alcohol, if any. And get regular
exercise. “The message is: Live a healthy lifestyle. If you
do, you reduce your risk of not only cancer, but also of
heart disease, diabetes, and many other serious
conditions.”
Sidebar:
What’s a Serving?
The National Cancer Institute (NCI) and other
cancer-prevention experts recommend eating at least five
servings of fruits and vegetables a day. If you’re not sure
of what constitutes “a serving,” here’s a handy guide.
According to the NCI, 1 serving is:
* 1 medium-sized piece of fruit (apple, orange, banana,
peach, etc.) or 1/2 cup of small or cut-up fruit (berries,
grapes, melon, etc.).
* 3/4 cup of 100 percent fruit juice.
* 1/4 cup dried fruit (raisins).
* 1/2 cup raw or cooked vegetables
* 1 cup raw leafy vegetables (lettuce, spinach, chard)
* 1/2 cup cooked peas or beans (lentils, pinto, kidney,
etc.)
Sidebar:
How to Calculate Your Body Mass Index
Don’t watch your weight. It’s better, scientists say, to
watch your body mass index (BMI). “BMI takes height into
account,” explains Colleen Doyle, M.S., R.D., director of
nutrition and physical activity programs for the American
Cancer Society. “If you weigh 150 pounds and you’re 5 feet,
2 inches tall, you’re overweight. But if you weigh 150 and
you’re 5 feet, 7 inches tall, you’re fine.”
To figure out your BMI, get out your calculator. Start with
your weight in pounds. Divide it by your height in inches.
Divide the result again by your height in inches. Then
multiply the result by 703. The final result is your BMI.
If it’s more than 25, you risk obesity-related health
problems, including cancer.
Example #1: 150 pounds, 5 feet, 2 inches (62 inches) tall .
150/62 = 2.419/62 = 0.0390 x 703 = 27.42. Greater than 25.
Overweight.
Example #2: 150 pounds, 5 feet, 7 inches (67 inches) tall.
150/67 = 2.239/67 = 0.0334 x 703 = 23.49. Less than 25.
Healthy weight.
Sidebar:
Attention Smokers:
Diet Changes Can Reduce Your Risk of Lung Cancer--And Help
You Quit
Austin, Texas, physician Tom Ferguson, M.D., thinks doctors
have been too hard on smokers: “The typical line is: ‘If
you smoke, quit.’” It’s a rare smoker who hasn’t tried to
quit--and failed, he explains, which fosters an defeatist
attitude: I just can’t do it. Dr. Ferguson is all for
smokers continuing to try to quit, but his book, The
No-Nag, No-Guilt, Do-It-Your-Own Way Guide to Quitting
Smoking (Ballantine), takes a different approach: Start
living a healthier lifestyle even as you continue smoking,
and your lifestyle changes can foster a more positive
I-can-do-it attitude that can help you cut down on
cigarettes and eventually quit. Diet is a good place to
start because many studies show that as smokers eat more
fruits and vegetables, their risk of lung cancer declines
significantly. Most recently, researchers in the
Netherlands followed a group of 62,500 women and 58,300
men, including many smokers, for more than six years.
Compared with the smokers who ate the fewest fruits and
vegetables, those who ate the most were significantly less
likely to develop lung cancer.