It's true any time of year: In order to do work, you've got to have fuel. If you leave for a morning run with an empty stomach, your body will eventually act the same way a car with no gas behaves; it'll attempt to do what it's being asked to do, but instead it'll sputter and stop. The same is true of swimming, weightlifting or cycling. Without fuel, it can't be done effectively.
This exercise truth is magnified in the winter due to the fact that you need heat to survive. Fortunately, the digestion process not only distributes energy throughout the human body, it creates heat as it does so [source: Clark]. It's the physiological equivalent of putting coal in a furnace. In fact, approximately an hour after eating, the body will create 10 percent more heat through a process known as thermogenesis [source: Clark].
The key is to find the fuels that burn quickly and easily but continue to provide energy for extended efforts. That energy can come from fats, protein or carbohydrates. About half of the energy used while exercising comes from carbs. Fats are also needed but in much smaller amounts. Finally, protein is crucial for rebuilding the microscopic tears in muscles during workouts [source: Snyder].
Click ahead to learn about the most effective foods to push you through a tough workout and keep you warm.