Fruit and Veg: No Cancer Prevention?
The results of the largest ever study of the relationship between diet and cancer have recently been published (April 2010) and show that eating fruit and vegetables have only a modest effect on cancer prevention.
Some have seized the opportunity to castigate ‘bossy’ nutritionists and health agencies for their well-intentioned advice to eat more fruit and vegetables.
The advice to consume at least five portions of fruit and vegetables a day has been promoted by the World Health Organisation since 1990, in the U.S.A. for nearly two decades and in the U.K. since 2003.
In 1997, the World Cancer Research Fund stated that there was ‘convincing’ evidence that fruit and vegetables helped to prevent cancers of the digestive tract. And in 2003, the World Health Organisation’s World Cancer Report claimed that the protective effect of fruit and vegetables against cancer was ‘the most consistent finding on diet as a determinant of cancer risk’.
This statement was made on the basis of the fact that 80% of the 250 or so studies at the time showed that eating more fruit and vegetables was associated with significant cancer prevention. Previous reports from this study (EPIC) also showed that fruit and vegetables helped to reduce the risk of cancers of the digestive tract and lung, but had no effect against breast, prostate or ovarian cancers, or lymphoma. Another large study (NIH-AARP) also established a link between eating lots of fruit and vegetables and prevention of cancers of the head and neck.
So, could we have got something so seemingly common-sense so wrong?
About the EPIC study
First, a little detail about the study.
Most previous studies into the link between diet and cancer prevention have been ‘case-control’ studies. This means that groups who contracted cancer have been compared retrospectively with those who were cancer-free. This method, although potentially instructive, means that the subjects might give inaccurate answers and their memories about their dietary habits might be unreliable. Also, typically, these studies may not have controlled for other important factors such as whether the subjects smoked or drank alcohol.
The study in question was led by Paolo Boffetta from the Tisch Cancer Institute at Mount Sinai School of Medicine, New York and was part-funded by Cancer Research U.K. Called the European Prospective Investigation into Cancer and Nutrition (EPIC) it was a prospective ‘cohort’ study. This means that detailed information about diet and lifestyle were gathered from seemingly healthy subjects and then they were followed over 9 years to see who developed cancer and what the risk factors were. The subjects were nearly 100,000 volunteers from 10 different European countries.
The results were adjusted for other risk factors such as smoking, alcohol consumption, the use of hormone replacement therapy (H.R.T.), levels of physical activity, body weight and the amount of fibre in the diet so that the results would reflect, as far as possible, the effects of fruit and vegetable consumption on cancer prevention.
Findings:
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Eating 200 g of fruit and vegetables a day (2.5 portions) resulted in 3% cancer prevention for all cancers
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People who ate 5 portions of fruit and vegetables a day, when compared to people who ate less than 2.5 portions a day, had a 9% lower overall risk of cancer.
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People eating more than 8 portions a day of fruit and vegetables, had an 11% lower risk of cancer.
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The effect of fruit and vegetable consumption was most marked preventing of cancers of the digestive system including mouth, oesophageal and stomach cancers.
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The consumption of vegetables was found to be more prevent more cancers than eating fruits.
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Heavy drinkers benefited most from greater fruit and vegetable intake which provided some cancer prevention against the more common cancers associated with drinking alcohol and smoking.
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The researchers noted that these cancer prevention results may be due to the fact that those that ate more fruit and vegetables also led healthier lives in other respects.
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The link between eating a large amount of vegetables and cancer prevention applied only to women.
So disappointing results then? Not quite.
The EPIC study: A riposte
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The people inclined to take part in this study – any study – may ultimately be self-selecting with the heavy drinking and smoking junk-food addicts disinclined to participate in a health study of any colour or hue.
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This study set out to look specifically at cancer prevention and not at other chronic diseases such as hypertension or diseases of the gall bladder, but more general results might have been instructive. The National Health Service in the UK maintains that eating five portions of fruit and vegetables a day reduces the risk of ‘some cancers’ including bowel cancer, and also heart disease, type II diabetes, strokes and obesity.
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Although the sample was large the study only followed the participants for under 9 years, whereas the cancer prevention effects over a lifetime might tell a different story.
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Cancer is regarded as a disease associated with ageing, and so it depends upon the age of the participants. Contracting cancer at 78 is surely a different issue to getting cancer at 28 years of age.
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Although the percentage effect of cancer prevention is small this still equates to approximately 35,000 avoidable cases of cancer a year in the US and 7,000 cases in the UK, which still translates to a lot of human misery and lives lost prematurely.
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With regards to eating fruit and vegetables as a cancer prevention strategy, unlike most therapeutic interventions, there is no appreciable risk – all is benefit, so there is literally nothing to lose.
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The Spanish arm of the study found that the people who ate the most fruit and vegetables reduced their risk of dying of chronic diseases at any age by 25% compared to those who ate very little, which surely is a significant finding.
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Cancer is a collection of over 200 diseases and if the cancer prevention afforded by eating more fruit and vegetables only really reduces the risks of a handful of cancer types, then you wouldn’t expect to see much of an impact against the overall umbrella figures for cancer.
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A link between bodyweight and cancer has been firmly established. Whether being overweight causes cancer or whether the two factors are linked to a third causative factor (toxicity?) remains unclear. In any event, eating fruit and vegetables helps to reduce and maintain body weight.
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In the EPIC study, the participants that consumed at least 5 portions of fruit and vegetables a day lowered their risk of coronary heart disease or stroke by 30 per cent compared with those on less than 1.5 servings a day.
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It also depends upon the rest of the diet. If you eat some fruit and vegetables but generally live on a diet of processed, convenience foods or take-aways then the health benefits of eating the fruit and vegetables may be seriously impacted.
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The results rest or fall on the diet and lifestyles as reported by the participants and there is a well-recognised tendency to underestimate the amount of alcohol consumed and the number of cigarettes smoked and to potentially overestimate the amount of fruit and vegetables eaten.
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There are often stories about ‘superfoods’ and this may lead to a skewed perception about the importance of eating a balanced diet containing a variety of fruits and vegetables. The more highly coloured fruits and vegetables are recognised to offer the greatest health benefit. Lycopene, for example, gives tomatoes and other fruits and vegetables such as red grapefruit their colour and is recognised to offer some prostate cancer prevention, while the sulphoraphane in cruciferous vegetables such as broccoli affords some bowel cancer prevention. So a love of turnips, for example, may be less beneficial than consuming copious amounts of tomato soup.
In this, as in all things, you need to read behind the banner headlines. As noble as the intentions of the study may have been, trying to quantify and isolate one aspect of human behaviour whist controlling for all other aspects is a very slippery eel indeed!
Further resources
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