I’ve complained that studies of weight loss diets have extremely forgiving definitions of “success.” Fatfu, in a terrific post that I only just now read (although it’s almost a year old), has a similar complaint. But she also whips out her calculator and tries to deduce one of Weight Watcher’s best-kept secrets — how many Weight Watchers clients lose weight over the long term?
38,000 people who reached goal weight per year sounds like a lot. But actually it turns out to be a really small number. I found a business article from back then that stated that Weight Watchers had 600,000 attendees in the U.S. in 1993. Divide 38,000 lifetime members per year into 600,000 and my calculator says that each year only about 6% of Weight Watchers members (give or take) reached their goal weight (presumably 94% failed).
Now before you get all impressed with Weight Watcher’s 6% success rate, let’s step back. For one thing, the successful 6% weren’t so fat in the first place. The 2001 study says that most were between a BMI of 25-30 (i.e. “overweight” but not “obese” – to use definitions I find silly). The 2007 abstract says the average starting BMI for that study was 27 – which is well below the average Weight Watchers participant. So in order to achieve goal weight the average lifetime member probably had to lose less than 10 lbs and would have to include a lot of people who had even less to lose. [...]
And what about the number we’re really looking for – how many people actually become “normal” weight long-term using Weight Watchers? It turns out only 3.9% of the golden 6% were still at or below goal weight after 5 years. By my calculations that means 3.9%*6.3% = 0.24% or about two out of a thousand Weight Watchers participants who reached goal weight stayed there for more than five years.
More recent numbers from Weight Watchers indicate that the rate might even be as high as 1 in a hundred. But that’s only after five years — and virtually all research on weight loss shows that “success” rates drop year after year. Just how low would the numbers be after seven years, or ten years? As Traci Mann wrote in an excellent American Psychologist article (pdf link) reviewing the evidence on dieting (hat tip to Fatfu):
Second, these losses are not maintained. As noted in one review, “It is only the rate of weight regain, not the fact of weight regain, that appears open to debate” (Garner & Wooley, 1991, p. 740). The more time that elapses between the end of a diet and the follow-up, the more weight is regained. [...]
Even in the studies with the longest follow-up times (of four or five years postdiet), the weight regain trajectories did not typically appear to level off (e.g., Hensrud, Weinsier, Darnell, & Hunter, 1994; Kramer, Jeffery, Forster, & Snell, 1989), suggesting that if participants were followed for even longer, their weight would continue to increase. It is important for policymakers to remember that weight regain does not necessarily end when researchers stop following study participants.
Dieting, for 99% or more of the people who try it, does not lead to long-term weight loss. Even the 1% who do lose weight, don’t typically lose enough weight to turn a fat person, into a person of average weight. So why is weight-loss dieting the advice given nearly all fat patients by their doctors?
Here’s something doctors don’t tell their patients: 41% of people who go on diets weigh more a few years after the diet, then they did before they began dieting.1 Since I’m a blogger, not a scientist, I’ll go ahead and make the irresponsible comparison: Dieting is significantly more likely to cause long-term weight gain than weight loss. That’s a Surgeon General’s warning that should appear on every diet program and product on the market.
- See page 224 of this article (pdf file). “Eight of the studies reported (or made it possible to compute) the percentage of participants who weighed more at follow-up than before they went on the diet. These rates averaged 41% and ranged from 29%… to 64%…” A few pages later: “From one third to two thirds of participants in diets will weigh more four to five years after the diet ends than they did before the diet began. This conclusion comes from studies that are biased toward showing successful weight loss… The true number may well be significantly higher.” [↩]