Thursday December 2 2010
A balanced diet helps to maintain a healthy weight
“Eating five fruit and veg a day ‘won’t help you to beat cancer’,” reported the Daily Mail.
The news story is based on a review of the effects of fruit and vegetables on the risk of a number of common cancers. The review concludes that, for many cancers, risk factors such as smoking, alcohol and weight are more important than fruit or vegetable consumption.
This was not a systematic review of the evidence itself, but the overview was based on other systematic reviews. The important message here is that risk factors such as alcohol consumption, smoking and weight have much clearer effects on the risk of cancer than eating fruit and vegetables.
Recommendations to eat enough fruit and vegetables are not just about cancer prevention, however, and a balanced diet is important for other health reasons, such as maintaining a healthy weight and to prevent heart disease. For more information visit our pages on healthy eating.
Where did the story come from?
The study was carried out by Professor Tim Key of the University of Oxford, who is funded by Cancer Research UK. It was published in the peer-reviewed British Journal of Cancer.
The author was a member of the Methodology Task Force for the World Cancer Research Fund systematic review.
What kind of research was this?
This was an overview of the evidence relating to the effect of fruit and vegetables on the risk of developing cancer. The author selected the studies that he included in this overview himself. This is not the same as conducting a systematic review, in which literature databases are searched methodically to identify all the relevant studies. The author has included other systematic reviews in his assessment, however, such as those conducted by the World Cancer Research Fund, which he was also involved in writing.
Smoking is known to be the predominant risk factor for a range of cancers including lung, mouth and oesophageal cancers. Alcohol consumption and being overweight have also been identified as significant risk factors for some cancers. The role of diet, however, is less clear.
Assessing how diet affects health is often difficult, as it is hard to get an accurate and unbiased record of people’s dietary habits, especially in the long term. Given that cancers may take years to develop, an understanding of lifetime habits is important. This review summarises some of the evidence for the role of fruit and vegetable consumption in reducing the risk of a number of cancers. The systematic reviews and meta-analyses conducted by the World Cancer Research Fund and others looked at a range of dietary factors in specific cancers, but this researcher has suggested further areas where more research is needed.
What did the research involve?
The author does not describe the criteria that were used to select the studies that were included, which in the main are large prospective cohort studies. He states that the report is a summary of the evidence on selected common cancers and that he has concentrated “on the results from large prospective studies or pooled analyses” as these are more reliable. The article does not describe how these were identified, or whether studies published in languages other than English were considered.
The results of these studies were not pooled for statistical assessment, as is done in a meta-analysis. This makes it difficult to come to any further conclusions regarding eating fruit and vegetables other than what the original studies found.
What were the basic results?
The review groups the findings by a number of cancer types:
(i) Cancers of the oral cavity and pharynx
The most significant risk factors for these cancers are smoking and alcohol consumption. The author says that the evidence that eating more fruit and vegetables protects against these cancers comes from comparatively small studies, which could be influenced by the fact that people who eat fewer portions of fruit and vegetables are also more likely to smoke and drink.
(ii) Oesophageal cancer
Smoking, alcohol consumption, obesity and gastro-oesophageal reflux are the main risk factors for oesophageal cancer. There are few prospective studies available, but there is some evidence that ‘adequate intakes’ of fruit and vegetables may reduce the risk of squamous cell oesophageal cancer, though this may still be influenced by correlations with drinking and smoking habits.
(iii) Stomach cancer
High salt intake and chronic infection with Helicobacter pylori bacteria are primary risk factors for stomach cancer. Large prospective studies have failed to support theories that risk of stomach cancer can be reduced by increased fruit and vegetable consumption in populations that are already well-nourished.
(iv) Colorectal cancer
Overall, diet is thought to be an important risk factor in the development of colorectal cancer, but the role of specific aspects of diet have not been established. The extent to which fruit and vegetables cut the risk of colorectal cancer through the increased consumption of dietary fibre is unclear.
(v) Lung cancer
More than 80% of lung cancers in Western countries are caused by smoking. Although some studies have indicated that lung cancer sufferers may report eating fewer portions of fruit and vegetables than healthy controls, the association is small compared with smoking.
(vi) Breast cancer
Reproductive and hormonal factors are the main influences of breast cancer risk, followed by obesity and alcohol consumption. In general, the evidence for the role of specific dietary factors in breast cancer risk is unclear.
(vii) Prostate cancer
In general, the causes of prostate cancer are not well understood. No conclusive associations have been reported between fruit and vegetable consumption and prostate cancer risk.
(viii) Overall cancer risk
Findings from a number of large prospective studies suggest that overall cancer risk has little association with fruit and vegetable consumption, except with respect to cancers already strongly associated with smoking and drinking. Clinical trials of dietary supplements of antioxidants have shown little benefit outside poorly nourished populations.
How did the researchers interpret the results?
The author says that, historically, the idea that eating more fruit and vegetables can produce important reductions in population cancer rates has been “unduly optimistic”.
He discusses the methodological difficulties that have affected research in this field, particularly with respect to obtaining accurate data on what individuals are currently eating or have eaten in the past. There is also the problem of ‘confounding’, which is the possibility that the apparent relationship between a risk factor and a disease is actually being caused by some other risk factor that has not been measured or is unknown.
The author recommends directions for future research and suggests that his findings do not “imply that there are no beneficial effects to discover, but future progress will depend on better understanding of the mechanisms by which cancer develops”.
He concludes by stating that “advice in relation to diet and cancer should include the recommendation to consume adequate amounts of fruit and vegetables, but should put more emphasis on the well-established adverse effects of obesity and high alcohol intakes on cancer risk”.
This review confirms the existing understanding that smoking, drinking and being overweight are all significant risk factors for developing a range of cancers. In its assessment of the influences of eating fruit and vegetables on cancer risk, the following points should be considered:
- This paper did not set out to use the robust methodology of a systematic review, in which all research that meets previously specified criteria is identified from a range of sources, regardless of study size, source or publication language. Although choosing to focus on the larger cohort studies may present the bulk of the evidence, it is possible that small studies could also contribute valuable evidence when combined together. A systematic review of this broad topic would be a considerable undertaking, however, and has been attempted by others.
- This study did not consider the full range of cancers, although the focus on the most common is appropriate from a public health point of view.
These limitations mean that the role of fruit and vegetables in the development of cancer cannot be completely discounted based on this overview alone. However, though the effectiveness of fruit and vegetables in preventing cancer may be debatable compared with other interventions, the wider benefits to health of a balanced diet (such as weight loss and reduced risk of cardiovascular disease) are well established.
Obesity and being overweight are known to be an important influence, not only on cancer risk but also on cardiovascular disease. Eating fruit and vegetables as part of a balanced diet is a good way to lose weight or maintain a healthy weight, so there are still indirect benefits.
The important message here is that other modifiable risk factors have much clearer effects on the risk of cancer, and drinking less and giving up smoking may be better ways to reduce risk than eating more greens. Further focused systematic reviews of the evidence for each of the cancers may shed more light on the relative role of dietary factors in cancer risk.