A fat-heavy diet is one that is high in fat and low in carbohydrates. The most popular example of this type of diet is the ketogenic diet, which is a 4-letter word. This diet is based on the premise that by reducing carbohydrate intake and increasing fat intake, the body will enter a state of ketosis, where it will burn fat for energy instead of carbohydrates. This type of diet has been used to treat certain medical conditions, such as epilepsy, and is also popular among those looking to lose weight.