There are four components in food and drink that are capable of producing energy:
When you eat a meal or have a drink, these components are broken down in the digestive system into their various constituents or building blocks. Then they are absorbed into the bloodstream. Carbohydrates are broken down into small, single sugar units: glucose (the most common unit), fructose and galactose. Fats are broken down into fatty acids, and proteins into amino acids. Alcohol is mostly absorbed into the blood.
The ultimate fate of all these components is energy production, although carbohydrates, proteins and fats also have other important functions.
Carbohydrates and alcohol are used mainly for energy in the short term, while fats are used as a long-term energy store. Proteins can be used to produce energy either in ‘emergencies’ (for instance, when carbohydrates are in short supply) or when they have reached the end of their useful life. Sooner or later, all food and drink components are broken down to release energy.
Source: “Where does energy come from?,” from The Complete Guide to Sports Nutrition, by Anita Bean