People throw around this idea as if it was simple.

Just the concept of calorie, which is bound to water temperature changes, is at most a vague second or third order approximation of a body’s energy intake. Petrol would be of super high calorie for instance, but it makes no sense for the body.

We have no clear idea of what any food gives as energy to a body, nor how each body reacts differently to any food, nor how much of that goes out, when that potentially stored energy is used in what amount (would CO2 production measurement help in this regard ? even if so but then it might be get rid of in unexpected ways)

So while the in/out idea may not be completely wrong, it’s effectively useless to anyone seriously thinking about diets.

If you absorb more calories than your metabolism burns, then you gain weight. Variables are:

- Calorie content of food.

- Ability of your body to absorb those calories. This includes what form the calories are in (petrol, obviously, doesn't work), but also may include your gut biome, and perhaps other person-specific issues.

- Your activity level (not just exercise, but non-exercise physical movement as well).

- What your food does to your metabolism.

Sure, if you eat too much of the wrong things, it hurts. If you exercise, it helps. But it's much more complicated than that.

No, it isn't. The biochemistry controlling nutrient uptake or release by different tissues is incredibly complex and nowhere near fully understood, which is part of the reason that studies like this one are interesting. We don't really know how any given calorie surplus or deficit maps to subcutaneous fat vs. visceral fat vs. thermogenesis vs. muscle vs. fatty liver. We don't really know why some people develop T2D at a normal weight, some people only develop it if they're obese, and some people never develop it despite being morbidly obese.

Pretending that this is a simple and solved problem doesn't really help anybody except authors of diet books.

People with bowel issues for example might actually consume vastly more calories than their peers but not gain as much weight due to their body being less efficient in converting caloric intake into fat. People with a fast metabolism might burn more calories per day at rest than others as well, and some of the complex interactions could involve interactions with caffeine.

If people are gaining weight rapidly it's always a good idea to look at the whole picture rather than just caloric intake because that can mask the true underlying causes.

FTFY

(of course I’m discounting weight gain due to fluid accumulation if you have circulatory problems or ascites)