Americans consume 10,000 times more cancer-causing chemicals naturally in their daily diet--in items ranging from alfalfa sprouts to meat to their breakfast toast--than from man-made pesticides, according to an article to be published today in Science magazine.

Scientists also have found that foods naturally contain a range of cancer-preventing chemicals, or "anti-carcinogens," according to Bruce N. Ames, a leading cancer specialist and chairman of the Department of Biochemistry at the University of California at Berkeley. Ames wrote the Science article.

Because scientists soon will be able to identify both the cancer-causers and the cancer-preventers in the human diet and will try to bring them into balance, an era is near in which people will be able to "fine-tune their diets" to avoid many major causes of cancer, Ames said. His article surveyed more than 179 recent studies of diet and cancer.

The leading known cause of cancer death is from the smoking of tobacco, he said, which accounts for about 30 percent of the approximately 350,000 cancer deaths in the United States annually.

Diet is believed to be the second major cause of cancer, with rates varying around the world, according to the local diet. People who migrate from one area to another tend to adopt the local diet and be subject to the local cancer rates, which may be higher for some types of cancer and lower for others.

Because of this variation apparently related to diet, Ames said, there is "hope that each major type of cancer may be largely avoidable."

In trying to sort out what it is in the diet that causes or prevents cancer, Ames writes: "Laboratory studies of natural food . . . and cooked food are beginning to uncover an extraordinary variety of mutagens chemicals that cause changes in cells and possible carcinogens and anti-carcinogens."

Though scientists have just begun to discover the natural cancer-causers and preventers in food, Ames lists some candidates:

* Fats such as those in meat, butter, milk and other foods may be a major problem because they can break down chemically in the body to create "free radicals"--chemicals with an extra electron and therefore an extra electrical charge that can disrupt chemical action in a cell.

* A variety of vegetables contain what Ames calls "natural pesticides" made by plants to protect against insects, fungi and animals. Ames gives 17 examples of vegetable groups with possibly dangerous chemicals. Included are alfalfa sprouts, oil of sassafras, mushrooms, celery, potatoes, rhubarb and cocoa.

* Burned and browned foods, including everything from caramelized sugar to toast, "contain a large variety of DNA-damaging agents and presumptive carcinogens," Ames said. Smokers, who consume burned tobacco, "have more easily detectable levels of mutagens in their urine than nonsmokers, but so do people who have consumed a meal of fried pork or bacon."

* Substances that appear to have some anti-cancer effects include vitamin C, vitamin E, a rare and toxic mineral called selenium, and beta-carotene, a chemical found in carrots and most green vegetables.

Ames said that a dietary-caused cancer could result from the lack of an anti-carcinogen in the diet rather than the presence of a cancer-causer.

"My point is not to single out some plants or foods, but to give examples of a whole new area to study," he said.

In his report, Ames said: "The 16 examples, plus coffee, illustrate that the human dietary intake of 'nature's pesticides' is likely to be several grams per day, probably at least 10,000 times higher than the dietary intake of man-made pesticides.

"The public has been focusing on man-made chemicals in recent years, when the reality is that natural hazards are enormously more prevalent than the man-made ones."

In a telephone interview, Ames said he is "very optimistic" because researchers are starting to have the tools not only to look at carcinogens but also to begin to separate the minor ones from the major ones.

"I think there will be a time in 10 to 20 years when we will be testing people to find out the levels of these chemicals in their diets, just as easily as we take blood tests now," he said.

Ames, who developed the chief test used as a first screen for cancer-causing chemicals, reviewed in his article new information researchers are finding about cancer and diet.

He wrote that "there are large numbers of mutagens and carcinogens in every meal, all perfectly natural and traditional. Nature is not benign. It should be emphasized that no human diet can be entirely free of mutagens and carcinogens, and the foods mentioned are only representative examples."

Identifying a food as mutagenic or carcinogenic is only the first step, Ames said. It then must be learned if the food poses a major risk.

He added that, apart from lung cancer caused by smoking, the rate of cancer has not changed significantly in the past few decades, despite an increase in the use of man-made chemicals. He said that exposure to these sources has caused cancers, but apparently not yet enough to affect the cancer death rate.