We all need a specific amount of food to maintain our many body functions, as well as additional calories to sustain our daily activity levels. It is no mystery that when we consume more food (energy) then the energy we expend, any additional calories will be stored as body fat. An accumulation of 3500 excessive calories will be stored as one pound of body fat. This accumulation can result in an unwanted change in body composition with a shift from a higher percentage of lean body mass to a higher percentage of bodyfat. The rise in body fat, which often comes without warning, is a battle many are struggling against today.

This increase in bodyfat spawns a search, often desperate, for reliable answers to innumerable painful questions. One of the re-occurring questions that accompany this quest to decrease bodyfat is should I limit or totally eliminate my consumption of carbohydrates? Other questions that fuel the confusion are how can I optimize my body’s fat burning ability? Do I have to work hard? Will I burn body fat at low workout intensity? We will discuss the factors that govern fuel selection and their relationship to exercise intensity and exercise duration to understand the key point in our bodies’ ability to decrease body fat.

Fuel Selection in Exercise
We will begin the discussion with protein, which contributes less than two percent of the substrate used during sessions, which last less than one hour. Although protein does play a minor role as a substrate during exercise, contributions can rise as high as fifteen to eighteen percent in activities lasting longer than 90 minutes in duration. The majority of energy is derived from the branched chain amino acids, leucine, valine and isoleucine. Several factors, including diet, exercise intensity and exercise duration, will determine whether fat or carbohydrate is the primary substrate preference during work.

Exercise Intensity
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
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.

Essential Point
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.

Therefore, when designing an exercise program to reduce bodyfat stores, it is vital to consider both (1) the total rate of energy expenditure and (2) the percentage of energy that is derived from fat metabolism. You must optimize the availability of fat to the muscle cells through selection of appropriate intensity and duration of exercise sessions. Since it takes approximately 20 minutes for lipolysis to occur, the session should exceed 20 minutes in duration for fat to be made available to muscle cells. At approximately 70% of VO2 max the availability of fat to the muscle cells diminish due to an increase in lactate production, which inhibits fat metabolism. Although engaging in activity at 20% of VO2 max will burn 60% of calories from fat as compared to working at 50% of VO2 max which would derive 40% of energy from fat, the total rate of energy expenditure is 2.5 times greater at 50% VO2 max. The absolute amount of fat metabolized is 33% higher during exercise at 50% of VO2 max. The ideal rate of work would be at 50% of VO2 max for duration of 60 minutes. Good luck with your training.

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