I don't think that's how it works. People you perceive as "naturally thin" probably simply:
* are used to not snacking between meals
* eat when they are hungry, and stop when they are not
* do not drink (excessively)
* are more-than-average active
Calories add up extremely quickly and it's very easy not to notice if you don't pay attention. Say you have one Mars bar per day, as a snack - that makes a difference of 10 kg of body weight per year as compared to someone who skips such a snack (a back-of-the-envelope calculation).
And what is "not natural" about any of that? Some people naturally regulate their intakes such that they don't get fat. It's not a conscious effort on their part. They eat what they want to and don't get fat. It's as simple as that.
There are certain breeds of dogs and cats that will eat until it makes them sick and easily get fat. Other breeds don't. Even with food readily available they just don't get fat. Humans are no different.
Yes, but some people's bodies will burn excess calories by generating heat while other's will immediately store it as fat. When it comes to nutrition, nothing is quite as simple as you hope.
I haven’t read much about the heat fact but the less body mass you have, the more you’d have to burn for heat (there is a relationship between body size and metabolic rate which is inversely proportional, small mammals and infants burn insane amounts of energy just keeping warm).
I did read a study on non exercise thermogenesis and how some people managed to burn quite a bit of calories just fidgeting throughout the day.
So while we all run around the same human body temperature and thermodynamics is all the same, different people arrive at different results because of different behaviors. To what extent they are genetic is up to debate but much is genetic like “a tendency to fidget” or “a tendency to like sweets”.
* are used to not snacking between meals
* eat when they are hungry, and stop when they are not
* do not drink (excessively)
* are more-than-average active
Calories add up extremely quickly and it's very easy not to notice if you don't pay attention. Say you have one Mars bar per day, as a snack - that makes a difference of 10 kg of body weight per year as compared to someone who skips such a snack (a back-of-the-envelope calculation).