George Bush may be a foe of broccoli, but the green leafy vegetable is fast becoming the haute cuisine of nutritional science.

"Think green," says Frederick Khachik, a research chemist at the U.S. Department of Agriculture's Human Research Laboratories in Beltsville. "Eat the greenest vegetables you can find."

In a new five-year program, the National Cancer Institute is spending $20.5 million to identify potential anti-cancer substances in common foods. High on the list is the cruciferous family of vegetables -- broccoli, cabbage, brussels sprouts and kale -- along with other greens such as parsley and spinach.

Khachik and his colleagues have detected concentrations of suspected anti-cancer compounds called carotenoids in about 40 green vegetables. "The darker green a vegetable, the more carotenoids it has," he says. Researchers also find that run-of-the-mill nutrients like vitamin C are concentrated in green leafy vegetables. In fact, a cup of broccoli contains more vitamin C than an orange.

This is not to say that other vegetables -- carrots, onions, tomatoes, turnips and potatoes -- do not have health virtues. They do. Population studies show that vegetarians generally have lower rates of cancer, heart disease and certain other chronic diseases than do meat eaters.

A 1989 National Academy of Sciences/National Research Council report, "Diet and Health," recommends eating at least five servings of fruits and vegetables a day of any type.

But broccoli chic seems to have taken over both in the kitchen and the laboratory as scientists probe the special benefits of green vegetables. As Tak Yee Aw, assistant professor of biochemistry at Emory University School of Medicine, says, "It's like your mother always said, 'Eat your greens to stay healthy.' Now we're beginning to understand scientifically why that is true." Greens vs. Radicals

For three decades, broccoli, along with other greens, has been linked to lower risks of cancer, notably of the stomach, lungs and colon.

Dean P. Jones, associate professor of biochemistry at Emory University School of Medicine, speculates that broccoli and other greens may reduce the risk of cancer by protecting cells against those ubiquitous renegade molecules called oxygen-free radicals. These molecules go on rampages, roaming the body, literally ripping cells apart and tinkering with their genetic material. Scientists theorize that these free radicals may help foster about 60 different diseases.

These destructive agents can be countered by other compounds called antioxidants, which prevent the oxygen-free radicals from binding to cells in the body. These protective chemicals range from vitamin C to beta carotene, one of the most potent carotenoids.

Another antioxidant compound that fascinates Jones is glutathione, which appears to be a major player in the body's detoxification process that helps neutralize cancer-causing poisons. According to Jones, glutathione is remarkably potent and in laboratory experiments can deactivate at least 30 different cancer-causing agents.

Topping the list of about 100 foods analyzed so far for glutathione content is the broccoli floret -- the flowerlike tip of the stalk. Parsley and spinach run a close second. All three greens possess two to three times more glutathione than oranges, green peppers, tomatoes and even the cruciferous cauliflower.

In addition,Terrance Leighton, professor of biochemistry at the University of California at Berkeley, has discovered another antioxidant in broccoli -- the molecule called quercetin, which he says may help inhibit the rapid growth of cancer cells in the body.

More recently, scientists have grown excited about the compound, indole, which is also found in cruciferous vegetables. In a recent issue of the Journal of the National Cancer Institute, researchers at the Institute for Hormone Research in New York City for the first time described how indole may have a protective effect against some forms of breast cancer.

Scientists know that certain types of breast cancer are sensitive to an excess of the female hormone estrogen. However, in laboratory tests of human patients, Jon Michnovicz of the Institute of Hormone Research found that moderate doses of a cruciferous indole actually accelerated the metabolic process by which the body deactivates the estrogen linked to breast cancer. Results of these experiments showed that the broccoli compound "turned up" the estrogen deactivation process by about 50 percent, says Michnovicz.

The combination of all these antioxidant compounds is thought to enhance the disease-fighting potential of green vegetables. At the same time, cancer specialists caution that eating broccoli or any vegetable is hardly a cure for cancer. The new laboratory findings on the different chemicals in greens are important clues to why a vegetable-rich diet is linked to lower rates of disease. Vitamin C Power

One powerful antioxidant is the old health-promoting standby: vitamin C, or ascorbic acid. It's not just that vitamin C may help prevent certain cancers, notably of the stomach. Scientists now are looking at a possible link between vitamin C-rich foods and heart disease. In test-tube experiments, Balz Frei, associate professor of biochemistry at the University of California at Berkeley, has found that vitamin C is able to destroy 100 percent of water-soluble free radical compounds that circulate in the blood. "It's remarkable and exciting," he says, "as are the implications for cardiovascular disease."

That's because a new theory of how arteries become clogged suggests that those same oxygen free radical-type compounds that are implicated in the development of cancer may contribute to the buildup of fatty deposits in blood vessels -- the first sign of heart disease. Scientists speculate that the LDL (low-density lipoprotein) or so-called "bad" cholesterol -- may not be particularly harmful to arteries unless it comes into contact with oxygen-free radicals, which oxidize the LDLs and make them toxic. Antioxidants, such as vitamin C, can help combat the oxidation process so that LDL cannot be as readily sucked into artery walls and transformed into plaque that clogs arteries, Frei says.

Although the artery-protecting power of vitamins C and E has not been proven in humans, it has been demonstrated in primates. In six years of experiments with monkeys, Anthony Verlangieri, professor of pharmacology at the University of Mississippi School of Pharmacy, has shown that vitamin C can prevent and even reverse clogging of arteries. In these studies, the monkeys were fed lard and cholesterol to give them heart disease. Some of the animals also received vitamins C and E. Results showed that the progression of artery damage was slowed and in some cases reversed in the animals given the two vitamins. Overall, the rate of heart disease was cut by 50 percent in the group treated with vitamins.

Meanwhile, a new USDA analysis found that broccoli has about 20 percent more vitamin C than previously believed. For example, 3 1/2 raw ounces have an average 112 milligrams of vitamin C -- not 93 milligrams as previously reported. But some parts of broccoli are richer than others. The floret has about 60 percent more vitamin C than the stalk.

Other green vegetables high in vitamin C are green bell peppers, kale, cabbage (including bok choy), mustard greens, Brussels sprouts, turnip greens and asparagus. Beyond Broccoli Chic

Not all the nutritional glory goes to broccoli. USDA's Khachik notes that some leafy greens possess more cancer-fighting carotenoids than does broccoli. In a USDA analysis of total carotenoids, raw kale was the superstar, with 78 milligrams per 3 1/2 ounces raw, compared with 36 mg in spinach and 4 mg in raw broccoli and Brussels sprouts.

Other scientific surprises come from collard greens. Several years ago, scientists at North Carolina's Research Triangle were testing certain vegetables to determine their anti-mutagenic activity -- how well they protected a cell's genetic material from damage by cancer-causing agents. They were surprised to note that collard greens had substantial anti-mutagenic properties, which may have a protective effect against cancer.

They passed this tip on to Diane Birt at Eppley Cancer Research Center at the University of Nebraska, who tested the greens in animal experiments. She found that feeding collard greens to mice thwarted the spread of implanted breast tumors. So did green cabbage. It was the first animal study to find that such plant compounds could not only block tumor formation but also stop the spread of cancer cells.

Many green vegetables are also rich in calcium, which may help ward off the crippling bone disease osteoporosis. In recent studies, researchers at Creighton and Purdue universities found that women can absorb as much, and sometimes more, calcium from cooked kale than from milk.

T. Colin Campbell, nutritional biochemist at Cornell University, notes that the Chinese rarely suffer from osteoporosis yet generally shun calcium-rich dairy foods and get all their calcium from plants.

However, some greens are better sources than others. Spinach, although it contains lots of calcium, is a useless source, because compounds in spinach called oxalates block the body's ability to absorb calcium. Among the best sources of calcium: mustard greens, kale, parsley, watercress and broccoli.

So your mother was right: eat your vegetables. Yet for all the excitement in the scientific community, there is concern that the greening of the American diet is still limited to a relatively small percentage of Americans.

The number of people who actually eat vegetables on a daily basis is "shocking," says Blossom Patterson of the National Cancer Institute. In recent surveys, she found that fewer than 10 percent of Americans ate three or more vegetables on a given day, as recommended by health authorities. Only 20 percent admitted to letting a crucifer like broccoli touch their lips. And 20 percent said they ate no vegetables at all.

Jean Carper is the author of "The Food Pharmacy" and writes a syndicated column about food and health.

To get the greatest number of potential disease-fighting compounds, eat your greens raw or lightly cooked. In lab tests, the anti-cancer molecule glutathione decreased about 25 percent in green beans boiled for five minutes. Canned vegetables had virtually none. Up to 50 percent of another protective compound, the indoles, were lost in cabbage and Brussels sprouts that were steamed or boiled.

Vitamin C and folic acid are particularly susceptible to destruction by heat. About half the vitamin C is lost by cooking a stalk of broccoli -- the amount of water used makes no difference. Best bet: the microwave. Only 15 percent of broccoli's vitamin C was destroyed by microwaving. And after five days in a refrigerator, about a quarter of fresh broccoli's vitamin C disappears.

On the other hand, cooking does not significantly affect the compound quercetin nor carotenoids, like beta carotene, that may have a protective effect against cancer. In fact, studies in humans have found that beta carotene is best absorbed from foods like broccoli when lightly cooked.