The study will involve an open randomised parallel group control trial with patients allocated either to the control or investment intervention group. All participants will complete measures of their beliefs about food, diet and exercise behaviour, intentions to change their behaviour and weight at baseline (2 weeks pre surgery) and at 3, 6 and 12 months follow up. Those in the investment group will complete carefully framed questions designed to raise the salience of the investment they have made in their procedure at baseline the 3 and 6 months after surgery. The content of this intervention will be tailored to the recent experiences of the patient (i.e. pre or post-surgery). This study has received favourable ethical opinions from the Bloomsbury Research Ethics Committee, National Research Ethics Service UK and the University of Surrey Ethics Committee. The structure and reporting of this trial will be guided by the CONSORT statement for clinical trials .
A pilot study has already been completed which illustrates that a low cost, easy to administer investment based intervention can change beliefs about food and improve weight loss in bariatric patients . This was based upon previous research and is grounded in research on the mere measurement effect, the impact of incentives and the placebo effect. The present study is an extension of this work using a larger sample, pre and post-surgery interventions and measurements and a longer term follow up.
University College Hospital (UCH) in London, UK, offers a NHS based standardised bariatric service for obese patients with a BMI over 40 (or 35 with serious co morbidities). Patients will be recruited if they have been approved for surgery and are attending the hospitals bariatric surgery pre-assessment clinic.
Patients will be included if they consent, are aged 18 or over, have attended the bariatric clinic at UCH, been accepted for surgery and have funding in place (i.e. the CCG has agreed to pay for their surgery). Recruitment will take place over a 14 month period.
The effectiveness of the proposed investment based intervention has been preliminarily examined in the pilot study conducted by Husted and Ogden . The study showed that with only 98 participants (48 in intervention and 50 in the control groups) there was a significant effect of the intervention on weight loss by 3 months follow up with a mean difference of 7 kg between the groups. This resulted in a small / medium effect size (d = 0.3). The present study aims to assess outcomes up to 12 months thus raising the risk of attrition over time. In addition, the study will have a baseline measure of weight pre surgery rather than after surgery. Therefore, assuming an attrition rate of 40% by 12 months with an alpha of 0.05 and a beta of 0.8 we propose to invite 200 participants (100 in each condition) to ensure that data from at least 120 participants is collected at the 12 months follow up (60 in each group). This should provide statistical power to detect a small to medium difference in weight (controlling for baseline weight) by 12 months follow up.
Two weeks before their operation patients attend the bariatric clinic for routine pre-operative tests. At this point all patients will see the researcher who will explain the trial, obtain consent and randomly allocate the patient to either the control or the investment based weight loss intervention condition.
Once a patient is consented the researcher will use the third party blinded randomization process provided by the clinical trial unit at the University of Surrey to allocate them to either the investment based intervention condition or the control group, using random number tables according to surgery type.
Those allocated to the control group will receive usual care and complete a questionnaire at baseline then 3, 6 and 12 months after bariatric surgery.
Investment intervention group
Those allocated to the investment based intervention will complete the investment based intervention at baseline (2 weeks pre surgery) then 3 and 6 months post-operatively. This intervention is based on the pilot study  and involves participants rating questions relating to the investment they have made in having bariatric surgery as a means to raise the salience of their investment. In particular, the questions will encourage patients to consider the ways in which the surgery has impacted upon them in terms of financial, social, personal and physical costs and focuses on factors such as pain, disruption to their family, social and work lives and financial burden. The investment intervention will be tailored to the recent experiences of the participants. To this end the questions and responses are framed in such a way as to emphasise investment in the process of bariatric surgery in order to optimise the outcomes of the procedure. They will also complete a questionnaire at baseline then 3, 6 and 12 months after surgery.
Primary outcome measures
BMI and weight: Patients’ weight will be obtained in the clinic to provide the primary baseline and endpoint measure of the trial. This will be collected preoperatively 2 weeks before surgery, immediately after surgery and postoperatively at 3, 6 and 12 months follow up. In addition baseline measures of age, sex, height, type of surgery, educational level, and ethnicity will also be taken.
Secondary outcome measures
Beliefs about food; including hedonic wanting using the Power of Food Scale (PFS) , hedonic liking relating to food palatability preference using written representations of food types replicating examples suggested in previous studies [20,21]. Behavioural intentions including patients’ intentions to eat foods high in fat and sugar content and diet and exercise behaviour using measures of snack and meals intake that have been used extensively in previous research [22,23].
The data will be analysed to explore the impact of the investment based weight loss intervention on patient’s weight and BMI controlling for baseline measures using ANCOVA. Furthermore the impact of the investment intervention will be explored with regard to patients’ beliefs about foods, behavioural intentions and diet and exercise behaviours using repeated measures ANOVA. Finally the data will be analysed to assess the role of changes in beliefs about food, behavioural intentions and behaviour in predicting weight loss by follow up using mediation analysis. Any baseline differences will be included as covariates in the analysis where necessary.