Fang Fang Zhang, an assistant professor at the Friedman School who specializes in nutritional influences on cancer risk, responds
A panel of experts at the World Cancer Research Fund and the American Institute for Cancer Research reviewed existing studies on this topic and published their findings in 2007. Their review, the most systematic one completed so far, concluded that the evidence that vegetables or fruits protect against cancer is less impressive than many people had thought.
Findings from a recent cohort study of 478,000 Europeans also suggest that eating two extra servings of vegetables a day had only little effect on preventing cancer. Now, shall we pull the greens off of our plates?
Not just yet. Keep in mind that not all fruits and vegetables are created equal.
We know that non-starchy vegetables are different from starchy vegetables in their calorie content. Fruit juice may have added sugar and reduced vitamins and fiber content compared to “solid” fruits.
If we study starchy vegetables and non-starchy vegetables as one group, we may not see a beneficial effect because the benefit and the harm may cancel each other out. In addition, vegetables may prevent cancers of specific sites, rather than all cancers. For example, many studies showed non-starchy vegetables protect against cancers of the mouth, larynx, esophagus and stomach, but not other types of cancer.
Moreover, compared to animal-based foods, vegetables and fruits contain fewer calories and less fat, which can help prevent excessive weight gain and obesity. The latter has been clearly associated with an increased risk of cancer in the esophagus, colorectum, breast (post-menopause), pancreas and kidney, as well as overall mortality.
With this in mind, we should continue eating vegetables and fruits to stay healthy.