We often advise our patients to lose weight with little guidance on how they should do that — other than telling them to “eat healthy” and “exercise.” However, while those behaviors are intrinsically beneficial on their own, often they do not lead to sustainable weight loss. A quick PubMed search will reveal hundreds of articles showing that even with various successful interventions, most weight loss programs only result in a short-term reduction in weight — oftentimes followed by long-term weight gain.

One of the reasons obesity is such a difficult problem to tackle is because it is not well understood. It is not just one behavior that leads to weight gain — the problem is multifactorial. Dietary factors are just one small piece of a very complex puzzle. Behavioral, hormonal, genetic, environmental, and biochemical factors also play a significant role in determining whether an individual accumulates fat or not. For example, in college, were you the one who could eat a whole pizza and not gain a pound, while your friend merely looked at a slice and gained weight? Or was it vice versa?

As physicians, we have long relied on the false premise that individuals store weight as fat when energy intake exceeds energy output (ie, the amount of energy needed for all cells, tissues, and organs to function). If a patient is overweight or obese, then we assume that at some point in his or her life there must have been an imbalance in that equation. As a result, his or her body started to store fat.

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Unfortunately, that equation is not as simple as it’s been made out to be. The equation itself can change according to dietary changes and alterations in energy expenditure (eg, exercise).

Consider the fact that most people don’t continuously gain or lose weight. Rather, everyone eventually plateaus if they keep their caloric intake stable. For example, a person who had maintained a relatively stable weight for the past several years who then decides to suddenly eat a pack of Twix every night for the rest of his life does not continue to gain weight indefinitely. With an additional 300 calories (kcal) per day — which equals an excess of 2100 calories per week – one might expect a weight gain of 0.6 lbs per week.1 

If that person continuously gained weight, he would weigh an extra 312 lbs in 10 years. However, he does not continuously gain weight because the body adapts its energy-balance system to account for weight gain or weight loss. It does this by adjusting the total energy expenditure of the system in the same direction as the caloric intake.

Based on studies by Leibel et al, we know that not all the weight gain is fat. If a person gains 10% of his body weight by overeating, approximately two-thirds of that weight will be body fat.1 The remaining one-third will be fat-free mass. Therefore, when someone gains weight, he is not just gaining fat. 

He also gains some metabolically-active fat-free mass that helps increase his total energy expenditure. In addition, his body’s metabolic requirements for all cells also changes, such that he burns more energy per pound of fat-free mass. As a result, he is able to compensate for the increased energy intake (ie, that nightly Twix bar).

This has important implications. First off, when an individual gains weight, he is not just adding tissue that his body must maintain. He is also changing how every cell in his body metabolizes energy.

Unfortunately, when individuals lose weight, about 20% of the weight loss is also in the form of fat-free mass, and thus total energy expenditure goes down. To make this clearer, consider that the relationship between total energy expenditure is on average 45 kcals to 65 kcals per pound of fat-free mass. 

When a person stabilizes at a 10% weight loss, he decreases his total energy expenditure — not only by losing fat-free mass, but also by changing the overall equation such that he burns only 40 kcal per kg of fat-free mass. Just as the person who ate the extra Twix bar plateaus at a higher weight, despite a fixed increase in daily caloric intake, so does the person who cuts his calories plateau.