"Food diaries are really good instruments because most Americans would be appalled at what they report"

http://www.science.tamu.edu/articles/976/

MODEL ANALYSIS: Texas A&M Statistician Probes Missing Link Between Diet and Cancer

COLLEGE STATION -- Cancer. It's a leading cause of death in the United States and a leading inspiration behind the lifetime research agenda of Texas A&M University Distinguished Professor of StatisticsRaymond J. Carroll.

According to the American Cancer Society, about one-third of the more than 572,000 U.S. cancer deaths that occur each year will be related to poor diet, physical inactivity or obesity and, thus, could be prevented.
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In addition to introducing error and uncertainty, food frequency questionnaires can't measure caloric intake, which Carroll says has a huge impact on obesity studies.

He considers the 24-hour recall method equally problematic, because it looks at a very small snapshot of dietary history when the long-term is more important. Moreover, it's expensive and, therefore, highly cost-prohibitive.

A better alternative to both methods is food diaries, in which respondents report actual eating habits for a week at a time on three or four different occasions throughout the year.

"Food diaries are really good instruments because most Americans would be appalled at what they report," Carroll adds. "They are a much better way of measuring, not to mention modifying."