Abstract: A total of three studies met the inclusion criteria, allocating 2971 participants to the intervention and control groups. The total number of participants randomised to the intervention groups was 1467, whilst 1504 were randomised to the control groups. The length of intervention was 9, 12 and 24 months in the different trials. The use of TTM SOC in combination with diet or physical activity, or both, and other interventions in the included studies produced inconclusive evidence that TTM SOC interventions led to sustained weight loss (the mean difference between intervention and control groups varied from 2.1 kg to 0.2 kg at 24 months; 2971 participants; 3 trials; low quality evidence). Following application of TTM SOC there were improvements in physical activity and dietary habits, such as increased exercise duration and frequency, reduced dietary fat intake and increased fruit and vegetable consumption (very low quality evidence). Weight gain was reported as an adverse event in one of the included trials. None of the trials reported health-related quality of life, morbidity, or economic costs as outcomes. The small number of studies and their variable methodological quality limit the applicability of the findings to clinical practice. The main limitations include inadequate reporting of outcomes and the methods for allocation, randomisation and blinding; extensive use of self-reported measures to estimate the effects of interventions on a number of outcomes, including weight loss, dietary consumption and physical activity levels; and insufficient assessment of sustainability due to lack of post-intervention assessments. The evidence to support the use of TTM SOC in weight loss interventions is limited by risk of bias and imprecision, not allowing firm conclusions to be drawn. When combined with diet or physical activity, or both, and other interventions we found very low quality evidence that it might lead to better dietary and physical activity habits. This systematic review highlights the need for well-designed RCTs that apply the principles of the TTM SOC appropriately to produce conclusive evidence about the effect of TTM SOC lifestyle interventions on weight loss and other health outcomes.

Plain-language summary: We included three studies in our systematic review. Altogether the studies evaluated 2971 participants, with 1467 participants allocated to the intervention groups and 1504 to the control groups. The studies had a length of intervention of 9, 12 and 24 months. This plain language summary was current as of December 2013. The use of the TTM SOC in combination with diet or physical activity, or both, and other interventions in the included studies provided inconclusive evidence about the impact of such interventions on sustainable weight loss (mean difference in favour of the TTM SOC was between 2.1 kg and 0.2 kg at 24 months). However, other positive effects were noted, such as changes in physical activity and dietary habits that included increased exercise duration and frequency, reduced fat intake and increased fruit and vegetable consumption. The studies did not report other important outcomes such as health-related quality of life, illness (morbidity) and economic costs. Overall, the quality of the evidence was low or very low. The main limitations included incomplete reporting of outcomes, methodological shortcomings, extensive use of self-reported measures and insufficient assessment of sustainability due to the lack of long-term assessments.