Dietary assessment instruments are susceptible to intervention-associated response set bias

Objective Evaluations of trials of the effectiveness of dietary intervention programs may be compromised by response set biases, such as those attributable to social desirability. Participants who receive a behavioral intervention may bias their reports of diet to appear in compliance with intervention goals. This study examined whether responses to standard dietary assessment instruments could be affected by a brief dietary intervention. Design We assigned 192 undergraduate students randomly to (a) see a 17-minute videotape on the consequences of eating a high-fat diet or a placebo videotape on workplace management and (b) receive preintervention and postintervention assessments or only postintervention assessment. Dietary assessments included 4 independent measures of fat intake. Results Among women, bias (intervention minus control) was -9.7 g fat (from a short food frequency questionnaire) and -0.6 high-fat foods (from a questionnaire about use of 23 foods in the previous day) (P<.05 for both). No results were significant among men or for 2 instruments that measured more qualitative aspects of fat-related dietary habits. Applications Even a modest dietary intervention can affect responses to dietary assessment instruments. Nutritionists should recognize that assessment of adherence to dietary change recommendations, when based on dietary self-report, can be overestimated as a result of response set biases.