The study of more than 18,000 men found that people with detectable amounts of chemicals known as isothiocyanates in their bodies had a 36% lower chance of developing lung cancer over 10 years than those without the chemicals. The chemicals are found in broccoli and other so called "cruciferous" vegetables.
Researchers from the National Institute of Environmental Health Sciences (NIEHS) in North Carolina along with colleagues in China performed the study in four small communities in Shanghai. That site is notable because of its population's high smoking rate and high level of cruciferous vegetable consumption, according to Dr. Stephanie J. London, an epidemiologist and the study's lead investigator.
The researchers recorded 259 cases of lung cancer during the study's follow-up period. The lowered cancer risk associated with isothiocyanate levels held up even after influences like smoking were factored out.
Patients' genetic makeup also significantly affected how much protection they got from isothiocyanates. Those who lacked genes that metabolize the chemicals--about 60% of the total study population--had more protection than those who carried the genes.
The study is the first to link biological measures of isothiocyanates with a decrease in cancer risk. Other studies have relied on dietary questionnaires that give no information as to how much of the chemicals actually are circulating in subjects' bodies.
While the results suggest that eating broccoli and related vegetables can lower cancer risk even for smokers, no one should assume that isothiocyanates would be enough to protect smokers from cancer, London warned.
Though the chemicals did lower cancer risk by 36% in this study, smoking alone increases lung cancer risk by as much as 10 times.
"The (cancer-causing) effect of smoking is so big, that it is not going to even come close to wiping out the smoking effect," she told Reuters Health in an interview.
Researchers think that isothiocyanates fight cancer by promoting the production of antioxidants and by inhibiting enzymes that allow carcinogens in cigarette smoke to damage DNA.
Isothiocyanates are not commercially available in pill form. But even if they were, researchers have no way of knowing how the more than 20 different isothiocyanates interact with each other and with the body to lower cancer risk.
Previous studies with pill forms of cancer-fighting chemicals--including beta-carotene--have found that doses higher than those found in foods may actually increase cancer risk.
"The epidemiologist lesson from that is 'just eat the vegetables,'" London said.
The study was published in the August 26th issue of The Lancet.