Dietitians are not taught obesity is solely the cause of the individual. Clinical dieticians often have to work with people who have something medically complicating the process.
But in general, yeah…. If you find the average dude on the street who is morbidly obese and you have them reduce their calories by 500 calories or so a week until they are around 2,000 or so calories, and you have them lift weights consistently they will start to lose weight and get back into a healthier and more fit body. Now if that 2,000 calories
Is junk food, they will still lose weight but will develop other issues. Like bad body composition and so on.
But if you had them where 90% of their calories was fresh fruit, vegetables, beans, and cut out most sugary drinks like soda and so on, and develop an actual meal plan, they will most likely be healthier.
If I were to suddenly start eating an extra 750 calories a day of ice cream and soda, and fried cheese balls, and I reduced the time I spent hiking and working out significantly I would gain weight.
Most people, not all, but most people, who are morbidly overweight, tend to eat way too many calories and don’t consistently work out. Same with other animals too like cats. I have three cats. Two of them are normal healthy weight. One of them is a bit chubby. The chubby cat eats and eats. The other two like to nibble a bit, leave and come back to it. But now, they leave and the other cat would eat it. They thinned out a bit and the chubby cat, Ngipin, got bigger. So now I use a food puzzle to feed them. It takes Ngipin about 10x longer to eat the same amount of food. So she’s eating less, my other two cats also is eating from the food puzzles and so they nibble, leave and return. I take Ngipin for an extra 15 minutes a day into my bedroom without the other cats, and play with her using the laser light. She’s starting to move towards a healthier weight.
My brother has a lab and it stays mostly indoors. He has no doggy door because they don’t want their cat getting out of their house. So the dog gets let outside several times a day. They use to just leave tons of food in the bowl and she would over eat. She would still be let outside. But when they reduced the food, and started taking the dog on 2 30 minutes long walks and had their kids go outside and play ball with them, they also got in better shape.