Can a diet rich in a particular nutrient really prevent cancer? The government is recruiting 32,000 middle-aged men to see if selenium or vitamin E can prevent prostate cancer, the biggest clinical trial yet to address such dietary questions.
It's just a first step toward what could become a major change in nutrition: Preliminary but intriguing genetic research suggests certain nutrients may prove more cancer-protective for one person than the next - suggesting that one day doctors might write prescriptions for diets to prevent tumors in certain people.
"The future is tailored recommendations," John Milner of the National Cancer Institute says about this fledgling new science, "nutrigenomics." "That's the excitement."
Cancer doesn't just arise overnight. A few tiny cells gone wrong slowly grow over decades. Whether the result is a life-threatening tumor depends on genes and environment - including food.
Up to 35 percent of cancers are related to dietary habits, says Milner, chief of NCI's research into nutrition and cancer prevention.
That doesn't mean an occasional cheeseburger or doughnut is dooming. But study after study links lifelong diets high in plant foods to lower cancer rates.
Also, people who eat lots of fruits and vegetables generally are skinnier. Obesity increases risks of cancers of the uterus, gallbladder and possibly colon and prostate, while a large weight gain after reaching adulthood is linked to breast cancer.
Armed with such provocative evidence, scientists now are trying to tease out which of the myriad nutrients and chemicals in different foods are most protective - and why, at a genetic level, they do the job.
It's exceedingly complex research. Not everybody gets equal benefit from nutrient-rich diets, a discrepancy that probably points to genetic variability. For example, scientists studying lung cancer rates in part of China found people with the lowest cancer risks also were genetically deficient in an enzyme that metabolizes certain nutrients in cruciferous vegetables.
In other words, those lucky people's genes seemed to make broccoli better for them.
Similar links to cancer are being explored with genes that metabolize alcohol, folate from grains and other food chemicals.
"In five years, we'll have a lot of information on how your gene profiles influence your response" to different foods, Milner predicts.
But first, scientists need hard proof of which of the many nutrients commonly considered protective truly are, and at what levels. Small studies promoting 12 cups a day of tea or three whole garlic cloves daily aren't too practical for many people.
Until now, most food and cancer research has focused on animals or merely monitoring people's diets and their later health, which gives only clues, not proof. Plus, too much of some nutrients can be dangerous.
Top of the federal research list: selenium, a trace element found in grains and meat. Previous studies suggest that eating 200 micrograms of selenium a day, about twice the national average, might lower the risk of prostate, lung and colorectal cancer, perhaps by slowing abnormal cell growth or activating tumor suppressor genes.
To prove the prostate benefit, NCI is recruiting 32,400 healthy men in their 50s to take for the next seven years either selenium; 400 milligrams of vitamin E, another nutrient linked to lower prostate risk; both; or a dummy pill.
Too much is toxic, so don't pop lots of selenium supplements, cautions NCI researcher Cindy Davis.
Lycopene, the chemical that makes tomatoes and watermelon red, is another top prospect. Cooking tomatoes with a little oil - think spaghetti sauce - significantly increases lycopene absorption. In one study, it decreased prostate cancer by 35 percent. The NCI has begun small clinical trials to find lycopene's maximum safe dose and see if giving it to prostate cancer patients before surgery helps stem their disease.
Despite lots of hype, research is much more mixed on other foods. Soy, for instance, is widely touted as protective against breast cancer, but women seem to get the benefit only if they eat soy before puberty, says NCI researcher Harold Seifried.
It will take years to sort out what are truly anticancer diets. For now, the American Cancer Society's best advice: Eat a wide variety of foods, including at least five servings of fruits and vegetables a day, and slim down.
By Lauran Neergaard
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