Carbohydrates are metabolized for use by the body before fats. Your body stores carbohydrates in the form of glucose inside the cells and muscle tissue, allowing it to be readily available for use. Fat, however, is stored in adipose storage sites that are farther away from working muscles, thus increasing the time it takes to convert it to usable energy. According to "Anatomy and Physiology" by Kenneth Saladin, carbohydrates are also digested and absorbed quicker than fats.
Digestion
In order for a nutrient to be used, it must first be digested and absorbed. In the race for digestion and absorption, carbohydrates win. In fact, carbohydrates start breaking down from the moment they enter your mouth. Salivary enzymes work to degrade carbohydrates and fats, also known as lipids, aren't digested until they enter the stomach. From the stomach to the small intestines typically requires about four hours of digestive work. If you eat a meal high in fat, however, gastric emptying can take as long as six hours. Once in the intestines, carbohydrates are absorbed by small capillaries while fat is taken up by lacteal cells.
Storage
Both carbohydrates and lipids are stored in excess around the body. Since carbohydrates are metabolized as glucose, molecules of glucose are put together and stored together as glycogen. Lipids are stored as a triglyceride molecule. Each triglyceride is made of three fatty acid chains and a glycerol. When a lipid is metabolized, your body must break down the triglyceride into a usable form. This takes more time and energy to do than breaking glucose off of glycogen.
Location
Glycogen and triglycerides are stored for later use. Physical activity increases your need for energy. Since speed is a necessity during activity, your body stores glucose in an easily accessible place -- inside your muscle. Carbohydrates, stored as glycogen, are found inside of your skeletal muscles, right where your body needs them. This makes them very accessible and a quicker form of metabolism than lipids. Lipids are stored in the various fat sites around your body - your hips, thighs, butt, stomach, even your arms. Lipid metabolism involves liberating it from a storage site, circulating it in the blood and finally the uptake by your working muscles.
According to "Exercise Physiology" by Brooks, Fahey and Baldwin, glucose metabolism is fast enough that it provides the necessary energy for high intensity exercise that is produced without the use of oxygen. The catecholamines necessary for fat liberation don't accumulate until about 15 minutes into endurance exercise.
Mechanism
Every nutrient that you eat is metabolized in the Kreb's cycle. The Kreb's cycle is a complex cascade of reactions from molecules that ends in the production of energy. When glucose enters the Kreb's cycle, it is converted into a glucose-6-phosphate and turned into pyruvate, which gains entry into the mitochondria, the portion of the cell in which metabolism occurs. Once inside the mitochondria, pyruvate is turned into acetyl-coA by an enzyme, acetyl-coA begins the Kreb's cycle. A lipid enters the Kreb's cycle in a more complex and time-consuming fashion. First, the lipid must be activated to a higher energy level before it can be taken into the mitochondria, this involves energy and time. Once activated, it is moved into the mitochondria where it is then oxidized. The beta-oxidation cycle is used to degrade the lipid and transforms it into acetylc-coA, and it can enter the Kreb's cycle.
References
- "Anatomy and Physiology"; Kenneth S. Saladin; 2004
- "Exercise Physiology"; George A. Brooks, Thomas D. Fahey, Kenneth M. Baldwin; 2005
- Colorado State University; Review of Food Chemistry; R. Bowen; April 2006
- Elmhurst College; Overview Metabolism; Charles C. Ophardt; 2003
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