http://www.theaustralian.news.com.au/common/story_page/0,5744,15192205%255E23289,00.html
There is no magic cancer bullet
Medical reporter Clara Pirani
May 07, 2005
HARDLY a week goes by without a new study claiming that some foods will protect
us against cancer. In recent months researchers have claimed that chilli and
broccoli may slow cancer growth, milk can reduce the risk of colon cancer,
oranges reduce the risk of mouth tumours, avocado extract protects against
prostate cancer, turmeric prevents bowel cancer and beans and soy protein may
stave off breast cancer.
However, cancer experts claim that while the link between a poor diet and heart
disease is well established, the evidence that some foods can prevent cancer is
far from convincing.
Dallas English of the Cancer Council Victoria's Cancer Epidemiology Centre
argues that rather than proving some foods prevent cancer, studies are
increasingly disproving the theory.
"I would say there is no definitive evidence yet that any food or nutrient is
protective against cancer. We've been studying diet and cancer for 30 years,
however the picture isn't much clearer now," Professor English says.
"For the longest time we've been saying eat your fruit and vegies, but in the
last five years the evidence that fruit and vegetables are directly associated
with a reduced risk of cancer is getting weaker, partly because a lot of
earlier studies weren't as good as the current ones."
According to English, many of the initial studies linking diet and cancer were
"case-controlled". Participants were divided into two groups, those with cancer
and a control group of healthy people without the disease, and both groups were
quizzed about their diet -- a system English claims is hardly reliable.
"If you are asking someone with bowel cancer about what they ate, the fact they
know they have bowel cancer may well affect their recall, and they may have
even changed their diet in recent years since being diagnosed with cancer," he
says. Also, the people who volunteer for the control group tend to be very
healthy, with a good knowledge of nutrition and are perhaps not reflective of
the broader population.
Most current trials, however, are large "cohort" studies where people who don't
have the disease are asked about their diet. Researchers then track the
volunteers, often for many years, to see how many develop cancer.
Even though these studies are more reliable, they are still not providing
consistent results, English says.
"In the past five years there have been several of those published and there
hasn't been any of them that have found any convincing evidence that fruit and
vegetables, as a whole, are protective."
Two years ago the International Agency for Research on Cancer, which is part of
the World Health Organisation, released an analysis of all the evidence about
diet and cancer to that time.
"Their conclusion was that fruit probably protected against three kinds of
cancer - stomach, lung and oesophageal cancer and that vegetables probably
protect against bowel and oesophageal cancer," English says. "But the word they
used was 'probably' and my feeling is that since that time the evidence,
particularly for bowel cancer, has gotten a bit weaker."
Terry Slevin, chair of The Cancer Council Australia Nutrition and Physical
Activity Committee, agrees: "The reason there's dispute about whether fruit and
vegetables might reduce cancer risk is that the big population cohort studies
are not producing clear results. It's not that the studies are not credible,
but there might be slight variance in the methodology they use, or the
population they apply it to."
Researchers conducting diet studies have several problems to overcome,
including how to determine whether it's the food people eat, or the food they
don't eat, that protects against cancer.
John Zalcberg, director of the division of haematology and medical oncology at
the Peter MacCallum Cancer Centre in Melbourne, says it's difficult for studies
to incorporate and assess all the dietary, lifestyle and genetic factors that
determine someone's risk of developing cancer.
"When it comes to diet, it's hard to control for just one variable," Zalcberg
says. "For individuals it boils down to assessing their own risk by looking at
things like family history and the number of things that we know increase the
risk of cancers, like smoking. It's very hard to say to someone there is one
particular diet you should have and you won't get cancer."
One large study in the US looking at cancer incidence among vegetarian
Seventh-Day Adventists is a case in point.
Even if the trial finds that vegetarians have a low incidence of cancer, what
does that really tell researchers?
"Vegetarians by definition eat more vegies and fruits and less fats, including
red meat," Zalcberg says. "So what is it (that prevents cancer), is it eating
the fruit and vegetables, or not eating the red meat?"
Researchers also face an uphill battle trying to pinpoint which nutrient within
a particular food may help prevent cancer.
English says there is some evidence that folate may prevent bowel cancer, but
only when it's taken as a supplement.
"The strongest evidence about folate involves supplements," he says. "It's not
shown in studies of leafy greens (which contain folate)."
A comprehensive study linking diet and food would need to analyse what people
consume, the quantity, how the food is prepared, the combination of foods, the
participant's family history of cancer, other risk factors such as smoking,
over a long period of time.
"The big cohort studies invariably are not that specific. We just ask about how
frequently people eat a certain food and perhaps the average portion size and
those answers are well known to not be very accurate, but in the big studies
those questions are the most we can afford to do."
Finding a food that could prevent cancer is big business.
According to the UK-based World Research Cancer Fund, the global incidence of
cancer is projected to rise from 10.3 million cases in 1996 to 14.7 million in
2020.
Much of the research is funded by pharmaceutical companies in a hurry to find
the magic nutrient that would earn billions of dollars.
Even if a widely available food is found to prevent cancer, companies know many
people would rather pop a supplement than adhere to a healthy diet.
"This kind of research is largely driven by people who are looking for a magic
bullet for cancer treatment, because that's where the money is," Slevin says.
"Companies want to find out which bit of the broccoli, which bit of the
capsicum, can prevent or cure cancer so they can put it into a pill and sell it
for millions of dollars. They know people will take a pill rather than have a
serve of broccoli."
So is there anything people can do to prevent cancer?
According to researchers there are some dietary and lifestyle changes that will
reduce a person's risk of developing the disease.
"The best diet to prevent cancer in one that keeps you at a healthy weight,"
English says. "There is evidence that obesity is a consistent risk factor for
colon cancer, post menopausal breast cancer, endometrial and kidney cancer."
Slevin says maintaining a healthy weight, not smoking, and being physically
healthy will help.
"Some of the things that we are confident about are that maintaining a healthy
body weight reduces the risk of a huge range of cancers.
"The second most clearly demonstrated phenomenon in epidemiology is the
relationship between being physically inactive and the increased risk of colon
cancer which is absolutely consistent," Slevin says. "Being physically active
also helps reduce risk of a whole range of cancers. But there is no magic
bullet."
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