(s)he should reply with " standing up will provide you more benefit then sitting
I should be careful with that term though. A small part of that group has this due of medical problem ( a known example is (night) eating syndrome where the person is consuming food without to know it ) But for the rest, it is ofc partially a matter of choice, but also a matter of life style.
a scientific research has reported that ppl with low salary has almost double higher chance to get obesity towards people with decent/higher salary ( towards average ). Companies like McDonald's and KFC are using "forcing consumption" marketing strategy to provide cheap food, and yet those ppl are being more attracted to this because they couldn't afford food variation, couldn't get more£/equal consumption feeling for same cost and other things.
a small addition ; there was a movie in the past where a person is going to consume only McDonald's food for a month long ( Super Size Me
). The result of this behavior is quite disastrous ... Not to mention that there are ppl which is doing this for years.
To not forget, besides food consumption as main factor, not enough movement is also a factor which leads to having obesity. If there is no medical cause for it, then it's not a disease because it's caused by the person itself.