Diet Studies Are Almost Never the Last Word
Think sticking to a diet is hard? Try studying one. Everyone is interested in whether different foods or nutrients affect our odds of getting diseases like cancer or of developing risk factors for those diseases, such as too much weight or high blood pressure. But there are many barriers to studying dietary change, which is why we still have no easy answers to the question of what, exactly, we should eat to be at our healthiest. It's also why you can be forgiven for often feeling whipsawed by headlines: Is coffee good or bad? What about alcohol, garlic, or chocolate?
This week researchers reported in the Journal of the American Medical Association that breast cancer survivors who cram their diets with fruits and vegetables are no more likely to escape a recurrence than women who stick to the usual five-a-day recommendation. Does that mean fruits and vegetables don't protect against cancer? No—just that in this specific group of women with breast cancer, the extra greens and additional apples didn't seem to help.
We asked researchers to explain why studies involving dietary changes are so hard to do—and what consumers should keep in mind when they read about them. Here's what the experts said:
You can't lock people in a room. Most diet studies take place in the real world. That means study subjects are keeping diaries of what they eat as they go rather than having their intake strictly controlled by someone else. You can give them meal advice, counseling, and how-to books up to their ears, but at the end of the day, they are on their own when it comes to what they put in their mouths. It's easier to get people to add something—like garlic, in the form of tasty sandwich spreads, or dark chocolate—than to take something away; no wonder a recent study comparing low-fat and low-carb diet plans found that almost no one was sticking to them by the end.
People lie about what they eat. In studies focusing on diet, including the recent study on breast cancer recurrence, the amount of calories subjects reported eating would have caused them to lose far more weight than they actually did lose. The misreporting isn't necessarily vicious, but the inaccuracies add up. Say you're phoned about your daily intake on a day when it was someone's birthday at work and you had a slice of cake. You may not report it, thinking that a typical day wouldn't include the cake...forgetting yesterday's "special occasion" piece of pizza, and the Big Gulp of the day before. Or, despite the portion size guides you get, you characterize your bagel from the deli as a 4-ounce standard serving when a 4-ounce bagel hasn't been sighted in any major city for a decade.
...so it's hard to verify that they're sticking to the plan. "You can't put a camera in everyone's belly and see exactly what they ate," says Christopher Gardner, a nutrition scientist at the Stanford Prevention Research Center who has recently published research on garlic and diet plans. You can get around this in some studies by taking objective measurements. Weight, for example, or if you're assessing intake of fruits and veggies, you can measure the level of pigments called carotenoids in the blood. In the breast cancer study, blood tests showed that the study subjects actually did eat more fruits and veggies (carotenoid concentration was 73 percent higher in those women after one year and 43 percent higher after four years). But objective measures can't definitively nail down whether someone is eating nutrients in certain proportions.
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."