Daily changes in social support preceded and accompanied daily changes in smoking particularly after a self-set quit date. Findings emphasize the need for a prospective daily diary approach to understand the dynamics of social support in smoking cessation.

Background: Most theories of health-behavior change focus exclusively on individual self-regulation without taking social factors, such as social support, into account. This study's first aim was to systematically test the added value of received instrumental and emotional social support within the Health Action Process Approach (HAPA) in the context of dietary change. In the social support literature, gender effects emerge with regard to the effectiveness of social support. Thus, a second aim was the examination of gender differences in the association of social support with dietary behavior. Methods: Participants were 252 overweight and obese individuals. At baseline and 12 months later, participants completed questionnaires on HAPA variables; diet-specific received social support and low-fat diet. Results: For the prediction of intentions 12 months later, instrumental support was more beneficial for men than for women over and above individual self-regulation. In terms of dietary behavior at T2, a moderate main effect of instrumental support emerged. Moreover, received emotional social support was beneficial for men, but not for women in terms of a low-fat diet 12 months later. Conclusions: Effects of received instrumental social support found in this study provide new evidence for the added value of integrating social support into the HAPA.

The results confirm the importance of examining both self-regulation and social factors in smoking cessation. This should be considered when developing future interventions for smoking cessation.

The findings are in line with previous results, indicating invisible support to have beneficial relations with affect. However, results emphasize the need for further prospective daily diary approaches for understanding the dynamics of invisible support on smoking cessation. Statement of contribution What is already known on this subject? Social support receipt from a close other has proven to have emotional costs. According to current studies, the most effective social support is unnoticed by the receiver (i.e., invisible). There is empirical evidence for beneficial effects of invisible social support on affective well-being. What does this study add? Confirming benefits of invisible social support for negative affect in a health behaviour change setting Providing first evidence for detrimental effects of invisible social support on smoking.

The results provide evidence for the phase-specific distinction of self-efficacy in the context of dietary change in an overweight or obese sample. Thus, differentiating between motivational and volitional self-efficacy beliefs should be considered when developing future interventions of dietary change.