Energy for endurance exercise is fueled mostly by carbohydrates and fats. Carbohydrate utilization increases as the intensity of the exercise increases. Since high-intensity exercise is carbohydrate dependent, carbohydrates are crucial to competitive endurance exercise performance.
Glycogen (stored, complex carbohydrate) is a primary source of energy for muscle movement, especially during high intensity exercise. Glycogen is found mainly in the muscles and the liver. In trained athletes, the equivalent of about 1,500 calories of glycogen is stored in muscles and 500 calories in the liver. Glycogen is made of glucose units that are released when glycogen is broken down. The conversion of glycogen to energy is more efficient than that of fat or protein.
The body typically has enough glycogen to fuel two to three hours of exercise at 65 to 80 percent of maximum heart rate. Storage capacity, even with proper diet and training, is generally insufficient to meet the demands of competitive endurance sports lasting longer than a few hours.
Muscle glycogen can only be used locally by the muscles that it is stored in. The liver is different; it can break down its glycogen into glucose and release glucose into the bloodstream to be used elsewhere.
Fruits and Vegetables
Carbohydrates from whole fruits and vegetables are generally coupled with fiber and loads of vitamins and minerals. Fruits are more carbohydrate-dense than vegetables; fruit generally packs more carbohydrates per serving than do vegetables.
There is mounting evidence that people who eat lots of fruit and vegetables are less likely to develop chronic diseases such as coronary heart disease and some cancers. These foods are healthy choices. Most of us don’t eat enough of them.
In addition to making high fiber fruits and vegetables part of each meal, use them for snacks. They can help keep the feelings of hunger away.
Choose a wide variety of colors and textures for all fruits and vegetables. Seek out deep greens, reds, oranges, and yellows. Choose from all: fresh, frozen, canned or dried. Variety, variety, variety…. And eat LOTS OF THEM.
The American Heart Association recommends enjoying lots of fruits and vegetables (five or more servings per day). These foods can provide fiber, vitamins, and minerals with few calories. They’re low in fat and sodium and contain no cholesterol.
Fiber helps make “good carbohydrate,” good. Fiber is an indigestible complex carbohydrate. It may be soluble (dissolving in water) or insoluble. Fiber is important for the health of the digestive system and for lowering cholesterol. Fiber slows down sugar absorption.
Foods containing fiber often are good sources of other essential nutrients. Depending on how they’re prepared, these foods can also be low in trans fat, saturated fat, and cholesterol. Fruits, vegetables, whole-grains, beans, legumes, and fiber-added foods are good sources of both soluble and insoluble fiber. Ready-made cereals can be a good fiber source; read the labels to make sure they provide at least four to five grams of fiber per one-ounce serving.
The American Heart Association (AHA) recommends eating a variety of food fiber sources. The AHA also recommends that at least half of grain intake come from whole- grain foods. Dietary fiber intake among adults in the United States averages about 15 grams. The Institute of Medicine recommends consuming 14 grams of fiber for every 1,000 calories ingested.
How Much Carbohydrate do Athletes Need?
Look at total grams of carbohydrate per day rather than the percentage of total diet as carbohydrate:
Aim for 5 to 7 grams of carbohydrates per kilogram of body weight per day (grams/kg/day) for general training. Since 1 kilogram equals 2.2 pounds, this is about 2 to 3 grams of carbohydrates per pound weight per day.
Aim for 7 to 10 grams/kg/day if you are an endurance athlete. Athletes training 10 or more hours per week are often fighting glycogen depletion. (See Table 1 below).
When glycogen is depleted during exercise, athletes will replace most of it if 11 to 12 grams/kg/day are consumed in 24 hours, although it may take up to 48 hours to completely restore them.
As total caloric needs increase, fat will help maintain weight (stay in caloric balance) and maintain intramuscular fat stores.
Once the recommended daily grams of carbohydrates are consumed, the remainder of your daily diet should be filled out with mostly good fat, lean protein, lots of fruits and vegetables. Don’t forget to drink your milk.
See Appendix A for a list of popular foods and their macronutrient distributions.