Starvation spurs the body into a metabolic states called ketosis, which means that it begins breaking down its own fat stores for energy, according to Lecia Bushak for Medical Daily. When there is no longer any fat available to be burned for fuel, the body begins to break down its own muscle proteins. The body's metabolism slows down considerably during this time in an effort to conserve energy.
The human body experiences progressive stages of metabolic change depending on how long a person goes without food, according to Bushak. Starvation officially occurs after an individual goes without food for three days. The state of ketosis that the body enters is the same phase that low-carbohydrate dieters seek to achieve during the induction phase of that diet.
Ketosis is not necessarily unsafe, but Bushak notes that the brain relies heavily on the intake of carbohydrates and has a very difficult time operating solely off of fatty acids. Therefore, starvation causes the brain to decrease its ability to function at peak performance. According to Bushak, the human body may be able to endure starvation for as many as three weeks, depending on the amount of a person's fat stores. In rare cases, people have been known to live for up to 70 days when they also had access to water.