When we stop eating the body makes use of the accumulated reserves. First it uses the carbohydrates. Once consumed, it begins to use stored fat. It is for this reason that obese people can survive longer without food.
The last step, after consuming carbohydrates and fats, is nourished with its own proteins. Literally the body eats itself. When proteins are depleted the death ensues irretrievably.
Being obese can prolong the agony, but the time it takes to disappear also depend on the degree of willing that we have when fasting. For example, an IRA prisoner who in 1981 fasted to death survived a total of 73 days (just over 10 weeks) which can only be explained by the fanatical involvement that he took in the process. Nor is the same to face a hunger strike physically fit and resting in the bed that assume the same working hard and in poor health. Also in a very cold enviroment much of our body reserves would be spent on maintaining body temperature which undoubtedly would accelerate the fatal outcome.
There are many stories circulating on the Internet that relate to people who have survived 100 days or more without food, or even years without eating anything by mouth. They suggest that the energy of their bodies comes from the sun or from similar 'energy sources'. The truth is that it has been never scientifically proven that no human being could survive without food, although it is true that years ago a person an affected from morbid obesity was not taking food for a whole year. This person had enough to use the fat reserves of his body, but he was medically controlled at all times (I think he even spent the whole time of fasting in a hospital ). Also he was supplied with other essential vitamins and mineral compounds to survive without serious health problems.
In a case of prolonged fasting the body part that suffers most is the brain. In fact much of the energy we consume is intended to support its operation. If the brain lacks energy it can induce hallucinatory states or lethargic, unequivocal sign that death is near.
Although there are thousands of evidence that people who do not eat die of starvation - obviously - there is a current of thought called Inedia (Latin word meaning fast) which claims that the human being does not need food and even water to survive, being sufficient to feed with prana, which for the Hindus is the life force.
News of persons appearing occasionally in the press claiming that they live without eating or feeding themselves with a tiny amount of calories are really dangerous.
Despite its obvious falsehood, all you get is that some of those who foolishly cling to that pseud belief end up dying of hunger and thirst, not to mention the collateral problems of anorexia and bulimia that can cause among adolescents.
As someone said, the human body is like a light bulb that needs electricity to provide light. And point.