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The evidence
No long-term study has shown that calorie counting is an effective way to lose weight and keep it off after the first few weeks. Depriving yourself for a few weeks will cost some weight even when the diet is a success.
How the body pushes back
The body's evolutionary mechanisms push back. Every week of deprivation brings more hunger, and the body goes into a sort of shutdown mode where the metabolism slows and fewer calories are burned.
A milder plan
A milder version of the plan, cutting daily calories by 10 percent, was supposed to produce weight loss under the old theory. Because the calories cannot be measured accurately, even that is virtually impossible to do. The rebound follows the same pattern.
How does the body slow metabolism during dieting?
Two things happen. Hormones shift, with leptin falling and ghrelin rising, and resting metabolic rate drops more than the lost weight alone would predict. In a 2011 study in the New England Journal of Medicine, Priya Sumithran and colleagues put 50 overweight people on a very low-energy diet for ten weeks. A year later, their leptin, ghrelin, and other appetite hormones were still shifted in ways that favor regaining weight, and they reported more hunger than before.
What happened to Biggest Loser contestants' metabolisms?
Kevin Hall's team at the National Institutes of Health followed 14 contestants for six years and reported the results in Obesity in 2016. Their resting metabolic rate dropped by roughly 600 calories a day by the end of the show. Six years later it was still about 700 calories below baseline, even though most had regained much of their weight.
What did the Minnesota Starvation Experiment show?
In 1944 and 1945, Ancel Keys at the University of Minnesota fed 36 young men about half their usual intake for six months. The men became obsessed with food, collected recipes, and saw their metabolic rates fall. When full feeding returned, many ate enormous amounts, and body fat overshot pre-experiment levels before drifting back.
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