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I have always wondered how this come about.

At work we hade a conference about this and some people say that some people get over weight because it´s genetics, even if they eat the same amount of calories as any other, they grow bigger right, that is if the intake is the same as others their fet cells would miraculously grow larger!! Most people leaned to the genetics explanation and I felt it was out of respect more than trough Empirical evidence.

Does the same amount of calories have different result on people?

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    Would you expect x number of calories to have the same effect on a 240 pound male athlete and a 95 pound elderly female? Why or why not? – Carey Gregory♦ Dec 19 '18 at 2:57
  • My question does not mention this, this is irrelevant I should have mentioned it in my q sorry – Erik Dec 19 '18 at 19:07
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    I'm voting to close your question as unclear and have also downvoted it because you are responding to input in comments and answers that is highly relevant to the question you wrote by saying it is "irrelevant" - if you meant to ask a different question please ask that one instead. – Bryan Krause Dec 19 '18 at 21:13
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There are various processes that influence weight gain based on calorie intake. It is in general clear that a severe reduction in calories (well below your body's basal needs) reduces weight in the longer term. I'm not sure any other scenario has consequences that are the same for all people.

An important factor is actual intake. Based on your genetics, diseases and your gut biome, you may not take up calories from the same foods as efficiently as someone else. Extremer examples are people with celiac's, crohn's or ulcerative colitis. Though many milder afflictions will also influence weight gain. Note that people with the above diseases can still be obese due to other factors. You can effectively be in a state of malnutrition and obese (sugar is one of the easiest things to take up, essential vitamins are less easy).

Another important factor is hormones and steroids. Imbalances in hormones and steroids can easily distort how you process sugar and fat, leading to a different reaction. Common example is too little or too many thyroid hormone. Too little causes weight gain (both due to fat and due to water swelling) and lethargy (exercising just got harder).

On top of that, you would also need to look at what is actually gained in weight. You could gain fat, water or muscle. All of these are influenced to some extent by the factors above. And also by what you eat exactly and what you do otherwise.

Note that most of these factors are influenced by both genetics and environment.

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The amount of calories someone consumes is just one side of the coin - you have to consider how many of those calories they are actually expending.

The most visible element of that is the amount of activity the person is doing - exercise for example. Imagine two otherwise identical people (Alex and Bob) who have both consumed say 2000 calories. Alex spends the rest of the day after eating sitting on the couch enjoying old episodes of Rick and Morty on Netflix (Keep Summer safe!) whereas Bob went out and ran a marathon, helped their elderly neighbor move their living room furniture around (Bob's a good guy or "sucker" depending on your perspective!) and then cycled 5km to the shop for cat food. Now who's going to have more calories available for storing in body fat - Alex or Bob?

They've both consumed the same amount of calories but Bob has burned more of those off then Alex.

Now you're no doubt thinking to yourself "well that's obvious - but I've seen Charlie and Derek who both spent the afternoon watching Netflix but Charlie is obese and Derek is a stick insect, what gives?"

Well that's because the majority (~ 70%) of our calorie burning is actually done by the basic processes of keeping the body running. This is called the Basal Metabolic Rate (BMR) and this varies between different individuals and while some of the factors dictating this difference have been identified and understood we don't have a complete understanding of it yet.

Additionally not only are we humans not all the same - not all calories are either. Different foods might have the same numeric calorie values but depending on how they are made up (fats, sugars, carbohydrates etc) the body will digest and process them differently and this can effect body fat levels. A person eating a diet high in saturated fats will gain fat faster than someone eating the same amount of calories from Low-GI sources.

So just looking at the amount of calories people consume is insufficient to fully answer the question of why some people are obese and some aren't.

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  • This is not relevant, of course if you burn your calories you dont gain weight. That is not what my question is about – Erik Dec 19 '18 at 19:02
  • The actual conclusion of the study is "There is growing recognition that replacing saturated fat with refined, high glycemic index carbohydrates increases postprandial glycemia and may be detrimental for weight control and predisposition to cardiovascular and inflammatory disease. In contrast, low glycemic index carbohydrates reduce risk". It seems to mainly look at high GI sources compared to low GI sources. It's not clear from the abstract that they actually looked at weight gain from saturated fats. Just that they looked at replacing saturated fats (for the standard reasons I assume). – HSquirrel Dec 19 '18 at 19:57
  • @HSquirrel you're correct.. I've posted the wrong link! On my phone at the moment so I'll correct it in the morning – motosubatsu Dec 19 '18 at 22:05
  • @motosubatsu No problem. I look forward to seeing the study you're referring to. – HSquirrel Dec 20 '18 at 23:38

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