When you follow a diet which drastically restricts the amount of carbs you consume, you will not have enough glucose in your blood to fuel your body. In this situation, your body turns to fats for energy. When your fats are broken down, small molecules called ketone bodies are produced, which act as an alternative source of energy. This condition is known as ketosis, and it is a natural state your body goes into. Since your brain requires a constant supply of energy, it would shut down if your body did not produce this alternative fuel source, but it cannot run on ketones forever. Low-carb diets or ketogenic diets, which help your body get into the state of ketosis, help you lose weight quickly but can adversely affect your overall health.