I don't agree there is always a disorder of some sort. I am slim, at age 44 now I only weight 3 kgs more than I did when I was 16. I have only ever fluctuated in weight by a few kgs either way naturally. This excludes pregnancy and the time I was on the depo injection but that to me was not my natural weight.
I do have IBS, but this only started about 2 years ago, so I wont say the ibs is linked to my weight. If I have a bad bout of ibs then I will lose some weight I agree, but once I'm ok again I will again just get to my usual weight and stay there.
I do think though, that food choices and exercise do play a part though. I don't eat cakes, pies, biscuits etc. And its not that I have high morals and wouldn't eat such a thing - I would, but I don't really like them. I don't have a sweet tooth at all. I don't put sugar on cereal or in tea/coffee, purely as I prefer it less sweet. I can though, polish off a whole bag of nuts and some salty crisps like some would eat a box of biscuits on a bit of a binge. I never ever have dessert. If we eat out I will have a starter and a main, where the rest of the table would have a main and a sweet.
Its genetics I am sure. My grandmother was the exact same and I am built and have the same shape as her. She was slim until the day she died, also never fluctuated weight over her lifetime.
Again, my ds is exactly the same as me. Doesn't eat much sweet foods, but will polish off crisps, nuts, and savoury foods the same as me. Dd, however, is just like her dad - she has such a sweet tooth. Ds is shaped just like be, a bit of a beanpole with more fat on a chip, but dd is more stocky and although she isn't overweight, is definitely more rounded with some meat on her bones.