The influence of time on dietary data: differences in reported summer and winter food consumption.

A pilot study was conducted to test whether or not the time of data collection affected subjects' responses to a dietary questionnaire and could therefore bias the results of a study. A 117 item food frequency questionnaire was administered to 43 subjects--16 in the summer and 27 in the winter. The summer and winter consumption per person per month was calculated and compared. Of the 117 foods, 105 (89%) showed differences between summer and winter consumption. While differences in summer fruit and hot cereal might be expected, this study showed that many other foods not usually thought of as seasonal were seasonally consumed by the population tested. Since food consumption is affected by supply and demand factors, and the reported memory of food intake is biased toward the present, the time of data collection could greatly influence reported intake for a great number of foods. Recommendations for enhancing dietary studies are given. These recommendations include the development of a gold standard of consumption for each food in a given population, incorporating the elements of person, place and time into study design, building the rate and length of data collection into the study plan and using the gold standard and improved design to produce a new generation of dietary studies.