Guest Post: Paul Appleby on Fruits, Vegetables, & Cancer
On April 7, I linked to an NPR report about a European Prospective Investigation into Cancer (EPIC) paper that found that eating more fruits and vegetables cuts the risk of cancer by only 4%. After reading his comments elsewhere, I invited Paul Appleby, Senior Statistician at the Cancer Epidemiology Unit at the University of Oxford, to share his thoughts on the paper.
Do fruits and vegetables prevent cancer? No. Does eating more fruits and vegetables reduce the risk of some cancers? Probably yes. To quote from the WCRF/AICR expert report on “Food, Nutrition, Physical Activity, and the Prevention of Cancer: a Global Perspective” (2007), “The strongest evidence, here corresponding to judgments of ‘probable’, shows that non-starchy vegetables and also fruits probably protect against cancers of the mouth, larynx, pharynx, oesophagus, and stomach, and that fruits also probably protect against lung cancer; and that allium vegetables, and garlic specifically, probably protect against stomach cancer.”
As I see it, there were two disappointing aspects to the EPIC paper in question (1). First, the authors examined the composite end point of all malignant cancers. Thus, the beneficial effects of fruit and vegetables at some cancer sites (see the WCRF/AICR list above) would be diluted by the inclusion of common cancer sites for which there appears to be no association with fruit and/or vegetable intake. As a result, the authors found only a small (but statistically significant) 3% reduction in overall cancer risk per 200 g/d increased intake of total fruits and vegetables. Although this would equate to the prevention of thousands of cancers every year, much of the media chose to put a rather negative spin on the story, overlooking the evidence for a beneficial effect of fruits and vegetables on other diseases such as cardiovascular disease (see, for example, ref 2 below). Interestingly, the latter study found that the “risk of coronary heart disease was decreased by 4% [RR (95% CI): 0.96 (0.93-0.99), P = 0.0027] for each additional portion per day of fruit and vegetable intake”, a benefit similar in magnitude to that found for cancer in the EPIC study. It is unrealistic to expect to see risk reductions of 10% or higher per extra portion of fruits and vegetables for such broad end points as all cancers or coronary heart disease.
The second disappointing aspect of the EPIC paper was the fact that the authors only considered total fruits and total vegetables (and the sum of the two) as their exposure variables, and did not look at sub-types of fruit and vegetables. As readers will be aware, different fruits and vegetables can have quite different nutritional characteristics (compare the vitamin C content of apples and oranges, for example) and it was a pity that the authors did not examine the associations between, say, citrus fruits or cruciferous vegetables and cancer risk. Such an analysis might have revealed stronger protective effects for certain types of fruit and vegetables that could have pointed the way to more nuanced public health advice to supplement the rather crude ‘5-a-day of any sort of fruits and vegetables’ message.
1. Boffetta P, et al. Fruit and vegetable intake and overall cancer risk in the European Prospective investigation Into Cancer and Nutrition (EPIC). J Natl Cancer Inst. 2010;102:1-9 Advanced access published April 6, 2010.
2. Dauchet L, Amouyel P, Hercberg S, Dallongeville J. Fruit and vegetable consumption and risk of coronary heart disease: a meta-analysis of cohort studies. J Nutr. 2006 Oct;136(10):2588-93.