As millions of Americans fill their plates with protein-rich steak and burgers rather than carb-heavy pasta or potatoes, researchers are reporting the strongest evidence yet that eating a lot of red meat increases the risk of colorectal cancer.

Those who ate the equivalent of a hamburger a day were about 30% to 40% more likely to develop cancer of the colon or rectum than those who ate less than half that amount.

Long-term consumption of high amounts of processed meat such as hot dogs increased the risk of colon cancer by 50%.

The findings, published in the Journal of the American Medical Association, join a growing body of evidence linking diet and certain types of cancer. By some estimates, as many as 3 million to 4 million cancer cases could be prevented worldwide each year simply through healthy eating and lifestyle changes.

However, meat lovers need not despair over thoughts of stocking their refrigerators with tofu burgers and vegetarian bacon.

"Our results do not suggest that one should eliminate consumption of red or processed meat," said Dr. Michael J. Thun, head of epidemiologic research at the American Cancer Society and one of the study's authors. "Rather, the message is to limit consumption."

The researchers defined high intake of meat -- including beef, veal, pork, sausages and bologna -- as three or more ounces a day for men and two or more a day for women (a McDonald's hamburger weighs 3.7oz., a Big Mac 7.8 oz.).

The study involved 148,610 adults of ages 50 to 74. All provided information about their meat consumption in 1982 and again in 1992 or 1993 as part of the cancer society's Cancer Prevention Study II Nutrition Cohort.

The median intake of red meat was just over 2oz. a day for men in the study and 1.4oz. a day for women, the researchers reported, but individual consumption varied widely. Among men, the heaviest consumers ate 10 times as much red meat as those who ate the least. The heaviest consumers among women ate 17 times as much.

Annual per capita beef consumption has risen since 1993, reversing a decline that began in 1976. According to the U.S. Department of Agriculture, the average American ate 114 pounds of red meat -- including 64 pounds of beef -- in 2000.

OTHER RISK FACTORS
Despite the link between meat consumption and increased colon cancer risk, Thun emphasized that other lifestyle factors boost the chance of developing the disease even more. Physical inactivity, for instance, doubles the risk.

"The level of risk with respect to overall health is not at all in the same league as the risks associated with smoking," he added.

Colorectal cancer is the third most common cancer in both men and women in the United States. The American Cancer Society estimates that 105,000 people will develop colon cancer this year, while about 40,000 will get rectal cancer. The two diseases will cause more than 56,000 deaths.

Researchers hypothesized decades ago that red meat might increase cancer risk after noticing that colon cancer rates in the U.S. and Europe, which have meat-based diets, were much higher than in Japan, where rice is the traditional staple and fish is a more common protein source.

A study last summer by researchers in Japan noted that a jump in colon cancer cases there coincided with an increase in red meat consumption that began 20 years earlier.

Mary Young, a registered dietitian who is executive director of nutrition for the National Cattleman's Beef Association, said the new findings should be interpreted with caution because they show only an association, not a cause-and-effect relationship.

She called beef a "nutrient-rich" food widely available in lean cuts that are low in fat and cholesterol.

"A 3oz. serving of lean beef packs in more than nine essential nutrients in the size of your computer mouse," she said, referring to zinc, protein and iron, among others. "It's a good or excellent source of more vitamins and minerals than any other animal protein -- more than fish, than chicken, than turkey."

Federal guidelines call for a diet balanced among the five food groups, including two to three servings a day from the group that includes meat, poultry, fish, beans and eggs. A revised set of guidelines, jointly published every five years by the Health and Human Services and Agriculture departments, has just been announced.

The JAMA study did not investigate why large quantities of red and processed meat appear to increase colorectal cancer risk. However, one theory is that cooking meat at high temperatures leads to the creation of so-called mutagens, which can damage DNA. Another theory holds that the high iron content in red meat produces free radicals, which likewise can harm DNA.

'BEANS AND TOFU'
"We've suddenly been handed a good piece of evidence as to what we can do in our own lives to reduce our (cancer) risk," said Dr. Neal D. Barnard, president of the Washington-based Physicians Committee for Responsible Medicine, who swore off smoking and meat during medical school in the late 1970s. "Suddenly, the beans and tofu are looking pretty good."

In a 1995 study, Barnard estimated that consumption of meat (including poultry) costs Americans between $29 billion and $61 billion in annual medical costs associated with high blood pressure, cancer and heart disease, among other conditions. Some research, he pointed out, has showed that eating white meat also increases colon cancer risk.

Two separate articles in the JAMA also investigate the link between foods and cancer. A team of researchers from the Johns Hopkins Bloomberg School of Public Health and Yonsei University in Korea reported that high blood sugar levels increased the risk of developing several cancers -- and of dying from them.

By contrast, Danish researchers found that eating fruits and vegetables did not reduce a woman's risk of breast cancer, as previous studies had suggested.

In an accompanying editorial, Dr. Walter Willett of the Harvard School of Public Health called the breast cancer results disappointing. However, he said eating fruits and vegetables has plenty of other health benefits, including lowering the risk of high blood pressure and cardiovascular disease.