WASHINGTON (AP) - When it comes to fighting cancer, not all broccoli is created equal.

Broccoli contains a compound, glucoraphanin, believed to aid in preventing some types of cancer. But the Agriculture Department studied 71 types of broccoli plants and found a 30-fold difference in the amounts of glucoraphanin. Some had virtually none of it.

The varieties of broccoli typically sold in supermarkets do not vary that much, but they do differ. Other fruits and vegetables, including tomatoes, garlic and carrots, also contain varying amounts of cancer-fighting chemicals, a fact many consumers probably do not realize.

Tomatoes offer lycopene, an antioxidant that attacks roaming oxygen molecules, known as free radicals, that are suspected of triggering cancer. The hotter the weather, the more lycopene tomatoes produce. Processing increases lycopene levels further.

"Nearly everything is highly variable in plants," said Paul Talalay, a specialist in cancer prevention at the Johns Hopkins University medical school. "You take the common experience of people who deal with marijuana or opium. They know you have to get the right strain" to get the desired effect.

The Agriculture Department hopes the broccoli research will lead to new hybrids richer in glucoraphanin.

"It's exciting to think about the possibility of identifying specific varieties that may have more cancer protection than others," said nutritionist Melanie Polk of the American Institute for Cancer Research.

"For right now, the bottom line is to eat your broccoli, but don't single it out as the only vegetable you eat."

The National Cancer Institute recommends eating at least five servings of fruits and vegetables daily.

Studies suggest that people who eat broccoli have a lower incidence of colon and rectal cancer. Sulforaphane, a product of the glucoraphanin in broccoli, induces the production of certain enzymes that can deactivate free radicals and carcinogens. The enzymes have been shown to inhibit the growth of tumors in laboratory animals.

The Agriculture Department research, published recently in the Journal of the American Society for Horticultural Science, identified high-glucoraphanin strains of broccoli that could be used in producing new hybrids.

"There's no question that we as plant breeders and geneticists can alter the levels of these compounds and have a beneficial effect," said Mark Farnham, who conducted the broccoli research at a department laboratory in Charleston, South Carolina.

But scientists face several challenges, just as they do in developing better cancer fighters among other vegetables. For one, it is not known how much of the various phytochemicals people need or whether too much could be harmful.

It also is difficult to breed new hybrids that do not lose the good attributes of existing varieties. Glucoraphanin is related to another chemical compound that causes the bitter taste of brussels sprouts, a cousin of broccoli. Bitter-tasting broccoli likely would not sell, even if it were better at fighting cancer.

Genetic engineering, which allows scientists to develop plants with specific traits, would make it easier to create new hybrids, but the technology increasingly has become controversial.

Talalay and other Johns Hopkins scientists developed broccoli sprouts that guarantee a consistent level of sulforaphane, as much as 20 times higher than the levels found in mature heads of broccoli. The sprouts are available in some stores under the trade name BroccoSprouts.

Copyright 2001 The Associated Press. All rights reserved.