Ketones (also called ketone bodies) are molecules generated during fat metabolism, when our bodies break down fat for energy. ketones are a super fuel that generate more energy (ATP) per unit of oxygen consumed than do carbohydrates or protein. The brain, heart muscle and kidneys, for example, prefer ketones to carbohydrates for fuel.
Low carb diets such as Ketogenic, Paleo and Atkins, have quickly grown in popularity and have proven to be significantly more effective for weight loss and prevention of heart disease than low fat diets (Annals of Internal Medicine).
Supplementation with exogenous ketones (ketones that mimic what naturally occurs in the body when fat is consumed) may help alleviate the fatigue and lethargy some experience while making the transition from a glucose (carb) to a ketone metabolism. For individuals on non-carbohydrate restricted diets, ketone supplementation may also be usefu in providing and additional energy source and may also limit exercise induced glycogen depletion. The greatest effects are seen in those on low carbohydrate diets because ketones replace carbs as a source of energy during physically demanding tasks.