FOOD FIGHT
New research lends credence to the argument that a sound diet can play a significant role in reducing cancer risk.
by Jon Nalick
The idea that eating the right foods can prevent the occurrence of certain cancers first gained prominence in 1981, when two British statisticians named Richard Doll and Richard Peto offered evidence that about 30 percent of American cancer deaths are diet-related.
Even so, how large a role diet plays in cancer has been a subject of intense study and debate, with often-conflicting study results that sometimes left scientists as perplexed as the public. But now, consensus is beginning to emerge about how some foods may help ward off certain cancers.
At the USC/Norris Comprehensive Cancer Center, researchers are running numerous clinical long-term studies to determine, for example, whether diets rich in fruits and vegetables or foods derived from soy can mitigate cancer risk.
Although those studies are not expected to produce definitive results for some time, preliminary findings here and at other research institutions lend credence to the argument that diet is linked to cancer risk.
Anna Wu, Ph.D., professor of research, says that proposition, if proven, is crucial because diet is something that people can generally change or modify.
"Many of the major risk factors for cancer are things that are simply out of your hands. You can't change your genes or your family history of disease — but you can change what you eat," she says.
Over the past two decades, Mimi Yu, Ph.D., professor of preventive medicine, has conducted many case-control studies in the U.S. and in Asia. The result: a number of findings that have suggested significant associations between cancer and specific foods.
She was one of the first investigators to firmly establish a link between salted fish, a staple in the diet of a specific population in Southern China, and cancer of the nasopharynx, a leading cancer in that population.
Wu is conducting several cancer studies including a seven-year project examining dietary and other factors affecting breast cancer risk in Asian women.
This group is especially interesting because women in Japan traditionally have one-sixth the breast cancer incidence as women in the U.S. — but when Japanese settle in the U.S., their breast cancer rates rise rapidly. By the third generation, they are identical to their Caucasian counterparts. The study, now in its fifth year, is designed to look at all the factors that may contribute to cancer risk, including diet, family history, physical activity and hormone levels.
"Our goal is to tease out the lifestyle factors that we believe may be important because their cancer risk is literally changing in front of our eyes," she says.
Some of the specific things that may contribute to the change include a possible drop in the amount of soy in the Japanese diet after coming to the U.S. Past studies by Wu and others have indicated that a diet low in fat may lower the levels of estrogen circulating in the blood; high circulating estrogen levels are associated with a higher risk of breast cancer. High intake of soy — which is rich in a type of plant estrogens known as isoflavones — has been suggested to lower the risk of breast cancer.
Still, she points out that even if soy proves to have anticancer properties, people need to understand that a person’s eating habits and overall lifestyle will likely determine their health and life expectancy.
“People do not eat just soy — they eat a complete diet. So we will never say, ‘Eat this one food to avoid cancer.’ There is no magic bullet,” Wu says.
Robert W. Haile, Dr.P.H., professor and director of the genetic epidemiology program, says scientists are fairly certain that the risks of colon and stomach cancers are affected by dietary factors.
“Evidence is strong that diets that include lots of cruciferous vegetables such as cabbage, broccoli and Brussels sprouts appear to protect against colon cancer. That's one of the most consistent findings. Right now we're trying to identify the mechanisms responsible for reducing risk ," Haile says
He notes that those foods are rich in a set of chemical compounds known as isothiocynates (ITCs) that appear to inhibit enzymes that convert certain chemicals in the human body into carcinogens. Additionally, ITCs may directly modify carcinogens, rendering them incapable of promoting cancer.
Further, a recent study published in the New England Journal of Medicine shows that calcium supplements can have a significant protective effect against colorectal cancers. A dose of 3,000 milligrams a day of calcium carbonate — about the same amount of calcium as in three Tums antacid tablets — can reduce the incidence of precancerous polyps by 25 percent.
Additionally, aspirin and folic acid are being studied as agents that may reduce colon cancer risk. And dietary fiber’s ability to reduce colon cancer risk remains unproven, although researchers continue to suspect that it may be beneficial in preventing some cancers, Haile says.
Other studies point to diets low in folate — found in raw fruits and vegetables — as potentially increasing colon cancer while diets high in folate may decrease risk.
Substances that may increase the risk of certain cancers include alcohol, whose heavy use — five drinks or more a day — is strongly linked to cancers of the mouth, esophagus, stomach, liver and breast. For breast cancer, even moderate use of alcohol — two drinks a day — has been shown to increase risk.
Some studies suggest that the risk of colon cancer can be increased by a diet heavy in red meat, especially well done meat. Any increased risk may come from the production of carcinogens in meat that is cooked at high temperatures, or possibly from the increased consumption of saturated fat, but as yet, researchers have reached no firm conclusions, Haile says.
In that vein, Haile warns against people attaching too much significance to any one study they hear about in the media or altering their diet based on a single news item. Noting that the media is rife with stories — often conflicting ones — about which foods cause or prevent cancer, he suggests that people pay attention to general trends established from confirmed results in a number of studies.
“But in the mean time, people should also know that there are several things we are quite sure about — and it is the common sense advice they've heard for years. Eat more fruits and vegetables, decrease saturated fats, and drink alcohol in moderation, if at all," he says.
Yu says that although remarkable strides have been made in the last decade in understanding how certain individual nutrients are metabolized in the body, “We have a lot more to learn. We still don’t understand all their main effects, let alone how they interact with each other. And that’s crucial because in every meal you put a lot of different foods in your stomach.”
Adding another layer of complexity is the fact that a person’s genetic makeup can have a huge impact on whether a chemical compound — beneficial or harmful — ever becomes activated, Yu says.
For example, some people have genes that code for a class of enzymes called glutathionine S-transferases, which can protect against cancers by neutralizing carcinogens, particularly those introduced into the body by smoking. But the same enzymes can potentially raise cancer risk by eliminating anti-cancer ITCs as well.
As a result of these kinds of reactions, researchers cannot guarantee that every person will derive the same benefits from foods thought to provide cancer protection.
”If I have a gene that causes me to excrete ITCs, I may have to eat twice as much broccoli, for example, to have the same effect as someone who lacks that gene,” Yu says.
She says that the next wave of research has already begun tackling these kinds of nutrient and gene interactions, with an eye toward being able to tailor diets for individuals that maximize anti-cancer properties based on their personal risk and genetic makeup.
Until then, Yu recommends eating a balanced diet for general good health and reducing cancer risk by altering behaviors such as smoking and excessive drinking.
“After all, diet is not like smoking — you can give up smoking,” she says. “You can’t give up food.”
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