Ketone bodies are synthesized from lipids during prolonged caloric deprivation and have pleiotropic effects on metabolism. They improve energetics and fuel sparing, and may be used to enhance human exercise endurance performance by mimicking advantageous aspects of starvation physiology. While exercising during endogenous ketosis may not be advantageous, the delivery of exogenous ketones creates a novel physiological state with high circulating ketone concentrations and replete carbohydrate stores, offering potential benefits for endurance performance.