But proving that particular foods protect against cancer has been difficult, says Walter Willett, a professor at the Harvard School of Public Health who spoke at the annual meeting of the American Society of Clinical Oncology.

Scientists long have been intrigued that people in developing countries, who tend to eat more plant foods and fish, have lower cancer rates than those in countries whose diets are dominated by fats and red meat, Willett says.

Recent studies, however, have dashed hopes for a variety of proposed anti-cancer strategies: reducing fat to prevent breast cancer, increasing fiber to ward off colon tumors and filling up on fruits and vegetables to avoid cancer in general, Willett says. These studies are convincing because they followed participants over time and in some cases randomly assigned people to follow particular diets.

Science has crushed enthusiasm for some dietary supplements as well. Beta carotene pills, for example, actually increased the risk of cancer in clinical trials. Studies found vitamin E failed to reduce cancer risk. Though these studies may have disappointed many people, doctors have learned a lot about cancer prevention:

• Early experiences may matter most.

Many long-term studies, such as the Women's Health Initiative, involved mostly women over 60. But midlife may be too late for people to reduce their risk of cancer through diet.

"If you are 50 years old and have a cancer diagnosis and you suddenly start eating well, that is not going to do anything," says Barrie Cassileth, chief of integrative medicine at New York's Memorial Sloan-Kettering Cancer Center, who will speak about nutrition at the meeting today.

Breast tissue may be most susceptible to outside influences before puberty. Older Japanese women exposed to nuclear radiation in 1945 did not develop breast cancer, but young girls did, Willett says. Carcinogens may do the greatest damage early in life, so diet may play its most important role during childhood, Willett says.

• The amount of food may be more important than the type.

A number of studies strongly show that people who burn more calories than they consume are less likely to develop cancer, Willett says. Evidence strongly links obesity to colon cancer, pancreatic cancer, postmenopausal breast cancer, liver cancer and others. Though eating vegetables may not reduce a cancer patient's risk of death, losing even a few pounds may benefit people with certain tumor types, Willett says.

Researchers continue to study nutritional factors that may increase the risks of cancer, such as high intake of dairy products and low intake of folic acid, calcium, vitamin D and lycopene, which is found in tomatoes.

Like many dieters, Janet Bright of Hendersonville, N.C., says she is often confused by the latest nutritional news.

"They say things like 'This is good' and 'This is bad,' and six months later they change their minds," says Bright, 62, a breast cancer survivor. "I just eat as healthily as I can and hope for the best."

• More articles on other Cancer Links

Reference Source 129
June 7, 2006