For reasons of cost and logistics, food frequency questionnaires have been the dominant method in nutritional epidemiology for the self-reporting of food consumption for the past 20 years.

But, writing in The Lancet, researchers argue that errors associated with food-frequency questionnaires (FFQs) are larger than previously thought, and suggest they may have “masked” a link between breast cancer and increased fat intake.

A team funded by the government’s Medical Research Council (MRC) and the charity Cancer Research UK compared FFQs with detailed food diaries from 13,000 women between 1993 and 1997.

By the year 2000, 168 women had developed breast cancer. Analysis of fat intake was assessed for each woman with the disease and compared with four healthy controls. They found that women whose daily intake of fat was more than 90g had twice the risk of breast cancer of those who had eaten less than 40g daily.

The researchers’ identification of a link between increased saturated fat intake and breast cancer was not evident with the use of the FFQ, but appeared following the assessment of the food diary.

Dr Sheila Bingham, from the MRC Dunn Human Nutrition Unit, Cambridge, 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.”

Dr Bingham accepts that the team’s discovery of a relation between fat intake and breast cancer cannot be proved to be free of error. But she says that its preliminary findings suggest that the food diary can detect the association.

Commenting on the study, Dr Lesley Walker, director of science information at Cancer Research UK, said, “This research highlights the importance of eating sensibly to reduce cancer risk.”

And in an accompanying commentary, Dr Ross Prentice from Seattle’s Fred Hutchinson Cancer Research Center in the US, said the report “highlights the importance of the methodological issues in nutrition and chronic disease”. The study provides “an example in which two valuable dietary self-reporting instruments give qualitatively different results”, he said.

Dr Prentice called for more objective information to be made available about the measurement properties of dietary assessment methods.

Reference: Bingham et al, The Lancet 2003; 362: 212-214

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