In his New York Times article "From an Apple a Day to 'Energy Balance' in Cancer Research", science writer George Johnson reviews both reports. He summarizes the differences between the two reports nicely: "where evidence had been found 'convincing,' it was demoted to a murkier status--'probable' or 'limited.' The strongest advice involved exercising more and reducing obesity, and eating more apples and oranges might mean eating fewer candy bars." The addition of "Physical Activity" in the title of the second report is telling.
Figure 1. Obesity Rates in the United States.
From 2007-2008 (not shown), 33.8% of U.S. adults were obese (32.2% for men, 35.5% for women).
As both the title of the newer article and this figure suggest, obesity is an ever-growing problem in the United States. The findings from the 1990's in support of good nutrition as a preventative measure for cancer are being seen more and more as invalid. Though no one would deny that eating more fruits and vegetables is good for one's health overall, researchers are now finding that there is not much of a direct correlation between cancer prevention and a diet heavy in fruits and vegetables; rather, a healthy diet is more successful in reducing obesity rates. The 2007 report discusses a newer phenomenon known as "energy balance," or how many calories a body consumes versus how many it expends. Researches now believe that cancer is caused when this balance tips to the point of obesity.
The studies conducted in the 1997 report were primarily case-control studies. Due to the backwards nature of these studies, bias is inevitable as patients can recall the past differently (in this case, both healthy and cancer patients could inaccurately report their previous diets). An article from the Journal of the National Cancer Institute discusses another issue: decreasing participation rates in surveys. Because of this decline, there is an overrepresentation, in this study specifically, of the benefits of high-fruit and -vegetable diets. Generally, people who agree to be the "healthy" subset of a study are already health-conscious people who eat more healthfully, exercise more, and smoke less than the average person. Thus, studies lead to an exaggerated conclusion of the benefits of eating large quantities of fruits and vegetables. Instead, prospective studies should be used to analyze the outcomes of eating large quantities of fruits and vegetables. This would allow the individuals in the study to be observed over a long period of time, so as to see the long-term effects of the diet.
This is not to discount the positive correlation between cancer prevention and a small subset of fruits and vegetables, or more specifically certain chemicals within them. Evidence does suggest, for example, that lycopene from tomatoes reduces the risk of prostate cancer. Antioxidants in fruits and vegetables have also proven to help reduce the risk of cancer when consumed at a young age. Ultimately, eating multiple servings of fruits and vegetables each day does have beneficial effects, just not necessarily in the context of overall cancer prevention.
"Food, Nutrition, Physical Activity, and the Prevention of Cancer: A Global Perspective." World Cancer Research Fund International (2007): n. pag. Web.
Johnson, George. "From an Apple a Day to ‘Energy Balance’ in Cancer Research." The New York Times. The New York Times, 23 Apr. 2014. Web.
Willett, Walter C. "Fruits, Vegetables, and Cancer Prevention: Turmoil in the Produce Section." Journal of the National Cancer Institute. Oxford Journals, 2010. Web.