Five-a-day won’t keep the doctor away

The American humourist Mark Twain said: ‘What gets us into trouble is not what we don’t know. It’s what we know for sure that just ain’t so.’ Twain’s famous words rang especially true a fortnight ago when the latest study on the link between fruit and vegetable consumption and cancer prevention landed on our desks.

It simply has to be true that eating fruits and vegetables helps to ward off cancer. After all, such purveyors of pristine science as the World Health Organisation, the National Health Service, Cancer Research UK and the American Cancer Society have all told us it is true. But behind these claims – and the catchy marketing campaign to eat ‘five a day’ – there is little solid science.

In a new study published in the Journal of the National Cancer Institute, the claim that eating daily amounts of fruit and vegetables can prevent cancer was revealed as nothing more than a piece of junk science. The study, led by Paolo Boffetta from the International Agency for Research on Cancer, followed almost half a million Western Europeans for over eight years in an effort to determine whether cancer can be prevented by high intake of fruit and vegetables. Strikingly, the study failed to find any significant statistical relationship between fruit and vegetable consumption and reduced risk of cancer. Eating fruit and vegetables simply did not protect one from getting cancer.

Boffetta strangely claimed at a news conference on the study that, ‘The bottom line here is that, yes, we did find a protective effect of fruit and vegetable intake against cancer, but it is a smaller connection than previously thought’, a claim echoed in the conclusion of the study, which notes that: ‘A very small inverse association between intake of total fruits and vegetables and cancer risk was observed in this study.’

But Boffetta’s claim doesn’t hold up. No matter how much spin the study’s authors put on their results, the fact is that the small inverse association found in the study is not statistically significant – that is, there was no relationship between intake of fruits and vegetables and a reduced cancer risk.

These results, of course, should not come as a surprise. Last year’s World Cancer Research Fund (WCRF) report, Policy and Action for Cancer Prevention, had similar results. It too claimed, for example, that eating fruit and vegetables can protect against cancer, even though the scientific evidence fails to support, if not directly contradicts, such claims. In the WCRF report, for instance, of the 17 cancers discussed, virtually all have statistically non-significant associations with every type of food, which means that they provide no evidence of any link between a particular food and a particular cancer.

In fact, the claim that eating fruit and vegetables protects against cancer is contradicted by the largest and most expensive randomised controlled studies of the link between eating certain foods and cancer risk ever carried out: the Women’s Health Initiative Dietary Modification Trial (WHIDMT). In the WHIDMT, almost 49,000 American women were followed over an eight-year period in terms of eating, weight and disease (breast cancer, colon cancer, heart disease, and stroke). The women in the intervention group ate ‘healthy’ diets low in fat and high in fibre with plenty of fruits and vegetables.

What were the studies’ results? There were no statistically significant differences between the intervention and the control group in the incidence of breast cancer, colon cancer, strokes or heart disease. In fact, the women following the healthy diet didn’t even weigh less than they did at the beginning of the study, or less than the group that continued to eat as they always had.

These respective cancer prevention studies help us to recall something else Mark Twain said: ‘Be careful about reading health books. You may die of a misprint.’ The same can be said of reading cancer studies. For when the researchers themselves are dismayed that their results are insufficiently glum, the truth may not always be found in their words; it can still be found, however, buried in their data.

Patrick Basham and John Luik are authors, with Gio Gori, of Diet Nation: Exposing the Obesity Crusade, a Social Affairs Unit book. (Buy this book from Amazon(UK).) Patrick Basham directs the Democracy Institute and is a Cato Institute adjunct scholar. John Luik is a Democracy Institute senior fellow.