Doctors aren't writing grocery lists on prescription blanks, and no one is advising: "Take two cabbages and call me in the morning." But a major effort is now under way in the United States--with tantalizing early results--to determine whether if certain foods can protect us from cancer.
Already a variety of fruits and vegetables and hundreds of chemicals in them have been identified as possible shields against various kinds of cancer:
Vitamins A, C and E.
Beta carotene, the coloring agent in carrots, and other carotenoids.
Selenium and calcium.
Flavonoids, indoles, coumarins and a lot of other multisyllabic chemicals found in plants.
"I liken this to where we were with cigarettes and lung cancer 20 years ago. We didn't know for sure, but there was a lot of circumstantial evidence," says Dr. Sherman Gorbach, cancer researcher and professor of community health and medicine at Tufts University School of Medicine.
"The preponderance of evidence, in cancer, favors a change in diet," he says. In Japan, for instance, where the breast cancer rate is low, women eat twice the fiber we do and have only 22% or 23% of their calories in fat.
"If you are really serious about preventing breast cancer," Gorbach advises, "you have to reduce fats below 25% of calories and increase your fiber."
He says that the fiber should come from soybeans and rye, which have components that block the cancer-causing effect of estrogen on breast cells; from crucifers such as cabbage, cauliflower and broccoli, which most experts believe may have protective qualities; and from fruits and vegetables high in beta-carotene and Vitamin C.
"We don't know which ones are best, so we recommend mixed fiber sources," he says.
Even so, he is more definite than many others.
At the National Cancer Institute's Diet and Cancer branch, the attitude is also wait-and-see. There, Herbert Pierson, Ph.D., heads a study of the cancer-fighting ability of garlic, citrus, licorice root, flaxseed and "umbelliferous" vegetables such as carrots, parsnips and parsley--so-called because they have umbrella-shaped flowers.
His project aims ultimately at collaboration with food manufacturers in the development of "designer foods," processed foods enhanced with protective chemicals.
For the present, however, Pierson is cautious:
"The epidemiological data suggest that adding fruits, vegetables, grains and spices to the diet has a beneficial effect," he says. "But most investigators don't want to commit themselves until the hard scientific data are all in place."
There is reason for the wariness. So far, the evidence comes from population studies, like the fat-and-fiber data from Japan, or the study of garlic consumption in China that shows less stomach cancer in places where people eat a lot of garlic.
Whether that information really establishes cause and effect and whether it can be applied in the United States--where food habits, lifestyle, environmental conditions and ethnic make-up are different--is still unknown.
Animal studies are similarly inconclusive. The effect of massive doses of suspected anti-carcinogens in short-lived mice may be entirely different from the effect of smaller doses in long-lived humans.
Some of the pathways by which the presumed anti-cancer foodstuffs work have, however, been decoded. It's believed that one group of protective factors, known as anti-oxidants, clears away potentially harmful waste products. Others prevent carcinogens from attaching to particular kinds of cells. And still others make the body produce extra quantities of powerful enzymes that deactivate certain carcinogens.
But it is possible that those extra enzymes could also increase the effect of other carcinogens. Even the much-touted crucifers--vegetables like cabbage, cauliflower, broccoli and bok choy--could, therefore, have a downside, because they contain factors with the enzyme-boosting effect.
Moreover, major dietary changes, aimed at cancer prevention, could affect other areas of health and nutrition.
A high-fiber diet, for example, is widely recommended because its indigestible bulk is thought to sweep wastes out fast enough to prevent carcinogenic contamination of the colon. Fiber also might sweep away some natural estrogenic compounds believed to be associated with initiation or spread of breast cancer, Gorbach says.
But fiber in the gut also tangles with dietary minerals, preventing absorption of necessary nutrients.
And while low-fat is desirable, no-fat is dangerous; the body needs some for normal function. Besides, fat is the dietary source of vitamins A and E, both of which are in the protective category because of their anti-oxidant activity.
Nevertheless, the data supporting the probability of food-borne cancer fighters has been mounting for decades, and has encouraged increasingly sophisticated research.