https://en.m.wikipedia.org/wiki/Conservation_of_energy
https://researchblog.duke.edu/2021/03/24/duke-researcher-bus...
I got back to a healthy weight by losing 15kgs. It can be done.
Interesting. I like to walk generally, but in the last year, with a fresh Garmin watch prodding me into more steps, I increased my walking by at least 50 per cent. I actually made some 5 million steps last year, at least 7 km every day, in every weather.
My weight fluctuated a bit, but basically didn't change. I am not fat, but I have some amount of belly fat I don't like.
What really changed and improved was my immune system. I am not totally immune to everything, but I tolerate a lot more environmental distress (cold, hot, sweating, getting soaked by rain etc.) before encountering first symptoms of having a cold.
What exercise helps with is making sure that the occasional binge, or change in circumstances, doesn't lead to permanent weight gain. If you're temporarily disabled for 2 weeks and can't move and put on some weight, your body then burns more of that fat when you go back to exercising, or regulates your appetite so you eat less.
But if you're not used to exercising and then suddenly increase your caloric intake, your body has less opportunity to burn the extra fat, so it leads to long term weight gain, and eventually your gut biome and body adjust to the new normal.
Walking and exercise make a HUGE difference in average weight. Just look at Canada, a place mostly analogous to the US, but with way more walkable cities and better public transit. They don't have this problem.
Regardless of how people get fat, we know for a fact that it's nearly impossible to get back to a healthy weight. Children of fat parents are more likely to be fat, both because of genetics and because of diet. Is it their fault that they were born that way?