[sn9]

Increasing activity increases calories burned, which can help produce a caloric deficit which will lead to weight loss.

Obesity is literally impossible without being in a caloric surplus over the long term.

Added sugar contributes to the problem, in that it is not very satiating so it makes it easier to overeat, but you can easily lose weight on a high sugar diet so long as a caloric deficit is maintained.

The actual physiology is well-understood. The only problem is changing habits and behaviors, which can be difficult but is certainly not even close to impossible.