It is established beyond doubt that dietary [
27] and physical activity [
28] behaviours, driven at a population level by complex environmental factors [
29,
30], ultimately give rise to excess body weight, and as such, any treatment for obesity addresses this accumulation of excess energy through ‘lifestyle modification’ in some way. Moreover, several large, methodologically robust randomised controlled trials have established the benefits of participation in structured lifestyle modification programmes in distinct patient groups, including those with cardiovascular disease [
31], pre-diabetes [
32] or established type 2 diabetes [
33,
34]. Regarded by many as the most rigorously conducted structured lifestyle intervention trial ever undertaken, Look AHEAD [
35] (Action for Health in Diabetes) is particularly relevant to considerations about how best to help patients with severe and complicated obesity through changes in behaviour. The trial recruited more than 5000 adults who were overweight (mean BMI 36 kg m
−2) with type 2 diabetes from 16 centres in the USA and followed them for 10 years, randomising them to a ‘usual care’ diabetes structured education programme versus an ‘intensive lifestyle intervention’, with an ambitious individual weight loss target of 10%, including meal replacement, if necessary, in the intervention group. Compared to controls, those in the intensive lifestyle intervention lost more weight and had improvements in many important health related outcomes such as diabetes control and medication usage [
36], but the primary trial outcome of major adverse cardiovascular events was no different, even after extended follow-up. This may have been due at least in part to the much lower than expected cardiovascular event rate in the control group, and it would be incorrect to dismiss the overwhelming benefits demonstrated by the trial, but it ought not to constitute grounds for mandating participation in such a programme for all overweight patients, even just those with diabetes. It is worth noting that a post hoc analysis of Look AHEAD participants found that those who had bariatric surgery lost on average 19.3% of their body weight compared to 5.6% in the intensive intervention group and 3.3% in the control group and were almost seven times more likely to achieve diabetes remission [
37]. Of course, the trial was not designed to determine the relative efficacy of surgery versus lifestyle, or whether lifestyle intervention before surgery is beneficial, but these findings are consistent with the observations from other trials of proven superior efficacy of bariatric surgery over lifestyle modification alone [
4].
Moreover, we know from ‘real world’ clinical studies that drop-out from intensive lifestyle interventions tends to be high [
38], weight loss is often modest [
39], sustained reductions over time are difficult to maintain [
40] and while 10% weight loss is generally regarded as a meaningful level with which to improve health [
34,
41], this is rarely achieved: In one large general practice-based cohort of UK adults with severe obesity, the annual probability of achieving 5% weight loss was one in eight for men and one in seven for women [
42]. Patients seeking clinical care for severe obesity would rather lifestyle modification alone over surgery if the two were equally efficacious [
43] and only a minority of patients who fulfil the clinical criteria will choose to have bariatric surgery [
44], but mandating participation in a structured lifestyle modification programme before surgery lacks any evidence base and is problematic for several reasons.