Worried Over Misinterpretations of Recent Studies, Cancer Researchers Come Together, Take Action

American Institute for Cancer Research
Thursday, 15 March 2001

Open Letter from 54 Experts Urges Public to Continue Diets High in Vegetables, Fruits for Cancer Prevention

In an open letter to the public, 54 prominent cancer researchers cautioned Americans against making changes in their diets based on reports of recent diet-cancer studies.

They warned health-conscious individuals not to draw conclusions from these latest studies, but rather from the preponderance of evidence accumulated over decades.

In particular, the researchers advised people concerned about cancer risk to maintain a diet high in plant-based foods despite three recent studies that cast doubt on the cancer-protective benefits of vegetables and fruits. Citing well over 200 studies that have revealed a strong association between vegetables and fruits and lower risk of cancer, they called on the public not to let a few recent studies deter them from striving to consume five to nine servings of these foods each day.

The letter points out that all studies are limited by their design. Intervention studies, in which individuals are guided in changing their diets for short periods of time, cannot tell us about the effects of long-term dietary patterns. Cohort studies track the reported diets of subjects over longer periods, but can only evaluate the effects of diets "high in vegetables and fruits" if they find significant numbers of subjects who actually consume five to nine servings of these foods each day.

The open letter, signed by 54 experts across several fields of cancer research, refers to three studies, one an intervention study, one a cohort study and one an analysis of nine previously published cohort studies. Due to necessary compromises made in their design and execution, "none of these studies determined how diets high in vegetables and fruits (five to nine servings/day), consumed over an extended period of time (more than four years) affect cancer risk."

The three studies to which the letter referred were:

Schatzkin et al, which appeared in the April 20, 2000 issue of the New England Journal of Medicine. The authors of this intervention study advised half of its participants (middle-aged subjects who had already had colon polyps removed) to eat diets high in vegetables and fruits for a period of four years. The authors found that subjects who were told to eat such diets had just as much colon polyp recurrence as those who were not advised to make dietary changes.

Important limitations of this study overlooked by reports:

- Short duration. In the letter to the media, the cancer researchers pointed out that "intervention studies …must be of limited duration. Yet we know cancer to be a disease that develops slowly, over the course of many years and even decades."

- Polyp recurrence, not cancer. The study looked at the recurrence of colon polyps among subjects who had already had colon polyps removed. Most polyps never become cancer, and the precise nature of the progression from polyp to colon cancer remains unclear.

- How many vegetables and fruits? Whether or not the participants in this study were actually eating the amount of vegetables and fruits they claimed to be has since inspired some scientific debate. Several researchers have suggested that the blood levels of carotenoids in members of the "high vegetables and fruits" group were much lower than would be expected.

Michels et al, which appeared in the November 1, 2000 issue of the Journal of the National Cancer Institute. Over the course of several years, this cohort study periodically asked its participants to remember what foods they ate. When the authors analyzed the subjects' questionnaires, which featured nearly 100 different dietary components, they found that those subjects who said they ate the most vegetables and fruits did not experience significantly less colon cancer than other subjects.

Important limitations overlooked by reports:

- Subjects ate very few vegetables and fruits. Because they track, but do not change, their subjects' diets, cohort studies can only measure the effect of diets "high in vegetables and fruits" if a significant number of subjects report eating such diets. But only two percent of the Michels study subjects said they ate more than four servings of vegetables each day. The average subject ate well below the five to nine daily servings of vegetables and fruits recommended for cancer prevention.

- Narrow range of intake. Because the amount of vegetable and fruit intake in the study was uniformly low, the statistical range of intake for the study was narrow, limiting its real-world relevance.

- Subjects reported diets from memory. Collecting data from a self-administered questionnaire, although an established technique, remains controversial. This method has been shown to give rise to certain biases. Of particular concern in this study is the fact that subjects tend to over-report "culturally desired behaviors" such as eating vegetables and fruits and under-report other factors like fat and sugar consumption.

Smith-Warner et al, which appeared in the February 14, 2001 issue of the Journal of the American Medical Association. This study was actually an analysis of eight previous studies that, like the Michels study above, tracked the reported diets of subjects using a questionnaire for several years. Once again, the authors found that those subjects who said they ate the most vegetables and fruits had no fewer cases of breast cancer than other subjects.

Important limitations overlooked by reports:

- Built-in limitations of cohort studies apply. Even though this study compiled data from several cohort studies, the fact that they were all of the same design means that the caveats associated with the cohort method (especially regarding self-reported diets and statistical range of intake) still apply.

- Subjects once again ate very small amounts of vegetables and fruits. Moreover, several of the previous studies that the authors analyzed neglected to collect data on apples, bananas, peaches, apricots, broccoli, brussels sprouts, cabbage, lettuce, spinach, string beans and tomatoes. Not only are these some of the most commonly eaten vegetables and fruits in the US, several of them (tomatoes and broccoli in particular) have also been shown to be powerfully cancer-protective.

In light of these important limitations, the letter signed by 54 cancer researchers from across the country concludes by placing the studies in a larger context.

Scientific investigation is an ongoing process. More studies on diet and cancer will be reported to the public. When it comes to something as important as personal health, the public should always look to the preponderance of scientific evidence, not simply the latest study. Evidence that has been building for decades swings heavily in favor of a long-term diet high in vegetables and fruits for reducing risk for cancer, as well as heart disease, diabetes and obesity.

For more information, or to contact American Institute for Cancer Research, see their website at: www.aicr.org