Do you know how your body transforms that slice of pizza into energy? The concept behind it is simple: energy is released whenever chemical bonds are broken. After digestion and absorption, food molecular bonds are broken, releasing energy to sustain body functions such as growth and repair, transportation of substances, and muscle activity. The human body uses mostly carbohydrates and fat for fuel. Protein typically offers only a small contribution toward energy.
All carbohydrates are ultimately converted into glucose and stored as glycogen in the muscles and liver. Glucose circulates in the bloodstream in order to be distributed to all cells. Liver glycogen is used to maintain blood glucose at a constant level. This means that when blood sugar falls, the liver breaks down glycogen, releasing glucose into the bloodstream. The glycogen stored in the muscles, on the other hand, is mainly used to fuel the muscle cells. The problem is that glycogen storages are limited and are easily depleted with high-intensity or prolonged exercise. (Muscles usually have enough glycogen to support one and one half to two hours of continuous exercise.) Thus, storages should be constantly replenished through the consumption of carbohydrate-rich foods such as breads, pasta, rice, legumes, fruits, and vegetables.
Dietary fats are broken down into smaller units (triglycerides, phospholipids and steroids) before being stored. When compared to carbohydrates, fat breakdown releases significantly more energy (9 kcal/g, while carbohydrates release 4 kcal/g). In addition, fat storages are substantially larger, which makes fat a potentially unlimited source of energy. It would be the perfect fuel for exercise, except that fat is less readily available for cellular use than carbohydrates. There are two main problems. First, to be used as fuel, fat stores must be broken down in order to release fatty acids into the bloodstream. Second, the body cells must have the appropriate enzymes to metabolize fats. These factors make fat harder to burn than carbohydrates. Moreover, fat can only be used for fuel in the presence of oxygen. This means that as exercise intensity increases (reducing your ability to breathe normally), the contribution of fat decreases.
Usually, dietary protein is not used for fuel. The body tends to spare protein to be used as building blocks for muscle growth, not for energy. However, when carbohydrate intake is inadequate, the body may use protein for energy, converting it into glucose to supply the brain. Unfortunately, this leads to an undesired muscle loss.
Fueling up for exercise
During the first 20 minutes of aerobic exercise (walking, running, and swimming, for example) the muscles mostly rely on glycogen for fuel. After that, fat begins to be used in order to spare muscle glycogen. As exercise progresses, fat contribution for fuel gradually increases while carbohydrate decreases. However, when glycogen storages are depleted (usually after two hours of continuous exercise), blood sugar starts to fall, mental function is reduced, and exercise becomes too hard to sustain. The body slowly shuts down in order to spare glucose for the nervous system. This is known as “hitting the wall.” That’s why endurance athletes that compete in long distance events must refuel every hour or so.
In addition, high-intensity bouts of exercise, such as sprinting and jumping, use mainly glucose for fuel, which may deplete the glycogen stores quickly. Low to moderate intensity exercise, such as jogging and walking, uses glycogen more conservatively. However, if exercise is sustained for long periods, glycogen storages will be depleted at some point.
Newbie exercisers may deplete their glycogen storages faster than trained athletes. With regular exercise, the body undergoes physiological adaptations that enable muscles to use more fat while conserving glycogen. This is because with training, the amount of fat enzymes inside the muscle cell increases. In addition, a trained body is able to release more fatty acids into the bloodstream. Thus, regular exercise improves fat utilization in the long run. This provides one more reason to stick to your workout.
References
Boyle, M. and Long, S. Personal Nutrition. Belmont, CA: Wadsworth Cengage Learning. 2010.
Wardlaw, G. and Smith, A. Contemporary Nutrition. New York: McGraw-Hill. 2009.
Wilmore, J., Costill, D. and Kenney, W. L. Physiology of Sport and Exercise. Champaign, IL: Human Kinetics. 2008.