During fasting, what sources of fuel does the body primarily use?

During fasting, the body primarily shifts its energy source from carbohydrates to a combination of fats and proteins. Initially, the body utilizes glycogen stores, which are carbohydrates, but these are limited and deplete quickly. Once glycogen stores are exhausted, the body transitions to using fats as a primary fuel source.

Additionally, during prolonged fasting, the body begins to break down proteins, particularly from muscle tissue, to produce glucose through gluconeogenesis. This process ensures that essential functions can continue to occur despite a lack of ingested food.

The utilization of multiple fuel sources—proteins, fats, and carbohydrates—enables the body to adapt to periods of fasting and maintain energy levels as well as metabolic functions effectively. This adaptability illustrates the body's remarkable ability to manage energy needs through various substrates depending on availability.