People who need to lose weight want to lose fat. Therefore, when they climb on a treadmill or stationary bike at the gym, they choose the workout that puts them in the "fat-burning zone", rather than the one that puts them in the "cardio zone". The fat-burning zone is a lower-intensity aerobic workout that keeps your heart rate between about 60% and 70% of maximum.
The cardio zone is a higher-intensity aerobic workout that keeps heart rate between about 70% and 85% of maximum. Which workout burns more fat?
The lower-intensity workout burns a higher percentage of calories from fat. But exercising in the cardio zone - for the same amount of time - burns about the same amount of fat, and more calories overall because it is more vigorous exercise. People who are tying to lose weight would therefore benefit more for the higher intensity "cardio zone" workout.
Fat is a major energy source during exercise, but it can be used only when oxygen is available. During moderate-intensity exercise (60% to 75% of VO2max), fat is the primary energy source for muscle contraction.
When fat is used as an energy source, glycogen stores are spared and exercise can continue for a longer period. A variety of dietary supplements, including carnitine and caffeine, promise to improve endurance by making it easier for the body to use fatty acids as fuel.
To be used for energy, fatty acids inside the muscle cell must be transported into the mitochondria. They are transported across the mitochondrial membrane with the help of the amino acid carnitine. Once inside the mitochondria, they are broken down to produce ATP. The rate at which the muscle can use fatty acids depends on how quickly they can be delivered to cells and then transported into the mitochondria.
Fatty acids used during exercise can come from adipose tissue, fat stored in muscle cells, fat consumed in the diet, or fat made by the liver. Most of the fat stored in the body consists of triglycerides found in adipose tissue. The breakdown of triglycerides in adipose tissue is stimulated by the rise in the hormone epinephrine that occurs as exercise begins. The resulting fatty acids are transported through the bloodstream to the muscle.
Fat stored within the muscle, referred to as intramuscular fat, can also be used as a source of energy during exercise. This fat already exists in the muscle tissue, so the fatty acids do not need to be transported in the blood. Higher-intensity aerobic exercise uses more intramuscular fat. Triglycerides consumed in the diet or made by the liver are a less important source of fatty acids for exercise.
They are transported in the blood in particles called lipoproteins. At the muscle, an enzyme cleaves the triglycerides and allows the fatty acids to enter the muscle cell. Because people generally do not eat a large, fatty meal before exercising, the amount of energy obtained from blood lipoproteins generally is small.
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