Obesity is caused by people consuming more energy than they expend and is associated with reduced life expectancy and many serious conditions including heart disease, stroke, diabetes, cancer, gallstones, fatty liver disease and sleep apnoea [1,2]. Currently almost two-thirds of UK adults are either overweight or obese with overall costs to society forecast to reach £50 billion per year by 2050 on current trends [2].

Although the most common form of obesity management emphasizes changes in diet and exercise, research indicates that weight loss surgery (WLS) is most effective and this approach is currently recommended for those whose BMI is greater than 40 kg/m2 (or 35 kg/m2 with comorbidities) [3]. A systematic review of WLS [4] concluded that the mean percentage excess weight loss (EWL) for the Roux-en-Y gastric bypass was 67% and for the gastric band was 42% at one year. WLS has also been shown to result in improvements in a number of other patient outcomes including quality of life, mood, subjective health status and perceptions of eating control [5,6].

Although WLS is currently one of the most frequently performed procedures in the US and Europe [7], questions have been raised about the long-term durability of weight loss as research indicates that a substantial proportion of individuals begin to regain lost weight over time [5,8-10]. In response to growing evidence that WLS does not work for everyone research has attempted to understand this variability. In particular, research has explored the mechanisms involved in successful and failed bariatric surgery to highlight how effectiveness could be improved [11,12]. The results indicate that less successful surgery is associated with feeling unprepared for the changes required after surgery, reporting being unsupported in the time following surgery and a sense that although surgery fixes their body, psychological issues relating to dietary control, self-esteem, coping and emotional eating remain neglected. These studies also explored successful weight loss following surgery and reported a role for a reduction in hunger, a decrease in the preoccupation with food and sense of improved control. Such research also highlights a central role for investment and this is supported by other studies using medical and behavioural interventions. For example, bariatric patients reported that success was associated with a feeling that they had made an investment in their operation in terms of factors such as time of work, financial cost, disruption to family and social life, the pain of the surgery and the process of recovery. As one participant said after surgery ‘the amount of pain, the operations performed. Don’t want to do any damage, don’t particularly want the stomach to enlarge anymore which it can do’. Similarly, research indicates that obesity medication may work through the greater investment needed for adherence due to unpleasant side effects [13]. Further, success from behavioural interventions is associated with perceptions of greater investment in attempts to lose weight such as joining self-help groups or attending organised slimming clubs. From this perspective greater investment into weight loss attempts increases the chances of success by motivating the individual to make the most of the efforts they have made so far and encourages them to maximise the consequences of these efforts.

Perceived investment therefore motivates change across all areas of obesity management. This perspective finds reflection in the body of work on the effect of financial incentives on behaviour change with greater costs either facilitating positive behaviours or deterring unhealthy behaviours [14]. It is also parallel to research on the placebo effect which illustrates that placebos which involved greater investment (i.e. bigger pills, expensive treatments, painful interventions, lengthy consultations, greater distances travelled) are more effective that those which are easier to take part in [15]. In line with this approach, a recent pilot study explored the impact of manipulating perceived investment in patients just after they had had bariatric surgery [16]. Patients (n = 98) were recruited from an online support group for bariatric patients and were randomly allocated to either the control or intervention group. Those in the intervention group then completed a simple series of carefully framed questions designed to encourage them to consider the investment they had already made in having their operation. For example, they were asked to rate the procedure in terms of their pain experienced, the time spent organising the operation, the disruption to their family and social life, the time needed to recover and the impact of the recovery process on their family and work. This approach was inspired by recent research exploring the ‘mere measurement’ effect which illustrates that simply completing framed questions can change the ways in which people think and behave [17,18]. The results from this pilot study indicated that the investment intervention resulted in an immediate change in participants’ beliefs about food and their intentions to change their diet and exercise behaviour. By three months follow up the investment group reported losing 7 kg more than the control group (a significant difference). This pilot study was small scale and involved patients post-surgery only. In addition, follow up data was only collected in the short term and attrition was high by this time point (probably due to the online nature of the study). The present study aims to develop this investment based intervention using a larger sample, with a longer follow up and involving assessments at both pre and post-surgery in order to generate a proper baseline. Bariatric surgery is therefore the treatment of choice for obese patients. Not all patients, however, lose the desired amount of weight. Research highlights a role for investment which lends itself to a low cost, easy to administer intervention that may promote successful weight loss in the longer term. Therefore the aim of this study is to evaluate the impact of an investment based intervention compared to usual care on patient weight loss, beliefs and behaviours after bariatric surgery.

Obesity is a risk factor for illnesses such as heart disease, diabetes and cancer. If effective, obesity surgery improves a patient’s health and reduces their need for NHS care. If unsuccessful then the costs include not only subsequent NHS costs due to these other illnesses but also the costs of the unsuccessful operation and the emotional cost to the patient. The investment based intervention aims to help improve the effectiveness of surgery which in the longer term is likely to be cost effective. This research is based upon a small scale pilot study that showed that a simple investment based intervention which encouraged patients to consider their personal investment in having weight loss surgery, improved weight loss and if this is reproduced in the current larger scale study then such a simple, easy to administer and low cost intervention could become routine practice for bariatric patients.