Despite 30 years of searching, scientists have not been able to nail down a clear link between human cancer prevention and consumption of broccoli, or any of the vegetables in the same family, says Dr. Walter Willett, chairman of the Department of Nutrition at Harvard School of Public Health.
But in the test tube, tantalizing clues keep emerging. In December, University of California, Berkeley toxicologist Leonard Bjeldanes and molecular biologist Gary Firestone reported in the Proceedings of the National Academy of Sciences how one component of broccoli, a compound called indole-3-carbinol (I3C), can slow cell proliferation, a hallmark of cancer.
In mice, the Berkeley researchers say, I3C was found to stop the growth of breast and prostate cancer cells, and the substance is now in clinical trials in humans. But Willett cautions, "We really can't connect these new findings from subcellular mechanisms to any effects of broccoli consumption in humans."
And even if I3C, which is available as a dietary supplement, does slow cancer growth, it's unclear how a person would take it. Stomach enzymes transform it into other chemicals, says Bjeldanes; research suggests injections might be more effective.
Karen Collins, nutrition adviser to the American Institute for Cancer Research, cautioned that "cancer patients should not start taking supplemental versions of I3C without full discussion with their doctors because we don't yet have clinical data about all effects possible."
Even if broccoli and similar cruciferous vegetables aren't a magic bullet for cancer prevention, says Willett, they "are excellent sources of many essential nutrients and good to include in our diets."
For what it's worth, Bjeldanes is convinced enough of the value of broccoli-type vegetables that he makes sure to eat one serving a day of them.