Leading Scientist Hails Progress Made To Date in Field of Diet and Cancer Research

American Institute for Cancer Research
Thursday, 31 August 2000

Award-Winning Scientist Kicks Off AICR Research Conference By Spotlighting Important Ways that Nutrition Influences Cancer Risk

Delivering the Keynote Address at the American Institute for Cancer Research's 10th Annual Research Conference, Dr. John Potter discussed the scientific efforts that are revealing numerous ways that diet and lifestyle affect cancer risk. Potter, Head of the Cancer Prevention Research Program at the Fred Hutchinson Cancer Research Center and Professor in the Department of Epidemiology and Community Medicine at the University of Washington, is widely considered one of the world's premier authorities on the connection between diet and cancer.

"In the last decade particularly, our knowledge of the cancer process itself has significantly deepened," he said. "At the same time, we've come to realize that what we eat and how we live can modify this process, for good or ill."

Potter pointed to an ever-growing body of evidence that diets high in fruits, vegetables, whole grains and beans have the potential to "short-circuit" the cancer process at several different stages. Research has also revealed how other factors such as high meat intake, lack of exercise, obesity, and smoking encourage cancer growth.

"We knew 25 years ago that cancer must be preventable because of the worldwide differences in cancer incidence. However, we have a much better idea today of how cancer can be prevented," he said. "But this vital knowledge did not come easily. It has taken thousands of studies in field studies in human populations, in labs, and in clinics around the globe; and the path along the way has taken many turns."

Early Research Focused Largely on Animals

Animal experiments formed the backbone of early diet and cancer research, Dr. Potter said. He noted, however, that relying exclusively upon animal studies to understand the cancer process offered scientists only a partial glimpse of the complexities involved. Those first studies showed how some specific components in foods such as fat or protein could modify the action of known carcinogens in increasing the likelihood of tumors.

"There was tremendous progress, but in focusing exclusively on the mutagenic potential of compounds in food, we were only seeing a part of the puzzle," said Potter.

Methods of Cooking, Processing, and Storage Linked to Cancer

Nonetheless, as the amount of data grew, incorporating research widely different in design and scope (laboratory studies, human cohort and case-control studies, controlled trials), certain dietary components were increasingly thought to increase cancer risk in humans. Further, many of the known or probable human carcinogens arose in food as a result of certain cooking, processing, or storage methods.

Heterocyclic amines are found in meat that is cooked at very high temperatures. Polycyclic aromatic hydrocarbons are products of combustion and occur in cooked foods and in meats that have been exposed to smoke. Aflatoxins are found in moldy foods, and N-nitroso compounds occur in cured and spoiled foods as well as alcoholic drinks. Alcohol, while having no direct DNA damaging capacity itself, is a recognized human carcinogen probably via one of its metabolic products, acetaldehyde.

All of these compounds or their metabolic products can cause the kind of genetic damage that is central to the cancer process.

Powerful Protective Components Discovered in Food

Scientists were also gradually learning that food was more than just a source for potentially dangerous carcinogens. It was now becoming clear that diet also played an important role in cancer protection.

"The power of diets rich in vegetables and fruit to reduce the risk of cancer emerged gradually over some time from epidemiologic studies," Potter said. "At the same time, studies in cell systems and in animals began to identify specific components of plant foods that could account for this lower risk."

In the 1950s, researchers began to identify enzymes that allow the detoxification and excretion of toxic and carcinogenic compounds from the body. Beginning in the 1970s, laboratory studies have shown that there a variety of components within vegetables and fruits that induce, or "turn on" on these enzymes. Thus, diets high in vegetables and fruits make it easier for the body to defend itself by detoxifying carcinogens before the cancer process begins.

Over the years, many naturally occurring plant components have been shown to play a role in protection. Some, like the antioxidant vitamins C and E, demonstrate the ability to prevent the kind of initial genetic damage that is important both early and late in the cancer process. Others, like folate, may be important in ensuring that DNA damage is less likely and even that DNA repair is facilitated. Still others, like retinoids and short-chain fatty acids, seem to play a defensive role later on in the process, by encouraging cancerous cells to become more normal and stop multiplying or to undergo a kind of suicide known as apoptosis.

Additional, More Indirect Dietary Influences Emerge

"In the laboratory," said Potter, "the cancer process from healthy cell to cancerous tumor is relatively well worked out. In the human body, however, it is much less clear. That's why we're still working to understand some of the more complex and subtle ways that our diets influence cancer risk. It is also why we need to continue to undertake human experimental studies with foods, dietary patterns, and physical activity."

In fact, Potter said, the human body does a remarkable job of protecting itself from cancer. Our cells are exposed to active, potentially carcinogenic, compounds every day, many of which are simply by-products of normal metabolism. However, the body has several lines of defense, which can be either aided or undermined by the kind of life we live.

Several of these defenses do not directly involve the protection or repair of genetic material itself. Instead, they alter the general cellular environment, making it more difficult for an already cancerous cell to grow and reproduce.

Regular physical activity, for example, seems to discourage cancer growth in some organs. Obesity, on the other hand, seems to have the opposite effect, increasing levels of growth hormones and similar factors that encourage the continued reproduction of cancerous cells. Likewise, diets that are "energy-dense" (high in energy, sugar, and fat) may provide conditions favorable to tumor growth.

"Cancer is often a product of nutrient-gene interaction," said Potter. "It's true that some people are born at higher risk for certain cancers, while some lead lives that raise their risk even further. But the important thing to remember is that many, perhaps all, of us can profoundly decrease our cancer risk by making changes to what we eat and drink and how we live."

The following dietary recommendations for lower cancer risk first appeared in the landmark AICR report, Food, Nutrition and the Prevention of Cancer: A Global Perspective. Dr. Potter chaired the international panel of experts that authored this definitive document which compiled, reviewed and analyzed over 4,500 studies on the link between diet and cancer.

Dietary Recommendations of the AICR Expert Panel for Lower Cancer Risk

1. Choose predominantly plant-based diets rich in a variety of vegetables and fruits, pulses (legumes) and minimally starchy staple foods (whole grains)

2. Avoid being underweight or overweight and limit weight gain during adulthood to less than 11 pounds

3. If occupational activity is low or moderate, take an hour's brisk walk or similar exercise daily, and also exercise vigorously for a total of at least one hour in a week

4. Eat 400-800 grams (15-30 ounces) or five or more portions (servings) a day of a variety of vegetables and fruits, all year round

5. Eat 600-800 grams (20-30 ounces) or more than seven portions (servings) a day of a variety of cereals (grains), pulses (legumes), roots, tubers and plantains. Prefer minimally processed foods. Limit consumption of refined sugar

6. Alcohol consumption is not recommended. If consumed at all, limit alcoholic drinks to less than two drinks a day for men and one for women

7. If eaten at all, limit intake of red meat to less than 80 grams (3 ounces) daily. It is preferable to choose fish, poultry or meat from non-domesticated animals in place of red meat

8. Limit consumption of fatty foods, particularly those of animal origin. Choose modest amounts of appropriate vegetable oils

9. Limit consumption of salted foods and use of cooking and table salt. Use herbs and spices to season foods

10. Do not eat food which, as a result of prolonged storage at ambient temperatures, is liable to contamination with mycotoxins

11. Use refrigeration and other appropriate methods to preserve perishable food as purchased and at home

12. When levels of additives, contaminants and other residues are properly regulated, their presence in food and drink is not known to be harmful. However, unregulated or improper use can be a health hazard, and this applies particularly in economically developing countries

13. Do not eat charred food. For meat and fish eaters, avoid burning of meat juices. Consume the following only occasionally: meat and fish grilled (broiled) in direct flame; cured and smoked meats

14. For those who follow the recommendations presented here, dietary supplements are probably unnecessary, and possibly unhelpful, for reducing cancer risk

At a separate session of the AICR Event, Dr. Potter was honored with the AICR Research Achievement Award in recognition of his many contributions to epidemiological and nutritional research.

For more information, or to contact American Institute for Cancer Research, see their website at: www.aicr.org