First, please refer to the following article. 
The Calorie Principle and Weight Gain; The Causality Has Been Obscure

According to Gary Taubes,  the author of Why We Get Fat (2010), in the early 1900’s, Carl von Noorden, a German diabetes specialist, first argued that we get fat because we take in more calories than we expend.

This view has persisted to the present day, leading many experts to firmly believe that excessive caloric intake and/or lack of exercise are the primary causes of weight gain[1].  This time, I would like to share the “law of thermodynamics,” which was said to be the basis for that theory. Mainly quoted from the book, it is so interesting that I think it is worth reading.
        

Because all mass－our fat tissue, our muscles, our bones, our organs, a planet or star, Oprah Winfrey－is composed of energy, another way to say this is that we can't make something out of nothing or nothing out of something.

This is so simple that the problem with how the experts interpret the law begins to become obvious.

All the first law says is that if something gets more or less massive, then more energy or less energy has to enter it than leave it.

It says nothing about why this happens. It says nothing about cause and effect. It doesn't tell us why anything happens; it only tells us what has to happen if that thing does happen. A logician would say that it contains no causal information.（*snip*)

If you asked me this question, and I said, Well, because more people entered the room than left it, you'd probably think I was being a wise guy or an idiot. Of course more people entered than left, you'd say. That's obvious. But why? And, in fact, saying that a room gets crowded because more people are entering than leaving it is redundant－saying the same thing in two different ways－and so meaningless.

Now, borrowing the logic of the conventional wisdom of obesity, I want to clarify this point. So I say, Listen, those rooms that have more people enter them than leave them will become more crowded. There's no getting around the laws of thermodynamics. You'd still say, Yes, but so what? Or at least I hope you would, because I still haven't given you any causal information. 

This is what happens when thermodynamics is used to conclude that overeating makes us fat. （*snip*)

"The first law states that energy can neither be created nor destroyed. In other words, energy can be converted from one form to another, but the total amount of energy in the universe remains constant. How might this law apply to weight management?

Suppose someone has stable weight over time. The first law dictates that, in theory, the number of calories consumed by this individual in the form of food is equal to the calories the individual expends during metabolism and activity. In other words, 'calories in = calories out’.(*snip*)

However, the first law of thermodynamics actually refers to what are known as ‘closed systems' －ones that can exchange heat and energy with their surroundings, but not matter. Is this true for human beings? 

Two authors have made excellent points about the relationship between thermodynamics and weight management. Based on those thoughts, I would also like to mention two points about the relationship between thermodynamics and my theory.
  

(2) When energy intake increases

Based on my intestinal starvation mechanism concept, even if a person who has maintained the same weight over the years, significantly reduces their usual caloric intake (e.g. about two thousand kcal daily) and the intake of carbohydrates, but meets the "three factors + one" criteria that cause intestinal starvation, they will gain weight (this means that the set-point weight itself has risen due to an increase in absorption ability).

[Related article] →Three (+one) Factors to Accelerate “Intestinal Starvation”

    
Of course, weight gain occurs when you return to your original diet afterward. In this case, since absorption efficiency itself has increased compared to before, both body fat and lean tissue contribute to the weight gain.

In short, even though you are eating the same amount of food (calories) as before, you are taking in more energy and nutrients into your body than before, which means you are getting bigger/fatter. In the words of Taubes, "a room crowded with ten people now has eleven people," and in this case, it is “intestinal starvation” that has caused it.