Most people don't announce to scientists that they've lost weight. Lot's of people
do lose weight and stay that way, it's just that there are even more who don't.
Consider that the rate of obesity continues to increase in the US. This shows that people are simply eating more and doing less exercise. And of course those that are already obese are the least likley to consume healthy ammounts of food and exercise as needed.
In other words, yes, the issue with overeating is in fact overeating. The only way to solve this problem for the whole population (as in, not for a single individual) is to change society and how it views unhealthy food. Off the top of my head, fast food advertisments should be made illegal. The entire point of those ads to get people to eat fast food. This single step could reduce obesity in the US by a noticable amount and requires no loss of freedom to any individual. A sugar tax or the like is a more extreme solution. But this idea is not unfounded:
>Sugar, rum, and tobacco are commodities which are nowhere necessaries of life, which are become objects of almost universal consumption, and which are therefore extremely proper subjects of taxation.
-Adam Smith
Book V: On the Revenue of the Sovereign or Commonwealth
Chapter III: On Public Debts
https://www.adamsmithworks.org/documents/chapter-iii-of-publ...