Or why you should put down that raw carrot and steam it instead.

Raw-food diets have been popular in alternative-health circles for decades. A Swiss regimen dating from 1964, called Instinct Therapy, recommended uncooked food as a cure for cancer. Its originator was forced to leave Switzerland--it seems that another of his regimens, which he called Metasexuality, had gotten him into legal trouble--but the influence of Instinct Therapy flourished. Today, alternative health clinics in Florida and California prescribe raw foods, while such restaurants as Raw Experience Living Foods in San Francisco and Maui serve up uncooked meals.

There's even a cookbook--Delights of the Garden: Vegetarian Cuisine Prepared Without Heat by Imar Hutchins (Doubleday Main Street). To make a pizza crust, it advises, grind almonds and pumpkin seeds and mix them with sun-dried tomatoes. For a topping, blend rehydrated sun-dried and fresh tomatoes, olives, onions, miso, zucchini and yellow squash.

Maybe it tastes good--I'll never know. But is it better for you?

As it turns out, no. In fact, cooking actually enhances the nutritional benefits of some vegetables, like carrots and tomatoes, and makes compounds that fight disease more effective. Even when cooking lowers nutrient levels (as it does with the vitamin C and folic acid in spinach), it doesn't deplete them. Cooked broccoli, for instance, has about 40 percent less sulforaphane (an anticancer compound) than raw. But because cooked broccoli tastes better than raw, you're likely to eat a lot more of it.

Boosting Nutrients

Some cancer-fighting compounds are liberated by cooking--for example, lycopene, a substance in the carotenoid family that turns tomatoes red. Studies have linked lycopene to lowered rates of heart disease, prostate cancer and colon cancer and have hinted that it might help ward off breast cancer. The compound is locked up in plant-cell walls; cooking breaks down those walls. And it's fat-soluble, which means it's more easily absorbed by the body when consumed with fat. Adding olive oil to cooked tomatoes, in other words, makes them more healthful. A 1995 Harvard University study found that men who ate at least 10 servings of cooked tomatoes, tomato sauce or pizza per week reduced their risk of prostate cancer almost by half.

Beta-carotene, the carotenoid in carrots, also seems to fight disease more effectively in its cooked form. "I don't want to discourage people from eating raw carrots--they're certainly better than potato chips," says John Erdman, director of the division of nutritional sciences at the University of Illinois, Urbana-Champaign. "But even steaming carrots for a short time virtually doubles the amount of beta- carotene the body can absorb."

Another such compound is quercetin, which gives yellow and red onions their color and can also be found in red grapes, apples, crookneck squash and Brussels sprouts. "It's a potent cancer fighter--stronger than vitamin E or C," says Terrance Leighton, a professor of biochemistry at the University of California at Berkeley. In raw vegetables, quercetin is bound to sugar molecules; cooking breaks those bonds. Acids also liberate quercetin. "People in Asia eat a lot of onions and other vegetables, but they don't often eat them raw--they pickle them," Leighton says. "That may help explain why Asians have lower cancer rates."

Protecting Nutrients

When cooking will destroy nutrients, certain strategies can keep those losses to a minimum. According to Barbara Klein, a professor of food science and nutrition at the University of Illinois, Urbana-Champaign, "If you bring a big pot of water to a boil and throw the vegetables in, then throw the water out afterward, you'll lose a lot more vitamin C than if you boil the vegetables in a small amount of water." Quick-cooking techniques, such as steaming and stir-frying, also minimize the loss of nutrients. Or make soup--almost all the nutrients that leach out of the vegetables will stay in the water.

ROBERT A. BARNETT wrote about food and health in Tonics (HarperPerennial).