Why do so many diets fail?

Weight and height are used in computing body m...

The vast majority of dieters regain weight as soon as they try to eat normally again. Do not blame yourself if this is your case, as your body cannot make the difference between dieting and starvation. When you start dieting, it therefore “thinks” that there is not enough food available, and it tries to conserve energy. Consequently, you will regain weight as soon as you are eating slightly more.

In February 2014, Eric Trexler and colleagues reviewed what we know about the changes in our bodies due to weight loss. They looked at it from the point of view of athletes, as many of them want to lose weight –or at least keep it under control. Long distance runners for example, want to keep their body fat percentage low. However, the mechanism is the same for everybody.

Weight loss and your hormones

A number of hormones, such as thyroid hormone, leptin and ghrelin, are important for the regulation of energy expenditure and intake. They act together to maintain the body weight you use to have, even if it is too high, by regulating the amount of energy you spend and your appetite. If you want to lose weight and keep it off, you will have to resist this mechanism.

The energy you are using

Your total energy expenditure includes your resting energy expenditure (energy needed to fuel the minimal requirements of your body at rest), your activity related energy expenditure and the energy you need to digest food.

Your resting energy expenditure is usually about 60% of your total expenditure. As you lose weight however, it decreases, which can thus lead to weight regain or to difficulties in further slimming down.

Your activity related energy expenditure also declines, as you will burn less energy to run or walk when you are lighter. Moreover, experts believe that your body becomes more efficient, which means again that you are burning less calories for the same workout.

Finally, as you are eating less, you need less energy to digest your food.

Your appetite

Your hormones will also increase your appetite. As if that is not enough, there is good evidence that they will stimulate you to eat food high in sugar and fat. On the other hand, high-protein, low-glycemic index diets are better to prevent weight regain, as they tend to make you less hungry.

What can I do?

You will have to watch what you eat well after the cessation of your diet. You might therefore want continuous support and advice.

It is likely that all these effects are proportional to the energy deficit. In other words: the fewer calories you take in, the more energy your body tries to conserve. If you reduce your food slowly and gradually, your body’s adaptations will be much less important.

Some people practice “refeeding”, whereby they eat more than required for a day or so in order to maintain their resting energy expenditure. However, we need more research to know it this is beneficial.

If you want to regain weight after dieting, you should increase the amount of calories very slowly. Studies have shown indeed that a rapid weight gain is usually only due to an increase in fat mass, while it is likely that you would like to increase your muscle mass.


Eric T Trexler, Abbie S Smith-Ryan and Layne E Norton. Metabolic adaptation to weight loss: implications for the athlete. J Int Soc Sports Nut. 2014; 11:7.

Erik S Blomain, Dare A Dirhan, Michael A Valentino et al. Mechanisms of weight regain following weight loss. ISRN Obes. 2013; 2013: 210524.

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