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Cancer and risk factors: the evidence

Thursday 1 November 2007

“The new rules for defeating cancer” was the front-page headline today in The Times - part of widespread press coverage that has been given to a report on diet and the risk of cancer.

The report, issued by the World Cancer Research Fund, has involved hundreds of experts who have reviewed all the evidence we have to date about the link between food, nutrition, weight gain, overweight and physical activity and the risk of cancer. The report presents findings and recommendations to decrease cancer risk.

The researchers conclude with 10 major "commandments"  (listed below). The predominant themes which feature in the news reports are that staying slim and avoiding processed meat and alcohol are key to avoiding cancer.

Professor Michael Marmot, an epidemiologist at University College London, chairman of the panel that produced this report, features in much of the coverage as suggesting that the direct link between increased weight and increased cancer risk was even stronger than that associated with smoking.

Essentially, the report is a series of 20 systematic reviews of all the evidence pertaining to food, nutrition, weight and physical activity on the risk of cancer, and is the best collection of all the research we have so far.

Importantly, readers should remember that cancer is not caused by single individual factors, such as eating bacon or drinking wine. Prevention programmes will need to take into account the variety of health determinants and the interplay between all the different risk factors.

Readers would be advised to interpret these findings as the panel has, as being soundly based, and that “when translated into effective public policy programmes and personal choices, [they] will reduce the risk of cancer”.

Where did the story come from?

This report was commissioned by the World Cancer Research Fund (WCRF) and the American Institute for Cancer Research. Hundreds of experts in several specialist working groups had particular responsibilities in the course of preparing the report. A methods group established a framework by which the systematic reviews could be conducted; peer reviewers assessed the methods and the draft and final reports, and advisers from international institutes and task forces were also involved.

The report is available to the public online (

What kind of scientific study was this?

The report was undertaken by the WCRF to build on a report they released in 1997 entitled Food, Nutrition and the Prevention of Cancer: a Global Perspective. This second report by the WCRF was commissioned to answer remaining questions and to consider new studies that had been published since the first report. It is a comprehensive consideration of all the evidence on food, nutrition, weight gain, overweight, obesity and physical activity on risk of cancer.

Essentially, the report is a series of 20 systematic reviews produced by nine scientific centres in Europe and North America. The systematic reviews were conducted using a methodology that had been especially developed by a panel of experts. This was to ensure consistency across the search for literature, its analysis and reporting. The systematic reviews contained all the evidence published up until 2005. Each systematic review was peer-reviewed twice by experts, at the beginning of the process and then at its completion. The 20 individual systematic reviews will soon be available on the website. Now they can be ordered on the CD that accompanies the report.

The systematic reviews formed the foundation on which the expert panel (“21 of the world’s top researchers in this area, with the support of independent observers”) graded the evidence (i.e. decided whether the evidence was good overall) and drafted recommendations around food and drinks, physical activity, growth, development, body composition, and determinants of weight gain, overweight and obesity.

What were the results of the study?

In the second part of the report, the results are presented as descriptions of the evidence and recommendations around all the possible risk factors. There are hundreds of pages of results, so we attempt a summary here. We do not report on the specific types of cancer to which the results pertain. When considering all the research together the panel made the following recommendations:

  • There is evidence of a convincing or probable causal link between certain foods and a reduced risk of some cancers including: foods containing dietary fibre, vegetables and fruits, milk, calcium supplements (which protect against colorectal cancer) and  selenium supplements (which protect against prostate cancer).
  • The following can increase the risk of cancer: alcoholic drinks (they say that evidence that alcoholic drinks are a cause of particular cancers has strengthened), red meat, processed meats (meats preserved by smoking, salting, or curing) which includes bacon, ham and salami (which are linked to colorectal cancer), a diet very high in calcium, salt and salty foods, beta-carotene supplements (high doses increase risk of lung cancer in smokers).
  • For other factors, there is limited evidence that the link is ‘causal’. For these, the researchers have decided that the evidence is so limited that no conclusions can be drawn. These include fish and food containing vitamin D and colorectal cancer, smoked foods and stomach cancer, milk for bladder cancer, total fat intake for lung cancer or postmenopausal breast cancer, foods containing animal fat and colorectal cancer; and butter and lung cancer.
  • For some factors, the researchers have concluded that an effect on risk is unlikely. With coffee for example the researchers say “it is unlikely that coffee has a substantial effect on the risk of cancer either of the pancreas or of the kidney”.
  • The panel concluded that there is convincing evidence that physical activity protects against colon cancer, and evidence that it probably protects against postmenopausal breast cancer and cancer of the endometrium. There is limited evidence that exercise offers protection against premenopausal breast cancer, cancers of the lung and pancreas. They also conclude that since physical activity protects against overweight, weight gain and obesity, that it also then “protects against cancers for which the risk is increased by these factors”. They say that “the evidence is consistent with the message that the more physically active people are, the better”.
  • When considering the evidence around body weight, the panel concluded that “body fatness” and “abdominal fatness” increase the risk of cancer (various types including oesophagus, pancreas, breast, kidney, colorectal, kidney, gall bladder). They say that the evidence that “body fatness is a cause of cancers of various sites” is more impressive now than it was in the mid-1990s.

What interpretations did the researchers draw from these results?

Based on the evidence, the researchers make 10 recommendations. They say that their recommendations do not take into account wider socioeconomic, cultural and other determinants.

Of relevance to the news stories, they recommend that people should be as lean as possible within the normal range of body weight, they should be physically active as part of everyday life, they should limit consumption of energy-dense foods and sugary drinks, eat food of mainly plant origin, limit intake of red meats, and avoid processed meats, and limit alcoholic drinks.

What does the NHS Knowledge Service make of this study?

In this huge report, hundreds of experts all over the world have made the best effort so far to assess, based on the evidence that is available through research, what factors increase the risk of cancer. They have tried to distinguish between evidence that is strong enough to suggest that the associations may be “causal” and that where there is not enough evidence to say that this is the case.

A follow up report to this one – the Policy Report – which will address how such behaviours are established over a lifetime and will provide advice and direction on what can be done to influence lifestyle choices, will be published in 2008. This report will consider evidence from two systematic reviews, one looking at what determines diet and physical activity patterns and a second that considers how effective existing interventions are. The panel was keen to stress that “weight gain, overweight, and obesity, and their antecedent behaviours, are critically determined by social, cultural, and other environmental factors”, which will be considered in the Policy Report to be published in 2008.

Some specific points:

  • Without looking in detail at all the reviews behind this report, it is not possible to make specific comments on the quality of the reviews. However, considering the teams of professionals involved in writing and planning this report, we expect that quality will be high.
  • The researchers note that their conclusions are based on the best evidence that is available, however, this may not be a ‘complete picture’ as ‘it reflects recent research priorities mostly in high-income countries’.
  • Also important to note is the fact that though the report is assessing the strength of the link between individual risk factors, such as “being overweight” and cancer risk, the risk factors themselves interplay with each other in complex ways. Approaches to cancer prevention will need to take into account all of these factors in a holistic way.

Readers would be advised to interpret these findings as the panel has, as being soundly based, and that “when translated into effective public policy programmes and personal choices, [they] will reduce the risk of cancer”.

Sir Muir Gray adds...

About 20 years ago Richard Doll and Richard Peto estimated that about one-third of all cancers were due to diet - about the same as cigarette smoking. Since then, the evidence has accumulated and become clearer.

One important point to make, is that the report should perhaps have been called diet, exercise and cancer, or modern lifestyle and cancer, because the motor car is as much responsible for obesity as our diet.

The balance of our diet is a matter for choice, but the amount we eat needs to compensated by the amount of exercise we take. Watch what you eat and take an excellent extra 3000 steps a day.

Analysis by Bazian
Edited by NHS Website