Consumers can't be blamed if they feel confused or even frustrated by the latest diet news, doctors say.
Today, for example, two studies show that vitamins E, C and selenium don't prevent cancer, even though earlier studies had suggested they might protect people from the disease.
Researcher J. Michael Gaziano says he's used to hearing from patients — including family members — who struggle to keep up with the latest research.
"Even my 93-year-old grandmother calls me to say, 'One day you guys say to eat oatmeal, and the next day to say don't bother,'" says Gaziano, who was involved in the new studies and works as a doctor at the VA Boston and Brigham and Women's Hospital.
Gaziano and other researchers offered to answer questions about diet research.
Q. Why do the new studies contradict the findings of earlier research?
A. Many of those earlier studies were preliminary. That means they usually can't prove cause and effect, Gaziano says.
Scientists are familiar with these limitations.
"Conflicting results are part and parcel of the scientific process," Gaziano says. "It's why we're often reluctant to make a clinical recommendation based on a single result."
But consumers — who pay much closer attention to diet news than to other areas of medicine — can easily get confused if they mistake these early findings for solid results, says Peter Gann of the University of Illinois at Chicago, who wrote an editorial accompanying Gaziano's studies in The Journal of the American Medical Association.
Q. So which studies should we pay attention to?
A. The strongest studies are called randomized clinical trials.
These are the best experiments for proving that one thing causes another, Gaziano says.
In these tests, researchers randomly assign one group of people to do one thing, such as take a vitamin, but assign a comparison group to do something else, such as take a placebo pill, he says. In a "double-blind trial" — the most rigorous kind — neither patients nor their immediate doctors know which pills they're taking. Only the researchers running the trial know who's in which group. An outside panel oversees the study to make sure that patients aren't harmed and may stop the study early if problems surface.
Q. Are all diet studies randomized trials?
A. No. There are many other kinds of studies, Gaziano says.
•Observational studies: In these studies, doctors observe different groups, such as people who choose to take vitamins and those that don't. They follow both groups for many years, then note how each group fares. They might measure which people develop cancer, for example.
This kind of study can find associations, such as the fact that people who take vitamins are less likely to get cancer, Gaziano says. But an observational study can't prove that vitamins actually prevent cancer. That's because there could be something different about people who choose to take vitamins compared to others. For example, people who take vitamins might be more likely to exercise and get regular checkups.
•Subgroup analyses: Doctors sometimes look through their data to see if there are other interesting trends, Gaziano says. These findings also can be misleading, though, because each subgroup may be so small that any trends could be caused by chance.
•Laboratory studies: Studies in animals or cells are interesting, but doctors don't usually base their advice to patients on these tests, Gaziano says. Instead, scientists use these tests to decide whether to do more definitive studies.
Q. What's the best way to research a health study?
A. Because all drugs — including vitamins and herbs — have side effects, it's worth taking the time to do some research putting them in your body, Gaziano says.
Gaziano suggests that consumers read news stories carefully to assess how doctors got their results. If a story isn't clear, patients should consider looking up the original article in a medical journal or asking their doctors.
Patients can also consult the websites of major medical organizations, such as the American Cancer Society,the Centers for Disease Control and Prevention or the National Cancer Institute. These groups sift through scientific evidence when making recommendations about diet, screening tests and other health issues.
Q. Why is there so much confusion about diet news?
A. Everyone likes good news, Gaziano says. So it's tempting for everyone — from researchers who want to further their careers to journalists looking for a scoop — to overplay preliminary results. Consumers are often quick to put too much faith in the latest "advance" out of wishful thinking. Lots of people, for example, would rather take a pill than go through the hard work of diet and exercise.
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