Metabolic Fuel Selection

     Metabolic fuel selection refers to the type fuel (fat or carb) your body metabolizes during exercise.  depending on the intensity in which you perform an activity will determine what percentage of fat is mobilized and burned as an energy source and what percentage of carbohydrate is mobilized and burned as a fuel source.

     This concept is generalized with the respiratory exchange ratio (R).  which is a ratio of the amount of CO2 expired compared to the amount of O2 consumed at a steady state ( CO2/O2).  There is a direct relationship with the difference in O2 consumed and CO2 expired to fuel selection.  During low intensity exercise  such as a brisk walk or jogging, CO2 expired is lower then O2 consumed and a respiratory exchange ratio of less then 1.0 will be obtained, where as during high intensity exercise such as running CO2 and O2 will be closer to  equal and a respiratory exchange ratio of near 1.0 will be obtained.  The closer the ratio is to 1.0, the more carbohydrate will be chosen as a fuel source, and your body will undergo glycolysis as its primary energy pathway.  If the ratio is lower, fat will be the main fuel source and your body will undergo more lipolysis.  Most activities fall in between the two extremes, and a combination of both fat and carbohydrates are chosen for fuel selection.  The intensity of the activity will determine what percentage of fat is burned and what percentage of carbohydrate is burned, for instance:  an activity such as sprinting may burn 85% carbohydrate and 15% fat where as an activity such as a brisk walk may burn 85% fat and 15% carbohydrate.

     To properly reach your goals, whether it be to lower body fat percentage of to increase your anaerobic ability, this concept will prove useful!  Adjusting your exercises to match this concept will help you reach your goals quicker; long, low intensity exercise promotes the breakdown and utilization of fat where as short, high intensity exercise promotes the breakdown and utilization of carbohydrates which is useful for athletes wanting to increase their anaerobic capacity.

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