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It sounds like a no-brainer that if you eat more you'll gain weight. Given that 1 kg of fat contains 8000 kcal of energy, you can estimate how much weight you'll gain if you eat more. But there are reasons to be sceptical here. There seems to be experimental proof against this simple model that assumes that the metabolic rate does not adjust itself to the energy take, see e.g. here.

Then there is a theoretical argument against the simple model. Animals living in the wild have to cope with whatever is avaialable to eat. If they find temselves in a situation where they need to expend a bit more energy to get to their food and what they then get isa bit less, they would eventually starve to death if their metabolic rate would not compensate for that. So, if you eat one sandwich of 100 kcal a day less and keep all the other factors the same, you should lose 1 kg every 80 days, which means more than 45 kg weight loss in ten years. But this obviously not going to happen.

It also cannot be the case that people will feel more hungry if they eat a bit less, because some people like me stick to a diet where they measure everything they eat and their weights typically does not fluctuate wildly on a time scale of years. Nevertheless, the calories in mines calories out = weight gain model is widely used, but it seems to me without any solid scientific basis.

My personal observations suggests that people who are obese don't eat all that much, rather they tend to eat unhealthy foods and they are physically unfit. In contrast, I eat a lot, about 4000 Kcal/day, but I'm physically very fit and I only weigh 57 kg. I eat a lot more than everyone I know, but I stick to eating only healthy foods.

I would be most interested in experiments where healthy, physically very fit people were given a lot more to eat, but only healthy foods. E.g. larger portions of brown rice at dinner, more whole grain bread at lunch, more vegetables fried in olive oil etc. while their exercise routine is monitored before and after the intervention so that the energy expenditure change due to voluntary factors is taken into account. A steady weight gain according to 8000 Kcal energy surplus = 1 kg in such an experiment would prove the simple model.

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I'll summarize from http://evidencemag.com/why-calories-count/

  1. "When people create a caloric deficit, they always lose weight." [...] "Whether they eat mostly protein, fat, or carbs makes no significant difference in how much weight they lose."

  2. "When People Eat More Calories than They Need — They Always Gain Weight".

This is supported by 10s of references and experiments, which I can expand this answer to discuss if anyone thinks this requires more detail.


About some of your points:

  • There seems to be experimental proof against this simple model that assumes that the metabolic rate does not adjust itself to the energy take.

    The calories in vs calories out argument doesn't ignore that metabolism or a person's activity level might change (mostly through "non-exercise activity thermogenesis"). It says nothing about what might happen to calories out. All it says is that you need to be at a caloric deficit to lose weight, and a caloric excess to gain weight. True, your calories in might affect your calories out, but that doesn't mean that calories in vs calories out is invalid.

  • If they find themselves in a situation where they need to expend a bit more energy to get to their food and what they then get is a bit less, they would eventually starve to death if their metabolic rate would not compensate for that.

    This is a false dichotomy. You have left out the option of the animal expending less energy on non-hunting activities to deal with having less food available. In either case (being less active during non-hunting activities, or a metabolic change), all you're saying is that the calories-out might change.

  • So, if you eat one sandwich of 100 kcal a day less and keep all the other factors the same, you should lose 1 kg every 80 days, which means more than 45 kg weight loss in ten years. But this obviously not going to happen.

    If you did manage to keep all the other factors the same, why is 1kg of weight loss every 80 days "obviously" not going to happen? Is it even possible to keep all the other factors the same? As you lose weight, your body will need to expend less energy (less calories out). Thus, your calorie deficit will not be 100 kcal for the entire 10 years. You can't ignore the change in calories out in the equation.

  • It also cannot be the case that people will feel more hungry if they eat a bit less, because some people like me stick to a diet where they measure everything they eat and their weights typically does not fluctuate wildly on a time scale of years

    This is a hasty generalization. Just because you can stick to a chosen diet by measuring and being disciplined, doesn't mean other people will. It also says nothing about the level of hunger felt by a person. Diet compliance during studies that rely on self-reporting is not good. (See Discrepancy between self-reported and actual caloric intake and exercise in obese subjects.

  • My personal observations suggests that people who are obese don't eat all that much, rather they tend to eat unhealthy foods and they are physically unfit. In contrast, I eat a lot, about 4000 Kcal/day, but I'm physically very fit and I only weigh 57 kg. I eat a lot more than everyone I know, but I stick to eating only healthy foods.

    Your personal observations are anecdotal and can not be taken to be representative or accurate. How did you track the amount eaten by the obese people that you have observed or the people that you know? Nothing you've said gives me reason to assume that the only difference between you others is the "health" of the food that you eat.

  • Please consider using some of these same references to tackle this question – Oddthinking Jun 16 '14 at 1:21
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    "When People Eat More Calories than They Need — They Always Gain Weight". That needs to be qualified. The calories also need to be absorbed. Some diets, eg meat, release more calories for absorption by the action of intestinal bacteria. – HappySpoon Jun 16 '14 at 1:34
  • Thanks for the detailed answer. I'll see if I can rephrase my question to make it more focussed on the problem that I think exists between the theory and the facts. – Count Iblis Jun 16 '14 at 14:21

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