Utilizing fat as an energy source is ketogenesis. Ketogenesis can occur in a fasting state and in people who eat a ketogenic diet. The ketogenic metabolic transition happens after about 24-36 hours of strict carbohydrate restriction. And we mean strict carbohydrate restriction, as in little to no fruit, bread, rice, beer, et cetera. If the body is truly deficient in carbohydrates, it will begin to break down its fat stores to use as energy. Stored fatty acids get broken down and converted to ketone bodies (acetoacetate and Beta-hydroxybutyrate) which are used by the body to make energy. The process ultimately leads to fat loss with no impact on lean muscle mass.