Surprise! Fruits and vegetables don’t prevent cancer

Last week I wrote about the evidence that vitamins don’t prevent cancer. Now a new study demonstrates that the fruits and vegetables that contain those vitamins don’t prevent cancer.

The study, Fruit and Vegetable Intake and Overall Cancer Risk in the European Prospective Investigation Into Cancer and Nutrition (EPIC), by Buffetta et al. appears in the latest issue of the Journal of the National Cancer Institute. The study has two major strengths. It is large (over 400,000 people) and prospective, meaning that it followed people over time instead of depending on long term recall of dietary habits. The editors of JNCI have responsibly included a summary for the press, to be sure that the results are reported correctly:

… Paolo Boffetta, M.D., M.P.H., of the Mount Sinai School of Medicine in New York, and colleagues analyzed data from the EPIC study (European Prospective Investigation into Cancer and Nutrition), which included 142,605 men and 335,873 women recruited for the study between 1992 and 2000. The participants were from 23 centers in ten Western European countries–Denmark, France, Germany, Greece, Italy, the Netherlands, Norway, Spain,Sweden and the United Kingdom. Detailed information on their dietary habit and lifestyle variables was obtained. After a median follow-up of 8.7 years, over 30,000 participants were diagnosed with cancer.

The authors found a small inverse association between high intake of fruits and vegetables and reduced overall cancer risk. Vegetable consumption also afforded a modest benefit but was restricted to women. Heavy drinkers who ate many fruits and vegetables had a somewhat reduced risk, but only for cancers caused by smoking and alcohol.

Most importantly, the press summary explains the interpretation of the findings.

The authors caution against attributing any risk reduction to diet and they conclude that any cancer protective effect of these foods is likely to be modest, at best.

In this population, a higher intake of fruits and vegetables was also associated with other lifestyle variables, such as lower intake of alcohol, never-smoking, short duration of tobacco smoking, and higher level of physical activity, which may have contributed to a lower cancer risk,” they write.

In other words, the tiny observed benefits are more likely to be due to confounding factors than to represent any protective effect of fruits and vegetables.

In an accompanying editorial, Dr. Walter Willet, from the Department of Nutrition at the Harvard School of Public Health that the data suggesting a cancer protective effect of fruits and vegetables was never strong.

During the 1990s, enthusiasm swelled for increasing consumption of fruits and vegetables with the expectation that this would substan­tially reduce the risk of many cancers. Potential reductions as large as 50% were suggested. The National Cancer Institute’s 5-A-Day program was developed in 1991 to promote increasing the average consumption of fruits and vegetables to five or more servings per day … However, the evidence for a large preventive effect of fruits and vegetables came primarily from case–control studies, which can be readily biased by differences in recall of past diet by patients with cancer and healthy control subjects. Even more problematic, participation rates in surveys have fallen sharply over the past 50 years so that those who agree to be interviewed as control subjects are likely to overrepre­sent health conscience persons who smoke less, exercise more, and eat more fruits and vegetables compared with those who do not participate…

Yet despite the weak evidence, lay people and professionals alike embraced the conclusions enthusiastically. Why? As a society we have a disturbing tendency to promote simple (and often unproven) answers to complex issues. It would be wonderful if cancer, a complex and multifaceted disease, could be prevented by eating fruits and vegetables. Such a simple answer sounds (and is) too good to be true, and should have been greeted with skepticism instead of uncritical acceptance.

Within the scientific community, it has been known for quite some time that the protective effect of fruits and vegetables had been vastly overstated. As Willet explains:

… In the late 1990s, the results of large prospec­tive cohort studies of diet and cancer began to accrue, and these did not confirm the strong inverse associations found in most case–control studies. Furthermore, a series of analyses that pooled the data from prospective studies for specific cancer sites confirmed the weak and non-statistically significant associations. In a comprehensive 2007 review, these findings led to the reversal of conclusions for strong benefits of fruits and vegetables for many cancer sites that had been considered convincing or probable in a similar earlier review 10 years before.

That message has not reached the general public and even many healthcare providers.

Fortunately, there was no harm done, and there was possibly a benefit in a decrease in cardiovascular disease as a result. Nonetheless, it is yet another example of a disturbing trend in preventive medicine, the tendency to issue public health recommendations on the basis of weak data. As a result, public health recommendations are often modified or even withdrawn after only a few short years, leading to distrust of science in general and the medical profession in particular.

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