What Fruits and Veggies Can and Can’t Do

Despite the results of a recent study, the fact remains that a diet rich in fruits and vegetables offers powerful protection against an array of chronic diseases -- including cancer

By: Dr. David L. Katz*

 

Results of the Women's Healthy Eating and Living trial -- published last week in the Journal of the American Medical Association -- suggest that increasing fruit and vegetable intake does not protect against breast cancer recurrence or death. Extensive media coverage of the study, including comments by its authors, would have us believe that the role of fruits and veggies as cancer fighters has been debunked. I don>t accept that interpretation, and remain convinced that a diet rich in fruits and vegetables offers powerful protection against an array of chronic diseases -- including cancer.

 

The study enrolled approximately 3,000 women -- mostly over age 50 -- who had been treated for early stage breast cancer. They were randomly assigned to (1) receive intensive dietary guidance for increasing fruit and vegetable intake, or (2) to a control group that received print materials about eating 5 servings of fruits and vegetables daily. At the end of the 7-year study, there was no major difference between groups for breast cancer recurrence, or death.

Of note, both groups of women reported increasing their intake of fruits and vegetables, and reducing their calorie intake. Yet both groups gained weight over time; the intervention group surpassed the control group. This raises questions about the accuracy of the dietary reporting since participants in a trial may report what they think investigators want to hear, and raises the possibility that weight gain offsets any potential benefits from eating more produce.

Another key consideration relates to timing. The 15th century father of modern toxicology, Paracelsus, is purported to have said, «The dose makes the poison.» If Preventive Medicine had a parent, he or she would say, «The timing makes prevention.» In the current study, the timing may have been all wrong for strong preventive effects, since all of the participants had cancer already.

Let's consider the implications of trying to prevent the consequences of an event that has already occurred. No one doubts that a smoke detector can help prevent your house from burning down, and reduce risk of burn injuries. But throwing a smoke detector into a burning house won't put out the fire. And a smoke detector won't prevent smoldering embers from igniting again, or prevent the complications of a bad burn.

Consider timing and heart disease. We know regular physical activity reduces heart disease risk, but imagine if a group of patients with advanced and unstable coronary artery disease was asked to engage in moderate exercise. It would very likely be lethal. Exercise helps keep healthy arteries healthy, but unhealthy arteries may not tolerate a sudden dose of exercise.

The recent breast cancer study is not the first time a dietary strategy for cancer prevention proved a disappointment. In 2000, two studies -- published in the same issue of the New England Journal of Medicine -- suggested that dietary fiber did not protect against colon cancer, as most experts had believed. In those studies, individuals who had pre-malignant colonic polyps removed were assigned to get extra fiber, or to a control group. The extra fiber -- from food or supplements -- did not reduce the rate of polyp recurrence.

Perhaps those results were predictable. Colon cancer, like breast cancer, develops over years to decades, and involves three stages: (1) initiation, (2) promotion, and (3) expression. Initiation is the induction of a genetic mutation with the potential to cause cancer; promotion is the slow, early growth of an abnormal cluster of cells; expression is the development of a clinically significant cancer.

Diet plays an important role in fighting both cancer initiation, and promotion. The many nutrients in fruits and vegetables, including powerful antioxidants, are thought to help protect our cells, and minimize the frequency of DNA damage that could lead to cancer initiation. Diet also protects against promotion by fueling the immune system to fight and contain any cellular abnormalities that arise. But once cancer, or pre-cancer, has been expressed -- in the form of a colonic polyp, or a breast cancer -- expecting a lot from diet is not unlike counting on that smoke detector after the fire. The damage has been done.

That does not mean there is no role for diet after cancer develops. Nutritional support is crucial for the body>s ability to fight a cancer, tolerate cancer treatment, and recover. Cancer can result in the loss of lean body mass -- the vital parts of the body that give it strength -- and a form of starvation called cancer cachexia is a major contributor to cancer deaths. A nutritious diet can help fight this, as can nutritional supplements designed specifically for this purpose.

Perhaps even after years of damage to the colon resulting in polyps, increased fiber intake may offer some small advantage. And increased intake of fruits and vegetables might offer some small protection against breast cancer recurrence, too.

Imagine if these strategies prevented one cancer recurrence in 3,000. At the population level, that translates into a very meaningful effect, potentially saving many lives each year. But in a study of only 3,000 people, such an effect would be invisible. In such a trial, only a very large effect could be seen -- and for the reasons noted, such a large effect is an unrealistic hope.

An ounce of prevention is worth a pound of cure, but the ounce must be applied early. The more advanced the damage is to our bodies, the more heavy lifting it takes to combat that damage.

A generous intake of fruits and vegetables is the cornerstone of a health-promoting diet, and healthful eating is a cornerstone of disease prevention. Recent trial data do not refute this, although they may suggest that starting sooner offers benefits that starting later simply cannot.

* David L. Katz, MD, MPH, FACPM, FACP; Director, Prevention Research Center, Yale University School of Medicine  and medical contributor for ABC News. He may be reached at www.davidkatzmd.com.)

Once cancer, or pre-cancer, has been expressed – in the form of a colonic polyp, or a breast cancer – expecting a lot from diet is like counting on a smoke detector after the fire. The damage has been done. But that does not mean there is no role for diet after cancer develops.