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Date Posted: 11:06:15 02/19/04 Thu
Author: J.R.Smith, c.f.t. ISSA
Subject: How to burn fat with Metabolic Activation Sprints or High Intensity Training Cardio


If you weighed 150 pounds and were to walk at a constant rate of 3 mph for 60 minutes you would burn approximately 4 calories per minute or 320 calories in 60 minutes. The energy to fuel the first 30 minutes would be predominantly glucose with a gradual shift of fat the remaining 30 minutes. The first 30 minutes might be an 80/20 carbohydrate to fat energy mix with the remaining 30 minutes switching to a 80/20 fat to carbohydrate mix.
If you were to use high intensity inteval training in a 60 minute period and walk at 3 mph for 45 minutes of the time and sprint for a total of 15 minutes (90 seconds walking and 30 seconds sprinting would = 45 mniutes walking and 15 minutes sprinting) at a time you would burn 4 calories per minute or 180 calories walking and approximately 16 calories per minute sprinting or 240 calories sprinting for a total of 400 calories burned rather than 320.

High intensity exercise simulates fast twitch muscle fiber development, while low intensity exercise results in slow twitch muscle fiber development. Fats are the primary substrate preference for muscle during low-intensity exercise (<30% VO2 max) whereas carbohydrates are the substrate preference during high-intensity exercise (>70% of VO2 max). Two principle factors are involved in the shift from fat to carbohydrate metabolism: (1) the recruitment of fast twitch muscle fibers and (2) an increase in levels of epinephrine.

As exercise intensity increases, more Type II fast twitch fibers are recruited. Fast twitch fibers function to metabolize carbohydrates rather than fat. This occurs because Type II fibers have an abundance of glycolytic enzymes but fewer mitochondrial and lipolytic enzymes. Lipolytic enzymes are responsible for fat breakdown and mithochondria are the organelles that oxidize free fatty acids within the muscle.

As noted earlier, a second factor that regulates carbohydrate metabolism is epinephrine. High levels of epinephrine increase muscle glycogen breakdown, carbohydrate metabolism and lactate production. Lactate production inhibits fat metabolism by reducing the availability of fat as a substrate. This results in carbohydrates as the main substrate used.

Exercise duration is another factor to consider in fat metabolism. During exercise sessions lasting longer than thirty minutes, there is a gradual shift from carbohydrate metabolism to fat metabolism. The process called lipolysis regulates fat metabolism. Our bodies store fat at adipocytes in the body in the form of triglycerides. Our bodies cannot use stored bodyfat for energy. These triglycerides must be broken down into one glycerol molecule and three free fatty acid molecules. It is the free fatty acids, which are metabolized via the Krebs cycle. This conversion cannot take place without the enzymes called lipases. At the onset of exercise the body releases the hormones epinephrine, norephinephrine, and glucagon, which stimulate the activity of the enzyme lipases. Lipolysis is a slow process and an increase of fat metabolism occurs after several minutes of exercise.

The major factor that determines the role of fat as a substrate during exercise is its availability to the muscle cell. In order to be metabolized, bodyfat (triglycerides) must be degraded into three molecules of free fatty acids and one molecule of glycerol. This splitting allows the free fatty acids to be converted to acetyl-CoA and enter the Krebs cycle. Hence, if fat is not available to the muscle cell then it can not be metabolized.

Fat can only be metabolized via Krebs cycle oxidation. It is essential to recognize that a reduction in Krebs cycle intermediates: whether it is the result of (1) low carbohydrate diets, (2) no carbohydrate diets, or (3) excessive prolonged aerobic sessions, resulting in a diminished rate of ATP production from fat metabolism. When carbohydrate stores are depleted in the body, the rate at which fat is metabolized is reduced. Therefore carbohydrates are essential in the ability to metabolize fat. It is only the free fatty acids that are metabolized via the Krebs cycle that are used in ATP production that go toward reducing bodyfat levels.

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