Quang said: If they claim that exercising isn’t exactly beneficial for losing weight, then can they explain why people who sit in front of their computer all day are more likely to be overweight that those who go out regularly?
The bottom line is you will need regular exercise no matter what.
Diet is still very important. If a person is exercising regularly, they might be able to eat more “junk” food (I’m including most carbohydrate-rich foods in this category due to their role in gaining weight, not just processed food and sugar) without gaining weight. Depending on what they are eating, however, it is still possible to exercise often and gain weight as fat rather than muscle.
If someone doesn’t exercise and eats the same foods, they will gain weight. But if they eat the right foods, that sedentary person would be able to get away with only a small amount of exercise (say, a walk around the block every day?) without gaining weight.