@TheLancetPH just published the first RCT of a commercial group weight-loss program —   Weight Watchers — with long-term (5-year) follow-up. Quick take: Pretty good RCT found minimal weight-loss impacts (2–4 lbs) at the 5-year mark that weren’t statistically significant. (Sizable impacts at the 1-year mark had mostly faded.) Continued below.

Program & Study Design:

  • The study sample comprised 1,267 overweight or obese UK adults, randomly assigned to a 52-week WW program, a 12-week WW program, or a control group. The study was mostly well-conducted but had differential attrition across the 3 groups (29% vs 31% vs 39%) that somewhat reduces confidence in the results.


  • Sample members averaged 211 lbs at baseline. At the 5-year follow-up, the 52-week WW group had lost 4 lbs more than the control group, & the 12-week WW group had lost 2 lbs more than controls. Neither difference was statistically significant, so they might be due to chance.

  • The study had found more sizable weight loss effects at the 1-year mark (especially for the 52-week WW group), but unfortunately these effects had mostly faded by 5 years — see graph above.


  • Unfortunately, weight loss is hard to maintain, as found in this and other studies. Here's the study report.

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