Losing weight seems so simple if we look at it from a purely mathematical point of view : in essence, we must burn more calories than we consume.
But why do so many people regain the weight they lost? From a scientific point of view, in fact, weight loss can be hard to maintain because the body is programmed to defend itself against it and regain weight as soon as possible. This is explained here by scientists Mark and Charles Brenner , directors of the Obesity Research and Education Initiative at the University of Iowa.

But what exactly happens? Weight loss triggers a series of biological and metabolic processes in the body: from an evolutionary point of view, it certainly prefers to regain weight or maintain weight than to lose it. When we lose weight, the metabolism goes down, leptin , the satiety hormone, goes down, making us feel dissatisfied, while at the same time ghrelin , the hunger hormone, goes up. The bacterial flora also plays a role in this.
According to recent studies, in fact, the bacterial flora of an obese person may not change with the diet, but rather lead to rapid gaining weight because the real changes on it can take five times longer than the period of the diet itself: in simple terms, at the end of the diet we don’t have a different bacterial flora, but a bacterial flora that continues to behave as it did when we were fatter.
In animals, studies of calorie restriction and weight maintenance after weight loss are much simpler than in humans, and Charles Brenner is known to have conducted some in mice.

In fact, animals know how to regulate themselves much better than we do with food , explains Brenner: an animal in the wild is hardly obese. Not only that: but despite the caloric restriction, animals are more easily able to maintain a good metabolism that allows them not to regain the weight lost as in humans. Finally, environmental and stress factors contribute to post weight loss happening differently between humans and animals.