Why «eat less, exercise more» won’t make you slim

(Versión en español: pinchar aquí)

«It’s quite simple. If you have a caloric deficit, you lose weight. If caloric balance is positive, you gain weight. Energy balance is a direct representation of the first law of thermodynamics, The One That says energy can neither be created nor destroyed. We’re not talking here acerca a hypothesis, or even a theory, but a physical LAW OF THE UNIVERSE.«(http://www.sixpackabs.com )

«In a perfect balance your energy intake provides exactly the required amount of energy according to our needs. If you take more, you gain weight; if you take less, you will lose weight » (said by a «dietitian»)

I have spent years trying to lose weight and I couldn’t. I ate less, I exercised more, and I did lost weight, but that only happened in the short term. Sooner or later my weight loss stalled. And when I ate a «normal» amount of food I recovered all the weight lost. That happened again and again. Maybe someone has managed to lose weight eating less and exercising more, but it’s not my experience nor I know anyone who has done that.

That failure is not caused by a lack of willpower, or because you withdraw the diet. Some people say that , but is not true. The cause is that «eat less, exercise more» does not work for long term weight loss, i.e. it does not work for weight loss.

Why «eat less and exercise more» does not work?

That idea is based on the first law of thermodynamics, a reinterpretation of the basic principle of energy conservation: energy can neither be created nor destroyed, it can only be transformed. Or in other words, the energy we eat will be burnt or stored as fat, but will not disappear by magic. Mathematically we can say:

What we store as fat = what we eat – what we expend

Or graphically, what we eat is stored as fat, or burned by exercising, or your body burns it in other ways:


If we eat less and do more exercise, keeping our current metabolism, will we lose weight? Yes, no doubt. Graphically, if we are able to eat less without changing our energy expenditure, we will reduce our fat storage.


The problem is that when we stop eating our metabolism slows down: our body starts to expend less and less energy and weight is not lost as predicted:


Our weight loss stalls: we are eating less but our body also expends less energy and the caloric deficit virtually disappears. Since we can not starve indefinitely, sooner or later we have to resume eating a normal amount of food and, as our metabolism is reduced, we will regain the lost weight:


«Eat less and move more» is a simple idea but it’s a hoax. It makes us believe that we are in control of the situation, it makes us believe that we can control our metabolism, but we don’t. Provided that we can eat less and we can exercise more, to lose weight is just a matter of willpower. But we are not in control: if we eat less we get hungry, if we eat less our metabolism slows down, if we exercise we do not only burn calories: our metabolism can be reduced even more and our appetite increased, etc. Although we can make an effort to control the black and blue bubbles in the schemes above, nobody tells us how the red bubble changes when the black and blue bubbles change. We are not in control.

Does that mean that the first law of thermodynamics doesn’t apply to humans? No, absolutely not. It always applies, but it is not useful for weight loss because it does not tell us how we can eat less without reducing our energy expenditure.

If for example a tennis player asks his coach what to do to win his next match and his coach answers that he «must win the last point of the match,» we can not say it’s untrue, but we know that this answer is useless for the player. Similarly, if a businessman asks his financial advisor what to do to earn more money and the advisor replies that he must «sell more products at higher prices and reducing costs,» we can not say it’s a lie, but that answer is useless. In the field of nutrition we are in the same case: we asked our doctor how to lose weight and he told us that we should «eat less and exercise more» (keeping or increasing our metabolism, but that vital detail is never said), but the answer is useless. It is true that if we do that we will lose weight, but we are not told how to do it, we are only told an obvious useless truth.

Are really things so simple and the way to lose weight is to cause a caloric deficit? Does it really make sense that the advice we are given to control our weight is «eat less than you expend»?

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