A ketogenic eating plan is a diet that is very low in carbohydrates, moderately low in protein, and very high in fat. Your body normally gets energy from eating carbohydrates. If you limit carbohydrates in your diet, your body will start to use stored fat as an energy source. When fat is broken down for energy, it enters your blood as a substance called ketones. A ketogenic eating plan relies mostly on ketones for energy. This eating plan can cause rapid weight loss because the body burns fat for fuel.