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Depending on numerous factors, your body utilizes varying ratios of fat, carbohydrates and protein for energy during exercise. Burning more fat than other fuels may seem beneficial for individuals who hope for weight loss. However, there are many factors that affect how your body processes fat during exercise.
Exercise intensity affects energy production more than any other variable. For example, physical activities performed at or below 50 percent of your maximum effort use greater amounts of fat than carbohydrates as fuel. The crossover concept, or switching from fat usage to carbohydrate usage, occurs as exercise intensity increases. For instance, carbohydrates may provide up to 80 percent of your energy when you exercise at 70 to 80 percent of your maximum ability.
Long duration exercise, lasting over one hour, can deplete stored carbohydrates within your body. As a result, fat becomes your main source of energy regardless of your exercise intensity. As you can imagine, your body cannot use carbohydrates if none are available. Carbohydrate depletion and subsequent reliance on fat for energy minimizes your exercise performance during long endurance activities. Endurance athletes, such as marathon runners, ingest carbohydrates throughout a race in hopes of maintaining carbohydrate availability and peak performance.
Training improves your fat burning ability during exercise. For instance, after two months of weekly endurance exercise, your body burns greater levels of fat per minute than before training. In fact, elite athletes continue to burn fat after exercising and during rest. Carbohydrate sparing, or an increased reliance on fat as energy, supports sports performance and improves endurance during long-duration activities.
Considerations for Weight Loss
Weight loss requires a caloric deficit, or burning more calories than you take in, regardless of where the energy comes from. Therefore, overall calorie expenditure becomes more important for fat loss than the source of calories expended. Low-intensity exercise burns fewer calories per minute than high-intensity exercise and consequently may be less beneficial for weight loss. For example, a 180-pound person burns 68 calories while walking 4 mph for 10 minutes and 136 calories while running 6 mph for 10 minutes. Although the usage ratio of fat to carbohydrate is greater during the walk, the person burns almost two times as many calories during a run in the same time span. As always, consult a doctor before you start an exercise program.
Fats are stored mostly in (subcutaneous) adipose tissue, but we also have small stores in the muscle itself (intramuscular triglycerides). At the onset of exercise, neuronal (beta-adrenergic) stimulation will increase lipolysis (the breakdown of fats into fatty acids and glycerol) in adipose tissue and muscle. Catecholamines such as adrenaline and noradrenaline may also rise and contribute to the stimulation of lipolysis.
As soon as exercise begins, fatty acids are mobilised. Adipose tissue fatty acids have to be transported from the fat cell to the muscle, be transported across the muscle membrane and then be transported across the mitochondrial membrane for oxidation. The triglycerides stored in muscle undergo similar lipolysis and these fatty acids can be transported into the mitochondria as well. During exercise, a mixture of fatty acids derived from adipocytes and intramuscular stores is used. There is evidence that shows that trained individuals store more intramuscular fat and use this more as a source of energy during exercise (1).
Fat oxidation is regulated at various steps of this process. Lipolysis is affected by many factors but is mostly regulated by hormones (stimulated by catecholamines and inhibited by insulin). The transport of fatty acids is also dependent on blood supply to the adipose and muscle tissues, as well as the uptake of fatty acids into the muscle and into the mitochondria. By inhibiting mobilisation of fatty acids or the transport of these fatty acids, we can reduce fat metabolism. However, are there also ways in which we can stimulate these steps
and promote fat metabolism?
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