What a large evidence review found about weight-loss programs

Why this study matters

Before any single trial, it helps to know what the whole body of evidence says. This review was prepared for the US Preventive Services Task Force, a panel that advises on preventive care. It pulled together randomized trials of behavioural programs and weight-loss medicines in adults. For Canadian readers, it gives a realistic average for what counselling-based programs achieve, and a benchmark for thinking about newer medicines that came later.

Who was in the study

The review included 122 randomized controlled trials with 62,533 participants, plus 2 observational studies with 209,993 participants. All were studies of weight loss or weight-loss maintenance in adults. The researchers searched medical databases for studies published through June 6, 2017, checked trial registries through August 2017, and watched key journals through March 23, 2018.

What the researchers did

This was a systematic review and meta-analysis. A systematic review searches for every relevant study using a set method. A meta-analysis combines the results of several studies into one estimate. Here, the researchers pooled results for behaviour-based programs using a random-effects model, a statistical method that allows for differences between studies. They did not pool results for medicine trials.

One reviewer extracted the data and a second confirmed it. The outcomes were health outcomes, weight loss or maintenance, reductions in weight-related conditions, and adverse events.

What they found

Compared with control conditions, people in behaviour-based programs lost 2.39 kg more weight at 12 to 18 months (95% confidence interval -2.86 to -1.93 kg), based on 67 studies with 22,065 participants (R32-C138). A confidence interval is the range within which the true value would likely fall. In weight-maintenance studies, people in behavioural programs regained 1.59 kg less (95% confidence interval -2.38 to -0.79), based on 8 studies with 1,408 participants.

A relative risk of 0.67 means the risk in the program group was about two-thirds of the risk in the control group.

Medicine trials also reported more weight loss or less regain than placebo at 12 to 18 months, with a range of -0.6 to -5.8 kg across trials. The review did not combine these into a single estimate, and this page does not either.

Most health outcomes such as death, heart disease and cancer were rarely reported. Some medicine trials noted small improvements in quality of life of unclear importance.

Side effects and people who stopped

The review found no evidence of harm, such as heart disease, from behaviour-based programs. In medicine trials, higher rates of adverse events were linked to higher dropout in medicine groups than in placebo groups. The summary does not give specific numbers.

What this study does not tell you

• The searches ended in 2018, before the current generation of weight-management medicines was studied. The medicine findings are out of date.
• The behavioural result is an average across very different programs and people. Any one program could do better or worse.
• The medicine range of -0.6 to -5.8 kg spans different medicines and trials. It is not a ranking and cannot be used to compare medicines.
• Long-term outcomes such as heart disease and death were rarely measured.
• Information on subgroups, such as older adults or particular ethnic groups, was limited.
• The summary we reviewed does not state who funded the review.

What it means in Canada

This review sets a realistic expectation for what counselling-based programs achieve on average. It predates today's medicines, so it cannot say how they compare. For current medicine information, see semaglutide in Canada and tirzepatide in Canada. For coverage, see public drug coverage by province. To find behavioural programs, see compare services.

Related reading

Other studies in this library that bear on the same question.

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