The bad news is that if you cook any kind of meat at hot enough temperatures, some of the protein will be converted into substances that can cause cancer.
The good news is that beef, pork, and possibly other kinds of meat contain natural anti-cancer substances. This is the first time chemicals that actively protect against cancer have been found in any animal product.
Scientists who presented the findings here before an international meeting of chemists said it was not yet clear whether the two kinds of substances might cancel out.
They did cite evidence that people who eat a meat-heavy diet have a much higher than normal incidence of cancer of the mouth, esophagus, stomach, colon and rectum. They noted, however, that this could simply be because people who eat predominantly meat diets also eat lots of fat, which is believed to increase the cancer risk, or because they eat fewer vegetables, which are believed to protect against cancer.
The findings were presented at the first major symposium on cancer-causing substances that are formed in food as a result of cooking. The symposium was part of the International Chemical Congress of Pacific Basin Societies, which has drawn some 4,000 chemists and related scientists from 45 countries bordering the Pacific Ocean.
Interest in the role of cooking in producing carcinogens emerged 20 years ago when it was found that charcoal broiling could place a powerful cancer-causing substance called benzopyrene in meat. This happens when fat drips onto the coals and is burnt, and the rising smoke adheres to the meat. It was found that a well-done one-pound steak could come to contain as much of the carcinogen as 300 cigarettes.
The new findings implicate a different process, one that affects any kind of meat as long as it is exposed to temperatures high enough to produce the chemical change. This includes most methods of frying, broiling and some methods of baking. Boiling or slow-cooking at low temperatures produce little or none of the chemical changes.
Shigeaki Sato of Japan's National Cancer Center Research Institute in Tokyo reported that, when large doses of the substances were fed to mice, they developed liver tumors. Rats developed cancer of the small intestine and colon. Sato noted that, although the animal doses were 5,000 times higher than those consumed by most people, humans are exposed to many kinds of carcinogens and that the total dose could be significant.
While studying carcinogens in meat, Michael W. Pariza of the University of Wisconsin said he happened upon substances extracted from beef that have the opposite effect.
"This was a real surprise," Pariza said, "especially because the substances appear to be present in rather high amounts in meat."
Pariza's findings are predominantly about beef in the form of fried hamburger but he said there is evidence that the anti-cancer substances also exist in pork and in other meats.
Proof of the beef extract's abilities emerged from experiments in which the skin of mice was painted with a known cause of skin tumors, a hydrocarbon called DMBA (dimethylbenzanthracene). After 12 to 14 weeks, each of the treated mice developed an average of 12 skin tumors.
But when the mice were painted with the beef extract minutes before receiving DMBA, only 70 percent of the mice developed tumors and the number of tumors averaged only four. The amount of extract used on each mouse was less than the quantity contained in one ounce of hamburger.
Pariza said that, although he had not yet fed mice to see whether the extract still worked when ingested, "I'd be very surprised if it didn't."
The active substance in the extract has not yet been isolated but it is known to be present in uncooked beef and to survive cooking.
Pariza said it was believed to work by interfering with enzymes in cells that act on carcinogens. DMBA, like all known chemical carcinogens, cannot cause cancer directly. It must first be "activated," or altered chemically by enzymes in the cell that are supposed to detoxify foreign substances. In the case of carcinogens, this process yields a chemical that can trigger cancer. Somehow, the beef extract blocks activation.
Despite the good news from the laboratory about meat, scientists here heard bad news from an epidemiological study of cancer victims and what they eat.
"The consuming of meats," said James R. Marshall of the State University of New York at Buffalo, "is associated with a significant increase in gastrointestinal cancer risk." He said the risk may be associated with an unusually heavy reliance on frying. Charcoal broiling did not appear to raise the risk above that of any other form of cooking.
Marshall's study compared the dietary habits of more than 1,600 gastrointestinal-cancer patients with those of nearly 2,000 other people of similar age, sex, and socioeconomic class.
In addition to the meat link, Marshal found a number of significant dietary factors. For example, people who added salt to their food were twice as likely to have cancer.
Higher cancer risks were also associated with adding pepper to food during cooking but not after and with the use of butter, vegetable oil, shortening and especially bacon grease.
Marshall said lower cancer rates were found among those who tended to eat vegetables raw and among those who usually ate fruits fresh rather than canned or frozen.