April 12, 2010
A Look At Fruits, Vegetables, And Cancer
By Michael D. Shaw
With so much confusing and often contradictory information appearing almost daily on how to prevent killer diseases such as cancer, how should we react to a study just published on-line in the Journal of the National Cancer Institute? The widely quoted results of this effort (Boffetta et al.), entitled “Fruit and Vegetable Intake and Overall Cancer Risk in the European Prospective Investigation Into Cancer and Nutrition (EPIC),” suggest that consuming lots of fruits and vegetables has little if any effect on preventing cancer.
As it happens, similar findings emerged in a number of studies done in the past five years.
Still, the Journal knew that the article would push some hot buttons, and felt it necessary to include an explanatory editorial by Harvard University MD/nutritionist Walter C. Willett in the same issue.
Boffetta et al. examined data collected for 142,605 men and 335, 873 women in the EPIC cohort, to assess relationships between intake of total fruits, total vegetables, and total fruits and vegetables combined and cancer risk during 1992-2000. The big pull quote from the study is that every two portions of vegetables and fruit consumed per day were associated with only a 2.5% lower risk of cancer. In other words, fruit and veggie haters can rejoice, and make fun of their more health-conscious friends, right?
Not so fast. Let’s hear what Susan Higginbotham, RD, PhD, Director of Research at the American Institute for Cancer Research has to say…
She first advises us that no one should greet this news as an excuse to return to the meat-and-potatoes mindset their parents and grandparents embraced. “We’ve known for some time that fruit and vegetable intake is probably protective against some, but not all cancers. So when you look at its effect against all cancers, as this study does, those overall numbers are going to look low.”
Higginbotham also points out that those study participants who were eating the most fruits and vegetables—six or more servings per day—had an 11% lower risk of all cancers than those who ate the least. “At the end of the day, that’s a significant decrease associated with eating the recommended amount vegetables and fruits,” she said. A “serving” is considered to be 100 grams (3.5 ounces).
Willett’s editorial notes that the enthusiasm that built up in the 1990s for fruits and vegetables to be cancer protective was based on case-control studies. In this type of study design, patients with a particular medical condition (the “case” group) are compared to a similar group (control) that does not have the disease in question.
Based on the non-random nature of at least the case group, case-control studies can lead to somewhat biased conclusions, and are thus typically followed-up with larger, more structured efforts. However, interesting and important trends are often first discovered with case-control studies, including the relationship between smoking and lung cancer.
The Boffetta work was a prospective study, whereby a group of similar individuals is followed over time with respect to certain factors under study, in order to determine how these factors affect rates—in this instance—of cancer.
However, “cancer” as an outcome can produce statistical results that are quite misleading. For one thing, there are many forms of cancer, and as Dr. Higginbotham states, it is well known that fruits and vegetables—that is, certain fruits and vegetables—are protective against certain cancers.
Moreover, lung cancer, one of the most common forms, is lifestyle related (smoking) in 90 percent of cases. One wonders what the results would have been if smokers who contracted lung cancer were removed from the study. After all, no one believes that eating fruits and vegetables can make up for a prohibitive risk factor like smoking, so why even include such individuals?
The Boffetta study claims to correct for confounding factors such as smoking, but I have my doubts that such a large effect can be adequately addressed though statistical manipulations.
By the same token, the study claims to correct for fat content in the diet, but again, this seems to be missing the larger point. No advocate holds that we can improve our health in any meaningful way by merely adding some good stuff to an otherwise bad diet.
With all due respect, a more useful study would have looked at the diets—and the cancers—in much more detail. My advice is to keep eating those fruits and veggies!