First, some people think in terms like "overeating always leads to weight gain" or "exercise causes weight loss," as shown in Figure-1. 

Exercise certainly consumes more energy, but a counter-regulatory function－that the body tries to regain energy that it has expended－should work.

In other words, exercise is essentially a force that pushes the body in the direction of gaining strength and ultimately, storing energy (weight gain) as it tries to stimulate energy circulation and re-energize the body. (In particular, it works more strongly in resistant exercises that target muscles.) 

As Dr. Briffa, the author of “Escape the Diet Trap,” says in his book, it is better to think that, "originally lean people start running marathons or playing soccer, and eventually become athletes[1].” It may be a cynical view, but I think it’s probably correct. 

They know they never gain weight even though they eat a lot, and most athletes eat three well-balanced meals, plus other nutritional supplements and snacks. 

On the other hand, there might be people who have gained 3–4 kg over the past few years because their work involves desk tasks or light physical activity, and they haven’t exercised recently. 

However, the real issue, I believe, is not the lack of exercise, but rather skipping meals, eating light meals, having an unbalanced diet relying too much on carbohydrates, or irregular eating habits.

Additionally, when athletes retire, their caloric expenditure decreases and opportunities to eat often increase, which can lead to a few kilograms of weight gain. I see this as the same mechanism that causes weight to rebound after dieting, where the body returns to its set-point weight. 

However, if there is a weight gain of more than ten kilograms over a few years, this is likely due more to changes in eating habits, as explained above, and can be attributed to weight gain caused by intestinal starvation.
            

Fighters and sumo wrestlers exercise, of course, but due to the nature of their sports, they sometimes need to increase their muscle mass or body weight. However, we often hear that it’s not easy for some fighters to gain muscle mass and weight even if they eat protein supplements in addition to their three meals. 

On the other hand, those who don’t want to gain weight sometimes put on weight quite effortlessly. This is because, as I have mentioned so many times, gaining weight (meaning an increase in one’s set-point weight) requires the induction of intestinal starvation.

During high-intensity strength exercises, like barbell exercises, the body’s regulatory mechanism to restore lost energy is even more powerful than with aerobic exercise.

However, if one tries to consume more calories and nutrients every 4 to 5 hours through meals or protein supplements, some undigested food tends to remain in the intestines throughout the day, which could ultimately hinder an increase in set-point weight. 

<A sumo wrestler's diet: a practical approach to increasing body weight>
        

■For details on how weight is increased when intestinal starvation is induced, please refer to the article below. 

[Related article]
 Gaining Weight by Intestinal Starvation; What Does It Mean?

(1)The relationship between diet and exercise is not simply an energy "in/out" relationship.
Exercise is essentially a force that works toward gaining strength and weight because the opposite reaction－that the body tries to regain energy that it has expended－should work (especially in the case of high-intensity exercise).

      
(2) However, the priority is in how we control our diet. Eating three well-balanced meals every day will help undigested food to remain in the intestines, and the set-point weight is less likely to increase. 

People who are originally lean start athletics, soccer, etc., and if they eat three well-balanced meals every day, they are less likely to gain weight and maintain the same body shape over the years.

      
(3) People tend to skip meals or eat less when they aren’t exercising or are only doing light physical work. In such cases, the body's regulatory function to absorb nutrients and store fat are weaker than during exercise, but in contrast, people end up feeling hungrier and intestinal starvation is more likely to be caused, resulting in an increase in one's set-point weight. 

      
(4)The way sumo wrestlers eat and exercise is a logical approach to increasing muscle strength and body weight. By eating digestible meals including a good amount of rice twice a day, they are more likely to induce intestinal starvation. Intense training further accelerates this effect.
         

"The experts who say that we get fat because we overeat or we get fat as a  result of overeating - the vast majority  - are making the kind of mistake that would (or at least should) earn a failing grade in a high-school science class.

They're taking a law of nature that says absolutely nothing about why we get fat and a phenomenon that has to happen if we do get fat - overeating - and assuming these say all that needs to be said."
(Gary Taubes. 2011. Why we get fat. New York: Anchor Books,  Page 76.)

The way Wade explained it to me, the animal doesn't get fat because it overeats, it overeats because it's getting fat. The cause and effect are reversed.

(*snip*)

The evidence that fat tissue is carefully regulated, not just a garbage can where we dump whatever calories we don't burn, is incontrovertible.(*snip*)

Those who get fat do so because of the way their fat happens to be regulated and that a conspicuous consequence of this regulation is to cause the eating behavior (gluttony) and the physical inactivity (sloth) that we so readily assume are the actual causes."
(Taubes.  Why we get fat. Page 89-90, 93-4.)

"Just prior to the Second World War, European medical researchers argued that it is absurd to think about obesity as caused by overeating, because anything that makes people growーwhether in height or in weight, in muscle or in fatーwill make them overeat.

Children, for example, don't grow taller because they eat voraciously and consume more calories than they expend. They eat so muchーovereatーbecause they're growing."
(Taubes. Why we get fat. Page 9.)