Does dieting make you fat?

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Years of dieting have irreparably damaged your metabolism and as a result, you’ll never be as slim as you’d like. Or at least, that’s what a number of doom-mongers would have you believe.

In 1984, Geoffrey Cannon published his bestselling book Dieting makes you fat. It sold so well that he re-wrote it and in 2009 released an updated version. Mr Cannon’s claim has become something of an internet meme: the more you restrict your food intake, the likelier you are to get fat in the future. If you loose weight by dieting, you’ll put it all back on, and more, when you stop. There are different explanations for why this paradoxical effect might happen. But regardless, a quick look at the scientific literature suggests it does seem to happen.

A number of studies have been designed to test whether dieting really does lead to subsequent weight gain. For example, Dr Eric Stice and colleagues of the University of Texas at Austin recruited 692 adolescent girls and asked whether they were currently making active efforts to loose weight. The researchers followed up their participants for four years and found that on average, those who had initially reported deliberate attempts at weight loss were now heavier than before.

The problem with studies like these is that scientists are using observational data to make causal inferences. This is often necessary, but always risky. After all, the number of fire engines that turn up to a fire in the city of London is directly related to the size of the fire, but it would be a mistake to claim that calling up an extra fire truck would cause the fire to increase in size. We could come up with a number of alternative theories to explain these findings, and these observational data can’t tell us which is right. For example, it may simply be that people who notice that they’re gradually putting on weight year by year tend to try dieting. Another group of people might never try dieting because they ‘never put weight on.’ If we compare these groups using an observational design, we see an apparent link between dieting behaviour and subsequent weight gain. In truth, it’s the struggling with weight fluctuation that’s causing the willingness to try new diets.
Cannon’s paradox is framed in a number of ways, and its proponents suggest a number of mechanisms, but the only one that should have us worried is the contention that dieting has fundamentally damaged your metabolism and you now burn fewer calories than your slimmer friends. After all, if the effects are readily reversed, why worry? To address this more specific question, as to whether any paradoxical effects of dieting might be long-lasting, there have been experimental studies, the results of which are much easier to interpret. A number of scientists have fed rats, mice and other lab animals very restricted low-calorie diets for some time, and then checked to see whether their metabolism recovered afterwards. For example, Drs McCarter and McGee of the University of Texas cut 40% of the calories out of their rats’ diets and found that the reduction in basal metabolic rate was only temporary. In fact, metabolic rates recovered even before they started giving the rats more food, once the rats had got used to the low calorie diet.
It’s surprising that these arguments about dieting are still being rehearsed. A number of excellent experiments in the 1950’s and 60’s ought to have put the question to bed. Dr Ancel Keys did a number of famous studies using ‘semi-starvation’ diets in the middle of the last century. In one study, he and his colleagues fed young men a very restricted diet and found the expected dip in metabolic rate, but they also found that it went back to the pre-experiment level once their diet was less restricted.
The results are fairly unanimous. Low calorie diets do tend to reduce basal metabolic rate, and soon after the diet stops, metabolic rate recovers. These findings turn up again and again in experiments across any number of mammal species, including humans. To be clear, in science, experimental studies trump observational ones. If you increase the number of fire engines attending the incident and the flames fail to lick ever higher then you’ve shown that the observed association isn’t caused the way you thought it was.
So don’t worry. Diets may not work very well, but they don’t damage your metabolism and make you fat.

Why does it look that way in observational studies? It’s more likely to be due to the paradoxical cognitive effects of restrained eating. More on that later.