Background: The Internet provides a widely accessible platform for weight loss interventions. Automated tools can allow self-guided monitoring of food intake and other _target behaviours that are established correlates of weight change. Many programmes also offer social support from the virtual community. The aim of this research was to assess associations between engagement with self-monitoring tools and social support, and weight loss in an online weight-control programme.

Methods: This paper describes a retrospective analysis of weight change among 3621 subscribers to a commercial Internet-based weight loss programme. Participants were all subscribers (2979 women; 642 men) joining the programme between July 2005 and November 2008 with two or more recorded weights spanning at least 28 days of participation in the programme. Engagement was indexed with frequency of using online diet and exercise diaries and with use of the social support forums.

Results: Programme engagement was associated with weight loss in both men and women after controlling for initial BMI and duration of participation. The three engagement variables accounted for 13% of variance in percentage weight loss in women (p < .001) and 19% in men (p < .001). In analyses including all the engagement variables, exercise diary use was an independent predictor of weight loss among men, but non-significant in women. In contrast, use of the online forums was associated with weight loss in women but not in men. Among participants who were overweight or obese, those in the highest tertile of engagement with food diaries (vs the lowest) were more likely to achieve clinically significant (> 5%) weight loss (men: OR = 3.45 p < .001; women: OR = 5.05 p < .001). Being in the highest tertile of engagement with exercise diaries was associated with clinically significant weight loss in men (OR = 3.48 p < .001) and, less strongly, in women (OR = 1.46 p < .05).

Conclusions: Use of self-monitoring tools and participation in online support are predictive of weight loss in the context of a commercial, online weight control programme.