Fat, breast cancer link raises questions on research methods

Related tags Breast cancer Nutrition

Scientists in the UK said on Friday they had found evidence of a
link between consumption of fat and breast cancer, a theory so far
poorly supported by human trials. Their study raises questions
about the methods of assessing dietary intake, showing that
food-frequency questionnaires could be less reliable than thought.

Scientists in the UK have found evidence to support the association between consumption of fat and breast cancer. They claim that imprecise methods of assessing dietary intake could be to blame for so far obscuring a link between a fatty diet and the cancer, a theory which has been suspected for some time, but remains unconfirmed by human trials.

Writing in a research letter in The Lancet​, the authors described how they assessed the relation between breast-cancer risk and fat intake with a food frequency questionnaire similar to those used in previous population (cohort) studies, alongside a seven-day food diary completed by women in the European Prospective Investigation of Cancer and Nutrition (EPIC) Norfolk study.

Women in the upper quintile (top 20 per cent) for saturated-fat consumption, eating more than 90g of fat per day, were at twice the risk of breast cancer than women in the lowest quintile (less than 40g of fat daily) when fat intake was assessed by the food diary; however no association was evident between increased saturated fat intake and breast-cancer risk with use of the FFQ.

Studies in which biological markers have been used to track dietary intake for selected nutrients suggest that the degree of error associated with food-frequency questionnaires (FFQs) is considerably larger than previously estimated, noted the authors. This could explain the lack of an association between increased fat intake and breast cancer in population studies.

Sheila Bingham from the MRC Dunn Human Nutrition Unit, Cambridge, UK, and Cambridge University colleagues studied around 13,000 women using the FFQ and food diary between 1993 and 1997. By 2000, 168 women had developed breast cancer. Fat intake was assessed for each of these women compared with four healthy controls matched for age and other factors to take account of possible bias.

Sheila Bingham said: "Inconsistency between experimental and epidemiological data on fat and breast-cancer risk could thus be accounted for by problems with methods used in cohort studies to measure diet. The food diary is more expensive to code for conversion into nutrients than the FFQ, but we have shown that its use is acceptable and feasible in large cohort studies."

"No biomarkers exist for fat intake, so none of the associations shown can be said to be free of measurement-error effects. However, our preliminary findings suggest that use of the food diary can detect relations between diet and cancer risk within a relatively homogeneous population,"​ she added.

Last week, however, a study by researchers from Brigham and Women's Hospital and Harvard Medical School reported a link between intake of animal fat and risk of breast cancer, in one of the first prospective studies to support the theory. The researchers used food frequency questionnaires to assess the risk.

In an accompanying commentary in The Lancet​(p 182), Ross Prentice from the Fred Hutchinson Cancer Research Center, Seattle, wrote: "Bingham and colleagues' report highlights the importance of the methodological issues in nutrition and chronic disease research by providing an example in which two valuable dietary self-reporting instruments give qualitatively different results. Their report will help to move the debate about measurement error in dietary assessment, which has been ongoing for several years, into a more practical arena."

He added however, that it would be useful to compare disease-risk associations with food diary and food frequency assessments of fat intake in additional cohort studies, "as the Norfolk-EPIC study is based on a modest number of cases of breast cancer - and especially because the estimated relative risks across fat intake quintiles (whether based on food diaries or food frequency questionnaires) seem larger than would be expected from the international correlational analyses that provided much of the stimulus for the fat and breast cancer hypothesis."

"More generally, the reliability and interpretation of cohort-study data on the controversial topic of an association between dietary fat and breast cancer will unfortunately remain unclear until further objective information becomes available about the measurement properties of the dietary assessment methods used,"​ he concluded.

Bingham and colleagues' results found the risk of breast cancer to be related particularly to saturated fat found mostly in high fat milk, butter, meat and some cereals such as biscuits and cakes.

Dr Lesley Walker, director of Science Information at Cancer Research UK, said: "Studying the link between diet and cancer is a complex business. The EPIC study continues to uncover more information about how the two can be connected. This research highlights the importance of eating sensibly to reduce cancer risk."

EPIC is part of a large European project studying 500,000 people in ten different countries to investigate the link between food and the development of cancer. It is jointly funded by the Medical Research Council, Cancer Research UK, The Europe Against Cancer Programme of the European Commission, the Food Standards Agency and the British Heart Foundation.

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