N° 18 | December 2007

The methodology behind the Second Expert Report

The first step of WCRF/AICR’s colossal Second Expert Report project was to establish a process for deciding which studies were relevant and how best to analyse these as a basis for drawing conclusions. There was no established methodology for assessing data in the context of causation of disease, including the influence of diet on cancer. Therefore the methodology for the report was drawn up by a task force of 20 experts from several disciplines, including nutrition, epidemiology, cancer, laboratory research, systematic reviewing, and public health. The specification for reviewing the scientific literature was then published in a manual for all of the systematic literature reviews (SLRs) to follow.

Nine independent teams of scientists from institutions in the USA, UK and continental Europe were charged with collecting the evidence by carrying out SLRs on 17 different types of cancer, as well as on cancer survivors, on the determinants of obesity, and on authoritative reports on other chronic diseases such as heart disease. The initial sweep found half a million studies, of which 7,000 were deemed relevant.

Many different types of studies are used to investigate the prevention of cancer; all have strengths and weaknesses, but none is perfect. Even the randomised controlled trial, which works so well with medicines, has limitations when it comes to the study of chronic diseases like cancer and complex lifetime exposures like food and nutrition. WCRF/AICR’s comprehensive Second Expert Report has therefore used a portfolio approach to synthesise the evidence, consulting all types of research and taking account of the advantages and disadvantages of each.

WCRF/AICR commissioned and funded the Second Expert Report, but the content was driven by an independent panel of 21 worldrenowned scientists. The Expert Panel worked for five years to assess and compare the studies reviewed by the SLR centres, and its conclusions and recommendations are based firmly on scientific evidence. It also had formal observers from six international organisations: the World Health Organization (WHO) and the Food and Agriculture Organization (FAO) of the United Nations (UN), the UN Children’s Fund (UNICEF), the International Union of Nutritional Sciences (IUNS), the UICC (Union Internationale Contre le Cancer), and the International Food Policy Research Institute (IFPRI).

The Second Expert Report, Food, Nutrition, Physical Activity, and the Prevention of Cancer: A Global Perspective, has provided the most upto-date recommendations for individuals and populations. In coming to judgement, each factor that might affect cancer risk was graded according to the strength, quality, and quantity of the scientific evidence. The Panel rated the likelihood that a particular factor causes cancer or protects against it, as ‘convincing’ or ‘probable’. Or, if there was not adequate evidence, the Panel gave ratings of either ‘limited – suggestive’, or ‘limited – no conclusion’. Occasionally it was possible to conclude that a substantial effect on risk was unlikely. Only judgements of ‘convincing’ and ‘probable’ formed the basis for WCRF/AICR’s 10 recommendations.

The Report is based on the best evidence available now – it includes relevant studies published up to the end of 2006. But WCRF/AICR are committed to looking to the future, to continue to interpret scientific evidence in the field of food, nutrition, physical activity, and cancer prevention. By establishing a continuous review programme to update the report on an ongoing basis, WCRF/AICR will be able to ensure that its conclusions and recommendations remain current and robust. A select Expert Panel will regularly review and analyse new studies and the results will be published online and in special publications.  (http://www.dietandcancerreport.org/cu)

While the Second Expert Report presents personal recommendations as well as goals for whole populations, setting these targets is just one step. Equally important, is understanding how to achieve them successfully. For this reason WCRF/AICR has commissioned a subsequent report, Policy for Cancer Prevention: Food, Nutrition, and Physical Activity – A Global Perspective, to be published in November 2008. Targeting policymakers, this will address why people adopt particular eating and physical activity behaviours over a lifetime. And it will look at the impact of interventions or policies on those behaviours.

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