Fuel for the human body takes three basic forms: carbohydrates (sugars), protein, and fat. Humans are capable of burning all three of these fuels, but do so at different times, rates, and under different circumstances. Using an extreme example, under starvation conditions the body burns its fat stores. Once fat stores are depleted the body begins digesting non-essential proteins and then finally essential proteins, which ultimately leads to organ damage and death. Thankfully, the body is efficient and uses the best available fuel first before it has to tap into essential reserves.
