TL;DR: It works well for some people, but not everyone.
The question is tricky because:
- Weight-Watchers is not a single, homogenised regime, but has changed over time.
- There are likely to be subclasses of patients with different initial conditions. (Is "patients" the right word? Possibly not, but I am going to stick with it, as I am treating it as a medical intervention, even if I am not intending to imply an illness.)
- Different patients are going to stick with the program for different lengths, and for different reasons. How do you judge the long-term effects? By the few that stick to the program or all the people who started, including the attrition?
- "Permanently" is difficult to confirm with a finitely-resourced study. "Short-term" isn't a clear range either (Is 6 months short-term? 1 year? 5 years?)
Let's look at some research anyway:
Four Weeks, Effectiveness
985 patients were randomly assigned to either Weight Watchers or self-help.
After 4 weeks:
Weight Watchers participants showed greater improvements than Self-Help participants on all measures; weight losses averaged 1.87 and 0.77 kg, respectively. The superior outcomes shown by Weight Watchers participants on measures of vitality and eating behavior were still evident when extent of weight loss was controlled. Weight losses across different sites, incomes, and levels of obesity were similar.
Twelve Weeks, Attrition
These studies show a majority of patients drop out, but those who do are more likely to have started out thinking they couldn't control their weight.
Fred R. Volkmar, MD; Albert J. Stunkard, MD; Joseph Woolston, MD; Robert A. Bailey, High Attrition Rates in Commercial Weight Reduction Programs Arch Intern Med. 1981;141(4):426-428. doi:10.1001/archinte.1981.00340040022010
In a study of 108 women enrolled in a commercial weight reduction program, we found very high attrition rates; 50% of the members dropped out in six weeks and 70% in 12 weeks. Similar attrition rates have been reported in five other programs in three different countries. Very high attrition rates diminish the effectiveness of commercial weight reduction programs, and they suggest caution in the interpretation of data based on weight losses of persons who remain in these programs.
They had a much smaller drop-out rate: after 12 weeks, 101 of 414 patients.
dropouts were significantly more likely than stayers to report low self-efficacy at the beginning of their memberships. Dropouts were also significantly less likely to feel successful in weight control and behavior change, even though their rates of weight loss did not differ significantly from those of the stayers
Six Months, Effectiveness
- Stanley Heshka, Frank Greenway, James W Anderson, Richard L Atkinson, James O Hill, Stephen D Phinney, Karen Miller-Kovach, F Xavier Pi-Sunyer, Self-help weight loss versus a structured commercial program after 26 weeks: a randomized controlled study
The American Journal of Medicine, Volume 109, Issue 4, September 2000, Pages 282–287
They had 423 patients, who were either given 2x20 minute sessions with a nutritionist and some printed materials, or Weight Watchers membership.
After 26 weeks, subjects in the commercial program, as compared with those in the self-help program, had greater decreases in body weight [mean (± SD) −4.8 ± 5.6 vs −1.4 ± 4.7 kg] and body mass index (−1.7 ± 1.9 vs −0.5 ± 1.6 kg/m2, both P <0.001) in intention-to-treat analyses.
A structured commercial weight-loss program is more likely to be effective for managing moderately overweight patients than brief counseling and self-help.
One Year, Effectiveness and Attrition
- Comparison of the Atkins, Ornish, Weight Watchers, and Zone Diets for Weight Loss and Heart Disease Risk Reduction: A Randomized Trial, Michael L. Dansinger, MD; Joi Augustin Gleason, MS, RD; John L. Griffith, PhD; Harry P. Selker, MD, MSPH; Ernst J. Schaefer, MD. JAMA. 2005;293(1):43-53. doi:10.1001/jama.293.1.43
This study assigned 40 people randomly to Weight Watchers (and 120 to other programs), and at the end of one year found the average loss was 3.0 kg (standard deviation: 4.9 kg), with 65% of the participants completing the program. (They made a "reasonable but imprecise" assumption that people who discontinued the study remained at the same weight.)
Fifteen Months After Hitting Goal Weight, Keeping the Weight Off
Stuart RB, Guire K. Some correlates of the maintenance of weight lost through behavior modification. Int J Obes. 1978;2(2):225-35.
This study was done on patients who lost their weight in the mid 1970s. Techniques have probably changed since then.
They followed up with 721 members of Weight Watchers classes, after they had reached their goal weight (at which point some stuck to the program and some didn't.)
They had all lost of a lot of weight before hitting their target weight:
The group averaged 73.2 kg (162.1 lb) before losing 19.0 percent of their body weight to reach goal weights averaging 59.6 kg (131.2 lb) in an average of 31 weeks.
Note: This is only looking at the people who achieved their dieting goal. Those who dropped out early are not included.
Most were within 5% of their initial goal weight:
Fifteen months after reaching goal, 24.6 percent were below goal, 28.9 were within 5 percent of their goal, 17.5 percent were from 6--10 percent above their goal and 28.9 percent were 11 percent or more above goal. Those who maintained their goal weight, as opposed to those who regained, tended to have several things in common: they attended classes after reaching goal weight in the same locations and with the same lecturers that assissted their weight loss; they had lower initial weights: they conceived of themselves as 'overweight' when they were approximately three pounds above goal; they had improved self-concepts; they made more lifestyle changes supporting weight maintenance; and they continued, after goal, use of many of the techniques used to reach goal weight.
So, longer-term, Weight Watchers remained effective for most of them, especially those who stuck to the program.
Five Years After Hitting Goal Weight, Keeping the Weight Off
Like the previous study, this one only looked at people who had achieved their target weight, ignoring any who had dropped out early. There were 699 in the sample, and an additional 217 used to oversample to calibrate.
The percentage of Weight Watchers lifetime
members who maintained at least 5% of their weight loss 1, 2 and 5 years after successful completion of the programme was 79.8, 71.0, and 50.0,
respectively. The percentage of participants who remained below their goal weight 1, 2 and 5 years after completion of the programme was 26.5,
20.5, and 16.2, respectively.
So, the bottom line, is 16% of people maintained their weight after 5 years, and 50% managed to keep at least a bit of the weight off. This suggests that 50% were NOT able to keep their weight off for 5 years.