Evaluating psychological accounts of diet-related mood improvements using novel control conditions

Several randomised controlled trials have now reported that improvements in diet quality can improve mood. Such trials are methodologically challenging to design, because of the dual difficulty in providing control conditions that match the treatment activity (e.g., planning, shopping, cooking), and that blind participants to the treatment condition. These concerns raise the possibility that well understood psychological processes may amplify or even drive treatment effects. In particular - self-efficacy - a person's belief in their ability to achieve specific goals (like cooking a meal) and - expectancy bias - a person's beliefs about the effects of food on mood, could both contribute to a treatment response. The current study sought to develop and undertake a preliminary test of diet control conditions, matched for self-efficacy and contrasting expectancy bias. Participants (n = 112) with low mood were randomised to one of four conditions, three involving diet-intervention: (1) fresh food (i.e., nutritional improvement); (2) canned/frozen food (i.e., nutritional improvement but with reduced expectancy); (3) cooking (i.e., diet change but without nutritional improvement); and (4) a wait-list control. Mood was measured at baseline and 3-weeks later post-intervention. The analysis revealed a significant improvement in mood in the three diet-intervention conditions, relative to the wait-list control, but with no difference between diet-interventions. Regression suggested changes in self-efficacy contributed to mood improvement. These findings imply that psychological factors are important to account for, and our approaches offer novel control conditions for future whole diet interventions.