The Data that Dieting Doesn't Work


Does dieting work? Well, it depends on how success is measured. And what the medical and diet industry mean by success is not what ordinary people mean. Let me explain.[1]

In the 1940s, success was measured by getting people to a “normal” BMI (an unscientific height/weight ratio),[2] but diets didn’t help most people get there. So in the 1950s they simply changed the measure of success to losing 40lbs, but 95% of people couldn’t do that.[3] As a result, in the next decades, they simply lowered the bar again to 20lbs.[4] But 20lbs is quite different for someone who is 150lbs than someone who is 300lbs, so in the 1970s, they changed it, yet again, to 10% of one’s starting weight. But since 80% failed at this,[5] in 1995, the Institute of Medicine lowered it to 5%.[6]

This is obviously nonsense. In a study of 130 female dieters, no one said they would be satisfied with losing 5%. These women averaged 218lbs, and their goal was to lose more than 70lbs (not even close to 5%!), and none of them reached this goal. Enough people lost 5% for the researchers to claim the diet was a success, but none of the participants thought they had succeeded.[7]

No matter what kind of diet you’ll try, many studies show that you’ll lose about 5-15 pounds over 4-6 months. But that’s too short of a range, since everyone wants to keep the weight off. The long-term studies do not bode well, and the best ones show that about half of dieters will gain weight within four to five years. The Weight Loss Industry refuses to make their data public because, they say, “Dieters will be discouraged if they are provided with realistic outcome data.”[8] Of course, they make lots of money on failure, indeed, the average Weight Watcher customer tries it 4 times![9]

There are a few decent long term studies that are revealing. One study starved obese people in a hospital for 38 days, and of those who were tracked for three or more years, 83% gained back more weight than they lost,[10] and in studies that tracked people even longer, the participants gained even more weight.[11]

Traci Mann and colleagues decided to track down every decent long term weight-loss study and see what the data showed.[12] They found that dieters, on average, had managed to keep off 2lbs and 40% gained weight from the diet! But, it’s even worse than that! They found that all of the studies were flawed, which suggests that the measly 2lb loss is optimistic.  

Here’s why. Most of the studies made people diet before the official study diet to see if they could do it. If they could not, they were not included in the study. Thus, those who had difficulty dieting, which is very common, were not included, skewing the success upwards. Furthermore, on average, 20% of the participants dropped out of these studies, and they have shown that if you fail, you’re far more likely to drop out because you don’t want to confess your failure to the researchers. Moreover, participants were almost never weighed in person – weight was conveyed by email or phone. But other clever researchers have called to ask people their weight and then shown up at their home with a scale, and, on average, obese people say they weigh 8lbs less than they do, and the rest say they weigh 5lbs less than they do. And last but not least, 20-65% of participants in these studies were on other diets at the same time. In other words, they had tried the study diet, failed, and then started a different diet when the researchers followed up, and thus their recorded weight-loss was really result of the wrong diet![13]

If you add these flaws into the mix, the supposed average puny loss of two pounds is optimistic and unrealistic. All in all, the net result of dieting is, in all likelihood, weight gain rather than weight loss.

All of this is to say that dieting does not work! Just say no to dieting! Dieting is too difficult: it is painful, miserable, hard work, and it is definitely not worth it. While this may seem depressing, we think it is liberating: you are much better off spending your energy simply trying to become healthy.



[1] The studies in this blog post are discussed in greater detail in Chapter 1 of Traci Mann’s excellent Secrets from the Eating Lab (Harper Collins: NYC, 2016).

[2] Thomas R. Knapp, “A Methodological Critique of the ‘Ideal Weight’ Concept,” JAMA 250, no. 4 (Jully 22, 1983): 506.

[3] A. Stunkard and M. McLaren-Hume, “The Results of Treatment for Obesity: A Review of the Literature and Report of a Series,”  Archives of Internal Medicine 1 (1959): 79-85.

[4] R. R. Wing and R. W. Jeffery, “Outpatient Treatments of Obesity: A Comparison of Methodology and Clinical Results,” International Journal of Obesity 3, no. 3 (1979): 261-79. Abstract here:

[5] Rena R. Wing and Suzanne Phelan, “Long-Term Weight Loss Maintenance,” American Journal of Clinical Nutrition 82, no. 1 Suppl. (July 2005): 222S-225S.

[6] Institute of Medicine, “The Nature and Problem of Obesity,” in Weighing the Options: Criteria for Evaluating Weight-Management Programs, ed. P. R. Thomas (Washington D.C.: National Academy Press, 1995), 55-58.

[7] Robert W. Jeffery, Rena R. Wing, and Randall R. Mayer, “Are Smaller Weight Losses or More Achievable Weight Loss Goals Better in the Long Term for Obese Patients?,” Journal of Consulting and Clinical Psychology 66, no. 4 (1998): 641-45.

[8] R. Cleland et. al., “Commercial Weight Loss Products and Programs: What Consumers Stand to Gain and Lose. A Public Conference on the Information Consumers Need to Evaluate Weight Loss Products and Programs,” Critical Reviews in Food Science and Nutrition 41, no. 1 (January 2001): 45-70.

[9] Weight Watchers International Business Plan, 2001. For an interesting discussion see,

[10] David W. Swanson and Frank A. Dinello, “Follow-up of Patients Starved for Obesity,” Psychosomatic Medicine 32, no. 2 (March 1, 1970): 209-14.

[11] D. D. Hensrud et al., “A Prospective Study of Weight Maintenance in Obese Subjects Reduced to Normal Body Weight Without Weight-Loss Training,” American Journal of Clinical Nutrition 60, no. 5 (November 1, 1994): 688-94,

; F. M. Kramer et al., “Long-Term Follow-up of Behavioral Treatment for Obesity: Patterns of Weight Regain among Men and Women,” International Journal of Obesity 13, no. 2 (1989): 123-36.

[12] Traci Mann et al., “Medicare’s Search for Effective Obesity Treatments: Diets Are Not the Answer,” American Psychologist 62, no. 3 (April 2007): 220-33; Traci Mann, A. Janet Tomiyama, and Britt Ahlstrom, “Long-Term Effects of Dieting: Is Weight Loss Related to Health?,” Social Psychology Compass 7, no. 12 (December 2013), 861-77.


And Here:

[13] Mann, Secrets from the Eating Lab, 11-15.