Right now, I believe the answer to diet/eating/health is the same as the nature vs. nurture issue- it all depends because it's highly interactive, and it varies in the same person in different circumstances. Ideally, as
@Snowman says, the body and appetites regulate themselves and it's easy and natural to stay in the green zone. However, when some switch gets flipped in some people for some reason, a cycle gets going with food (especially carbs) that gets out of hand. With time, the ability to metabolize fat and the blood sugar levels go way out of whack. For many in this stage, bread, sugar, etc. become more like an addiction and much harder to control. Listening to the body for them in that stage is going to lead to extra body fat, and eventually diabetes, etc.
At this stage, some intervention is needed to get everything back on track. There needs to be a way of letting the body get back into the natural cycle of fat metabolism, less reliance on blood sugar to feel good, and avoiding the starving feeling when the body reduces the blood sugar suddenly with insulin. This is where some form of intermittent fasting seems to work well- I've had success with the 3 meals a day in 12 hours, with no snacking habits of Georgie Fear's book, others use other plans, but they all serve to get the body back to being able to manage hunger by metabolizing fat and not relying on frequent hits blood sugar to feel good.
Exercise helps with the "demand" and "efficiency" of the system, but it is, as almost everyone seems to acknowledge, a nutrition issue. If anyone's accumulating extra body fat and craving starches and sugars, figure out a some plan you can live with that gets you out of that loop. Cut way back on the starches and sugars for awhile, find a way to go more hours without eating, and let your body go through cycles of satiety and hunger, and you'll lose the cravings and your body will regulate itself quite well. Then you can probably enjoy nice piece of cake or toast or whatever, in moderation, when appropriate, without binging or gaining weight.