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Two widely used food preservatives boosted levels of a natural cancer fighter in laboratory animals and appear to do the same thing in humans, a researcher says.

Advocates of natural foods have long decried the use of preservatives, but Dr. Andrew Dannenberg of Cornell Medical College found that the preservatives BHA and BHT "revved up" the gene for an enzyme that helps destroy carcinogens before they trigger tumors.When the genes are cranked up, they produce more of the enzyme, providing better protection against cancer-causing substances in the environment, Dannenberg reported at the International Conference on Cancer Prevention at Rockefeller University in New York on Thursday.

BHA and BHT are synthetic additives used as preservatives in cookies, crackers and a wide variety of other foods.

The results do not mean that foods should be pumped up with even more preservatives, he said. The findings are important because they uncover a cancer prevention mechanism that appears to be part of the explanation for the well-known anti-cancer properties of broccoli, cauliflower and brussels sprouts.

"They are amazing vegetables," Dannenberg said. "They have an amazing array of anti-cancer compounds."

His research shows that at least part of the effect of those compounds comes from revving up the same gene affected by BHA and BHT.

The gene produces an enzyme with the mile-long name UDP-glucuronosyltransferase, or UGT. The study found elevated levels of the enzyme in the liver, kidneys and small intestines of rats fed higher doses of BHA and BHT than are normally found in foods, Dannenberg said.

He then found preliminary evidence that the substances do the same thing in humans. Dannenberg said he has also found that sulforaphane, an anti-cancer agent recently isolated in broccoli, exerts its action partly in the same way, by revving up the gene for UGT.

In a separate animal study, Dannenberg found that a widely prescribed anti-ulcer drug called Prilosec also appears to protect against cancer by causing genes to turn up their production of enzymes.

The results appear at a time when researchers have become widely convinced of the value of fruits and vegetables in preventing cancer, but have made little headway at figuring out exactly why that's true.

"I think it's going to be decades before we sort this out," said Dr. Walter Willett of the Harvard School of Public Health, a leading authority on nutrition and cancer. "But this doesn't mean you can't do anything. You can eat more fruits and vegetables."

One of the problems is that many people avoid the foods with the most potent anti-cancer effects, he said. Those include not only broccoli, brussels sprouts and cauliflower - the so-called cruciferous vegetables - but also spinach, kale and collards, which are rich in folic acid, another potent anti-cancer fighter, Willett said.

The National Cancer Institute is conducting more than 20 studies of diet and cancer, many focusing on chemical relatives of vitamin A called retinoids, said Dr. Lee Wattenberg of the University of Minnesota.

Those studies may ultimately show that such feared illnesses as breast cancer are preventable, Wattenberg said.