Do Fruits and Vegetables Prevent Cancer? Or Not?
By Suzanne W. Dixon, MPH, MS, RD
We’ve been told again and again that eating more fruits and vegetables will protect us from cancer. But headlines about recent findings from the European Prospective Investigation Into Cancer and Nutrition (EPIC) study (1) suggest otherwise:
“Fruits and Vegetables Have Only Weak Effect in Cancer Prevention”
“Cancer Protective Effect of Fruits and Vegetables May Be Modest at Best”
“Fruits and Veggies Have Small Effect on Cancer Risk.”
What gives? Do fruits and vegetables protect us against cancer or not? It turns out the devil is in the details.
Contrarian Views Sell More News
When you read media stories about medical and nutrition science research, it helps to keep in mind that contrary headlines sell. For example, a low-fat diet has been touted for years as the best way to beat heart disease. If a study shows that a low-fat diet is not useful for decreasing heart disease risk, this is big news.
You can bet that the media will trumpet these headlines, which are contrary to conventional wisdom.
Looking Beyond the Headlines
Another consideration?
Most health and science journalists are not trained scientists. They are not trained in complex statistics. They are not trained to fully understand the strengths, weaknesses, and caveats of different study designs. And they typically do not report on putting the new study into context with other research on the topic.
As Dr. Ralph Moss, PhD, who is a research scientist, points out, “According to the EPIC study, conversion to a moderately high fruit-and-vegetable diet could ideally save hundreds of thousands of people from getting cancer each year. This astonishing fact was hardly conveyed by the negative press reports on the EPIC study.”
Dr. Moss, author of the respected website Cancer Decisions (2), further explains, “if the subjects had increased their fruit and vegetable intake by just 150 grams per day, they would have reduced their risk of getting cancer by 2.6 percent (men) and 2.3 percent (women). Now, 150 grams is the weight of one apple.”
Nearly 1.5 million men and women were diagnosed with cancer in the United States last year. The EPIC study results suggest that one apple per day could prevent approximately 36,000 of these cancer cases every year. “Is it a small thing to keep more than 36,000 Americans from getting cancer at such a minimal cost?” asks Dr. Moss.
Which Cancer?
When assessing whether it’s worth the effort to eat more fruits and vegetables, we also need to look at the type of cancer studied. It turns out the type of cancer was “every type.”
According to Susan Higginbotham, RD, PhD, Director of Research at the American Institute for Cancer Research (AICR), “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.”(3)
By lumping together every single type of cancer into one study, including the ones that are least likely to be related to diet, we can’t see much of an effect of fruits and vegetables on risk. If we consider the cancers that do appear to be related to diet, the percentages are significantly higher.
In fact, the World Health Organization states, “Dietary factors account for about 30% of all cancers in Western Countries and approximately up to 20% in developing countries; diet is second only to tobacco as a preventable cause.”(4)
The cancers most convincingly related to diet include those of the mouth, pharynx and larynx, stomach, esophagus, lung, and colon and rectum. Colorectal cancer alone takes the lives of approximately 50,000 Americans every year. It is the second leading cause of cancer death in this country.
Straight From the Horse’s Mouth
Whenever I see media reporting that downplays important nutrition research, I consult my friend and colleague Diana Dyer, MS, RD. I value her opinion. Diana is not only a scientist and a dietitian; she’s also a 3-time cancer survivor. After one childhood cancer and two breast cancers (the last one over 10 years ago!), she speaks with authority on issues of cancer and nutrition. In her blog about the recent EPIC study, Diana raises points that those touched by cancer should consider. (5)
“Cancer is a tough task master. Not all cancer is preventable. Risk reduction is the name of the game. Do not put all your eggs (or fruits and vegetables) in one basket. Don’t look for one ‘magic bullet’. Cancer risk reduction needs to be multi-focused by creating a healthy lifestyle that consists of quitting (please don’t start!) smoking, working toward achieving or maintaining a healthy weight, daily exercise, finding an enjoyable way to handle the stresses in life (we all have ‘em!), and eating a healthy diet filled with healthy foods.”
Wise words indeed! To this she adds, “Small percentages are real, and I’ll take them. I would rather be nuancing over the variety of apple… than debating the side effects of various chemotherapy regimes that may also offer only a few small % points of potential benefit…(ugh – been there, done that, twice, not fun).”
Real Life is Messy
As a final note, we should bear in mind that EPIC is an observational study. This type of study has major flaws and weaknesses. For many reasons, it’s often the best we can do.
A randomized controlled trial is the "gold standard." Unfortunately, nutrition research is messy. It does not lend itself to this type of study. Of the several large-scale, randomized nutrition studies of the last few decades, nearly all have had problems keeping the intervention group on the intervention diet.
Sadly, our food environment is so toxic and difficult to navigate that almost nobody can be assigned to a healthy diet and actually stick to it.
And nutrition studies cannot be blinded. Simply by changing their diet, the people in the intervention group are fully aware that they are in the intervention group.
This doesn’t even touch on the problems we might see in the “control” group. This is the group of people assigned to go on living life normally. They are not supposed to make changes to their diets. Unfortunately, in this type of controlled nutrition trial, the control group commonly becomes “contaminated.” They end up attempting to follow the healthier diet themselves!
People in the control group may think, “If the intervention diet reduces disease risk, I want that advantage too.” In the end, both groups end up consuming a fairly similar diet. The study won’t find anything significant because the two diet groups aren’t significantly different from one another.
So, we are left with observational studies, which are, admittedly, imperfect.
With nutrition and disease we have to consider the totality of the evidence and the consistency of the findings from other studies on the topic. And for now, the totality of the evidence very strongly points to fruits and vegetables reducing risk of several types of cancer (mouth, pharynx and larynx, stomach, esophagus, lung, and colon and rectum). And heart disease. And stroke. And hypertension. And diabetes. The list goes on.
For more information or article references visit http://www.appleboost.com/newsletters/2010/04/20/issue-20-believing-this....
Suzanne Dixon, MPH, MS, RD, is an internationally recognized expert in nutrition, chronic disease, cancer, health and wellness as well as the Executive Editor of Nutrition Intelligence Report, a free natural health and nutrition newsletter. For more information, past issues or to sign up for a free subscription, visit http://www.appleboost.com/.