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Vegetables, fruits and cancer: where are we now?
Perhaps the most recognised of health messages is “at least 5 a day” for portions of vegetables and fruits, based on epidemiologic and laboratory studies linking higher intake of these foods with lower risk of heart disease and some cancers.
The 1997 expert report from the World Cancer Research Fund (WCRF) and the American Institute for Cancer Research (AICR) found convincing evidence that higher intake of vegetables and fruits reduced risk of several cancers. In 2007, the second WCRF/AICR expert report re-evaluated the evidence. This review – the most comprehensive and rigorous of its type – was more cautious in ascribing causal effect, but nevertheless considered the evidence strong enough to make recommendations1.
The evidence supporting these (and other) recommendations is now being updated as part of the WCRF International Continuous Update Project (CUP), and is judged by an independent expert panel, which will draw conclusions and make recommendations.
The most recent conclusions from the independent Expert Panels for the 2007 report and the CUP are that higher intakes of vegetables and fruits probably reduce risk of cancers of the mouth, larynx and pharynx, oesophagus, lung and stomach. These cancers have well-established external causes – tobacco, alcohol and infection. While the most well known mechanisms proposed for a beneficial effect of vegetables and fruits focus on their antioxidant potential, there is increasing interest in possible interactions between several plant constituents and infectious agents. For example, evidence is accumulating for a subtle impact of folate status in modulating the response to infection with the human papilloma virus, a known cause of some head and neck cancers as well as cervical cancer2.
The methodological challenge
Better description of the molecular mechanisms underpinning cancers, in particular those related to infection, will bring about a more sophisticated understanding of the complex interactions that eventually lead to cancer. But this is just the beginning of an enormous shift in how the roles of individual food components, foods or patterns of diets in health in general and cancer in particular are perceived.
There are imperfect tools for establishing levels of consumption of vegetables and fruits and their various bioactive constituents. What is required is to establish such exposure over decades, and to discriminate between different types of vegetables and fruits, with precision and accuracy. The tools currently available enable strong conclusions to be drawn for broad categories of vegetables or fruits, but are far from being able to pinpoint specific molecular effects. In the future, integration of molecular techniques into epidemiologic and clinical studies has great potential to increase this understanding.
A holistic approach
In the face of these obstacles, what can be said? Most dietary characteristics are in some way associated with each other – people who eat more of one type of vegetable are more likely to eat another; those who eat plenty of plant foods often eat less animal products; and people who follow a healthy dietary pattern tend to smoke less and be more physically active. This suggests a cautious approach to making recommendations – WCRF recommends at least fi ve portions of non-starchy vegetables and fruits a day as part of a broad approach, to “eat mostly foods of plant origin”, taking account of the impact of whole grains and pulses, and of plant foods in general being low in energy and useful for body weight maintenance. This recommendation, together with recommendations to limit intake of meat, alcohol and salt, is itself part of a more holistic approach to a healthy pattern of diet and other behaviours including physical activity.
The evidence that vegetables and fruits help prevent certain types of cancer is strong enough to recommend their consumption, but confi dence in the impact will be increased by adopting as many recommendations as possible, as part of a pattern of healthy behaviour. Several studies have shown that the more people follow the WCRF/AICR recommendations, the lower is their risk not just of cancers but also other chronic diseases. Vegetables and fruits are a key marker of this healthy lifestyle.
- WCRF/AICR. Food, Nutrition, Physical Activity, and the Prevention of Cancer: a Global Perspective. Washington DC: AICR, 2007
- Moody M, Le o, Rickert M et al. Folic acid supplementation increases survival and modulates high risk HPV-induced phenotypes in oral squamous cell carcinoma cells and correlates with p53 mRNA transcriptional down-regulation. Cancer Cell International 2012, 12:10-21