Diet Studies Are Almost Never the Last Word
You can't tell study subjects to eat badly. "We couldn't encourage women to eat less than five servings of fruit and vegetables a day," says John Pierce, first author of the study out this week on the link between lots of fruits and veggies and breast cancer recurrence. One, it isn't ethical, since the foods are healthy for other reasons, and two, no one would be in the study if they thought it might hurt them. So instead, he had to look at whether eating more than the five-a-day recommendation added any benefit. In this particular population—that is, women on the average in their early 50s previously treated for early-stage breast cancer who were already eating more healthfully than the typical American—it did not. Note: That does not suggest that fruits and veggies are useless for preventing the recurrence or incidence of breast cancer in everyone.
It's hard to separate out the effects of food and weight. In this week's study, the women were supposed to aim for lower fat intake in addition to eating more fruits and vegetables; while the intervention group did eat less fat then the control group, both groups were getting more of their daily energy from fat after six years than when they started. They didn't lose weight, either. So if they'd done that, would there have been a survival benefit? A study last year found that breast cancer survivors who stuck to a low-fat diet saw a 24 percent lower chance of a recurrence than those who did not, but it wasn't clear whether the drop in fat consumption or the resulting 6-pound average weight loss was responsible. If you're a breast cancer survivor, the distinction doesn't much matter: Slashing fat can help you lose weight and cut your chance of the cancer coming back. Let the researchers worry about exactly what the mechanism is.
Food is complicated—but we don't need to totally understand the interactions to act. Rather than trying to isolate the effect of particular parts of a diet, maybe it's better to recommend entire food patterns such as the Asian or Mediterranean diets, says Gardner. If the Asian diet is healthier because it contains soy, does it matter if it's because there's something protective in soy or because soy milk displaces dairy milk? And if a study shows that the Mediterranean diet—heavy on fruits, veggies, "good" fat like olive oil, and lean sources of protein—cuts the risk of a second heart attack by half, might it not be better to adopt the diet and get the full effects of the ingredients' interactions than to emphasize one or two individual components? "We may need to look at the bigger picture," says Susan Gapstur, an epidemiologist and expert on cancer prevention at Northwestern University. "Perhaps the more important questions are to ask about the full lifestyle."