Fasting vs. Eating Less: What's the Difference?
The upshot is that it is much more difficult to just eat less, especially if what you're doing is eating primarily carbohydrates. The reason is interesting: Insulin tells your body not to burn fat. So if you're eating enough carbohydrates to stimulate a significant amount of insulin (as the volunteers did in the starvation experiment — around 1000 calories of carbs a day) it isn't enough fuel for your body, but it prevents your body from accessing your stores of fat, so you starve. If they had eaten no food, they would have been able to burn more calories and would have suffered less.