Archive for the ‘Cancer’ Category

Fruits and Vegetables Fight Cancer! Or Do They?

February 21, 2011

The American Cancer Society recommends eating at least five servings of fruits and vegetables per day to help prevent cancer. They have evidence to back this up, right? Um.

I found this study today, which looked for a link between fruit and vegetable consumption and cancer risk in 478,000 European subjects. It’s a prospective epidemiological study, meaning the researchers gathered information about subjects (in this case, how many fruits and vegetables people were eating), waited a few years, and then followed up to see how their subjects fared (in this case, who got cancer). The large number of subjects and the prospective nature of the study made it potentially very informative. Unfortunately for the veggies-prevent-cancer theory, the results were underwhelming at best.

The researchers found that a 200 grams/day increase in fruit and vegetable intake correlated with a statistically significant but minuscule reduction in cancer risk (about a 3% risk reduction). Assuming a cause-and-effect relationship (which is actually a pretty big assumption), this would mean that by increasing intake of fruits and vegetables by 200 grams/day, a person with a 30% probability of getting cancer could reduce that probability to 29.1%. Why am I not running to grocery store to buy more veggies?

Furthermore, even in a large study like this one, such a weak correlation has to be taken with a grain of salt. Other lifestyle factors could be at play that the researchers did not account for. People who eat more fruits and vegetables also tend to smoke less, exercise more, eat fewer french fries, etc. The researchers tried to factor out these variables, but it would be impossible to catch them all. There are probably variables scientists don’t even know about yet that affect cancer risk.

Walter C. Willett, M.D., Dr.P.H., of the Harvard School of Public Health, wrote in an accompanying editorial that “this study strongly confirms” the findings of other prospective epidemiological studies, which show weak or non-existent correlations between produce intake and cancer risk.

The studies that originally sparked the belief in fruits’ and veggies’ cancer-fighting properties were retrospective case-control studies, which tried to obtain dietary information from people who already had cancer and compare it to information from cancer-free controls. The problem is that a cancer diagnosis tends to skew a person’s memories about their dietary habits. Sick people are more likely to remember eating junk food while healthy people are more likely to remember eating nutritious food. Prospective studies, which gather information ahead of time, are free of this 20/40 hindsight bias.

Does this mean people don’t need to eat fruits and vegetables? Well, it certainly disproves the notion that a diet low in fruits and vegetables causes cancer. In later posts I’ll look into the possible connection between fruits and vegetables and heart disease. Either way, they’re not a panacea. But they’re probably good foods to eat because they provide energy and bio-available vitamins and minerals, without any apparent harmful effects on metabolism. Considering the overall quality of our food supply, that’s a pretty big deal.