A retrospective analysis was conducted to assess weight change among 3621 subscribers (2979 women; 642 men) to a commercial internet-based weight loss program. Program engagement (indexed with frequency of using online diet and exercise diaries and with use of the social support forums) was associated with weight loss in both men and women after controlling for initial BMI and duration of participation. These engagement variables accounted for 13% of variance in percentage weight loss in women (p<0.001) and 19% in men (p<0.001). Exercise diary use predicted weight loss among men but not women and the use of online forums predicted weight loss among women but not 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 (p<0.001 for both men and women). Being in the highest tertile of engagement with exercise diaries was associated with clinically significant weight loss in men (p<0.001) and, less strongly, in women (p<0.05). These results suggest that the use of self-monitoring tools and participation in online support are predictive of weight loss. Int J Behav Nutr Phys Act. 2011 Aug 2;8:83. PMID: 21810222
