Effectiveness of slimming clubs tested

“Weight-loss courses such as Weight Watchers should be used by the NHS as a weapon to tackle the obesity crisis,” according to the Daily Express. The story is based on research that found that overweight and obese adults referred to Weight Watchers lost twice as much weight over a year as those who received standard advice on losing weight at their local GP surgery. Participants on the Weight Watchers programme also had greater reductions in waist size and body fat, which are both changes associated with a lower risk of cardiovascular disease and diabetes.

This was a well-conducted study. Although it had some limitations, the overall results should be reliable. It should be noted that in the trial, which was sponsored by Weight Watchers, participants received free access to the programme, which may mean that their behaviour was not typical of people having to pay for the course themselves. Another limitation of the study is that it only lasted 12 months, and therefore does not address the common difficulty of maintaining weight loss in the long-term.

Where did the story come from?

The study was carried out by researchers from the Medical Research Council Human Nutrition Research Laboratory, Cambridge; the University of Munich, Germany and the University of Sydney, Australia. It was funded by a grant from Weight Watchers International to the UK Medical Research Council. The researchers say that the sponsor had no role in the study design, data collection, data analysis, data interpretation or writing of the report. The study was published in the peer-reviewed medical journal, The Lancet.

In general, the media reported the study fairly. In its story the Daily Express mentioned another commercial organisation, Slimming World, which could be misleading since this programme was not assessed by the trial. The Daily Mail included comments from an independent expert and mentioned the source of funding – a pertinent point which many other newspapers left out.

What kind of research was this?

This was a randomised controlled trial (RCT), involving 772 overweight and obese adults. They were assigned to receive either 12 months of standard care for weight loss (as defined by national guidelines) or 12 months of free membership to the Weight Watchers commercial weight loss programme. The researchers aimed to assess weight change in both groups over a 12-month period.

One thing to note is that the trial was not blinded – the participants knew which group they were in, as did some of the researchers. Given the nature of the interventions being investigated, the lack of blinding was unavoidable, but it means that knowing which treatment group they were in could have unconsciously influenced participants’ motivation and therefore the amount of weight they lost. The randomisation procedure (how the participants were randomly allocated to each group) was concealed from the researchers by use of an online database.

The researchers say that there is an urgent need for interventions to deal with the global health problem of obesity, as excess weight accounts for 44% of the global burden of diabetes, 23% of heart disease and 7%-41% of some cancers. They also highlight that for overweight individuals a weight loss of 5%-10% is associated with significant health benefits. They suggest that partnerships between primary care and commercial organisations could be used to deliver weight-management programmes on a large scale, but say that prior to their study there had been few RCTs of commercial weight-loss programmes and that their effectiveness has not yet been compared with standard care.

What did the research involve?

Between September 2007 and November 2008, researchers recruited 772 overweight and obese adults from primary care practices in Australia, Germany and the UK. Participants were 18 or over, with a BMI of 27kg-35 kg/m2. They had at least one additional risk factor for obesity-related disease, including “central adiposity” (a waist circumference of more than 88cm in women and more than 102cm in men), type 2 diabetes or mild to moderate high cholesterol (dyslipidaemia). They had initially recruited 1,010 potential participants but excluded 238 on the grounds of recent weight loss of 5kg or more and various health and medical disorders.

Participants were randomly assigned to receive either 12 months of free membership to Weight Watchers, or standard care, as defined in national treatment guidelines for obesity. The Weight Watchers system places emphasis on a balanced diet based on healthy eating principles, increased physical activity and group support. In this research participants received 12 months of free access to weekly community-based Weight Watcher meetings, which involve weigh-ins, group discussion, behavioural counselling and motivation. Participants are also able to access internet-based systems to monitor food intake, physical activity and weight change, to join community discussion boards and to access recipes and meal ideas.

Participants in the standard care group received weight loss advice from a health professional at their local GP surgery, based on national treatment guidelines. It is not clear how often people in this group met with health professionals or how much support they received.

Researchers followed the two groups for a period of 12 months. They measured body weight, fat mass, waist circumference and blood pressure at at the start of the study and at 2, 4, 6, 9 and 12 months. Blood samples were also taken to measure blood sugar, insulin and lipid levels at 6 and 12 months.

As well as recording weight change, the researchers also looked at changes in fat mass, waist circumference, blood pressure and markers of cardiovascular risk. They analysed their data using validated statistical methods.

What were the basic results?

  • Of the 377 participants assigned to the commercial programme, 230 (61%) completed the 12-month study. Of the 395 assigned to standard care, 214 (54%) completed the study.
  • Participants on the Weight Watchers programme lost twice as much weight on average than those in the standard care group.
  • The average amount of weight lost at 12 months was 5.06 kg for those in the commercial programme, compared to 2.25 kg for those receiving standard care. This equated to a difference of 2.77 kg [95% CI −3.50 to −2.03].
  • Over the 12 months of the study the Weight Watchers participants were three times more likely to lose at least 5% of their initial body weight than those assigned to standard care (OR 3.0, 95% CI 2.0-4.4). They were also three times more likely to lose 10% or more (3.2, CI 2.3-5.4) of their initial weight.
  • Participants in the commercial programme also had larger reductions in waist circumference and fat mass, greater improvements in insulin levels and improved cholesterol ratios.
  • Small reductions in blood pressure were recorded in both groups at 12 months.
  • Participants reported no adverse events related to participating in the trial.

How did the researchers interpret the results?

The authors say that referring selected patients to commercial weight loss programmes providing group support and dietary advice can present a “clinically useful intervention” for managing the weight of overweight and obese people. They also say that these programmes can be delivered on a large scale.


This was a well-conducted study and its findings are likely to be reliable. Some points are worth noting:

  • There were high drop-out rates in both groups (40%-50%), which may have affected the study’s results. Although the researchers say that they anticipated this possibility when calculating the sample sizes needed to obtain meaningful results, the difference in drop-out rates between groups could have affected the reliability of the results.
  • The lack of blinding was unavoidable given the nature of the two interventions being tested. It is possible that participants knowing which treatment they were assigned could have had an influence on results. However, the objective measurement of weight makes this less important as it does quantify the effect of these interventions.
  • The researchers ensured that the allocation of participants at randomisation was concealed. This means that allocation could not be influenced by the researchers or the participants, and this is an important feature of this well-designed trial.

One aspect not addressed by this report is the cost-effectiveness of the different approaches. Although the more intensive support organised by Weight Watchers (including weekly weigh-ins and group support) resulted in more weight loss than standard guideline advice from a GP, more research will be needed to assess the financial costs to achieve this extra benefit. Also, as participants were given free access to the Weight Watchers programme, it is unclear from this study how having to pay to attend might affect the amount of weight lost or the chances of dropping out.

Finally, the study does not address a common problem with weight loss: the difficulty of keeping the weight off in the long term. Future studies could look at this too, particularly in people who discontinue the programme once they have achieved their target weight.

NHS Attribution