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Your body is an engine. It runs on a mix of carbohydrates, fats, and a very small amount of protein. And it’s a finely tuned engine, using the best fuel for every need.
» Carbohydrates Broken down into sugar by digestion, they provide quick energy. But for high-intensity activities, your body can’t store much more than a two-hour supply (hello, gel shots).
» Fatty acids This is by far your largest fuel source, stored as adipose tissue (that is, fat). A 200-pound man with an average 20 to 22 percent body fat has about 45 pounds of adipose tissue, or enough fat to fuel the body for 73 days. But to provide energy, fatty acids must be refined into ATP, the “gasoline” that fires muscle cells.
» Protein It’s used in small amounts when carb reserves are depleted, but don’t worry about burning too much muscle— unless you’re literally starving.
» Pace The faster and harder you go, the more carbs—and less fat—enter the mix. At maximum effort, maintainable for only a few minutes, the body operates on pure carbs. But during long, slow aerobic activities like backpacking, the body burns fuel that consists of up to 60 percent fat.
*BMI CHART Source: WebMD; calculate your precise BMI number at webmd.com/diet/calc-bmi-plus. Note: BMI numbers can be misleadingly high for fit athletes.
**TARGET HEART RATE This number will vary for individuals. Rule of thumb: If talking is an effort, you are going too hard.
***GO THE DISTANCE Numbers are for a 40-year-old,180-pound, 6-foot man carrying a 40-pound pack over steep terrain.