A methodological report from the Malmö Diet and Cancer study: development and evaluation of altered routines in dietary data processing

Background: In the Malmö Diet and Cancer study, information on dietary habits was obtained
through a modified diet history method, combining a 7-day menu book for cooked meals and a diet
questionnaire for foods with low day-to-day variation. Half way through the baseline data
collection, a change of interview routines was implemented in order to reduce interview time.
Methods: Changes concentrated on portion-size estimation and recipe coding of mixed dishes
reported in the menu book. All method development and tests were carefully monitored, based
on experiential knowledge, and supplemented with empirical data. A post hoc evaluation study using
"real world" data compared observed means of selected dietary variables before and after the
alteration of routines handling dietary data, controlling for potential confounders.
Results: These tests suggested that simplified coding rules and standard portion-sizes could be
used on a limited number of foods, without distortions of the group mean nutrient intakes, or the
participants' ranking. The post hoc evaluation suggested that mean intakes of energy-adjusted fat
were higher after the change in routines. The impact appeared greater in women than in men.
Conclusions: Future descriptive studies should consider selecting subsets assessed with either
method version to avoid distortion of observed mean intakes. The impact in analytical studies may
be small, because method version and diet assistant explained less than 1 percent of total variation.
The distribution of cases and non-cases across method versions should be monitored.