Self-monitoring frequency in increasing fruit and vegetable intake: A randomized experiment

Objective: According to psychological theories including Social Cognitive Theory and Control Theory as well as empirical evidence, self-monitoring of diet is an effective behavior change technique, yet adherence is often low. Indeed, daily self-monitoring may not be necessary for clinically meaningful outcomes, though evidence on what constitutes enough self-monitoring is observational. This preregistered study experimentally tested whether different frequencies of self-monitoring fruit and vegetable (FV) intake produce differential changes in FV intake, body weight, and psychological constructs.Methods: Participants were randomly assigned to three conditions: 18, 24, or 30 self-monitoring days across a 30-day period. In total, 166 participants (70.5% women; age: M = 25.3, SD = 9.5; BMI: M = 23.8, SD = 3.0) completed the study. Psychological constructs included dietary self-efficacy and the perceived ability to identify patterns in one’s diet, set goals accordingly, and recognize which changes are needed.Results: A mixed ANOVA found a main effect for time, p < .001, but not condition, p = .123, and a non-significant interaction effect, p = .280, indicating that FV intake increased in all three conditions and that changes did not differ between conditions. No weight changes were found. Significant declines in psychological constructs over time were observed in at least one condition (ps ≤ .003), with particularly strong and consistent declines in dietary self-efficacy.Conclusions: The study provides experimental evidence that daily self-monitoring is not required to achieve meaningful changes in FV intake. Lower-frequency self-monitoring may represent a feasible and less burdensome approach to dietary behavior change.