I hate the word diet. It denotes eating different for a certain period of time to lose weight. What the vast majority of these people need is not a diet, but a lifestyle change. They need to eat less portions and healthier foods.
30 years ago as a Marine drill instructor I ran the physical conditioning platoon for a while. It was called various things, the fat platoon, the pork chop platoon. These are the recruits that cannot make it physically and are sent to PCP until they can make it physically or drop out. A large majority of them are usually overweight and many of them do end up dropping out of boot camp. The ones that survive are better for it.
I can tell you from my experiences there, fat shaming (though we didn’t call it that back then, we called it giving the recruits the proper motivation), rigorous exercise and forcing a lifestyle change in eating habits on them works. Everyone of those fat recruits ended up losing weight no matter if they stayed in boot camp or dropped out.