Eating for Exercise
From Elizabeth Quinn,
Your Guide to Sports Medicine
FREE Newsletter
Should I Eat Carbohydrate or Fat for Exercise?
When it comes to eating for exercise there are several things to consider while meal planning. Carbohydrate,
fat, and protein all contribute to the fuel supply needed by working muscles, with carbohydrates and protein
providing 4 Calories per gram and fat providing 9 Calories per gram. And all nutrients get converted to energy in
the form of adenosine triphosphate or ATP. However, each nutrient has unique properties that determine how it
gets converted to energy. Carbohydrate is the main nutrient that fuels exercise of a moderate to high intensity,
while fat can fuel low intensity exercise for long periods of time. Proteins are generally used to maintain and
repair body tissues, and are not normally used to power muscle activity.
Because the body can not easily store ATP (and what is stored gets used up within a few seconds), it is
necessary to continually create ATP during exercise. There are three main pathways to convert nutrients to ATP
and it is the intensity and duration of the exercise that determine which method gets used.
The first path only supplies about 10 seconds worth of energy and is used for short bursts of exercise such as a
100 meter sprint. After this, either aerobic or anaerobic metabolism is used to continue to create ATP. Then
major difference between aerobic and anaerobic metabolism is the presence of oxygen to create ATP.
Aerobic metabolism requires oxygen to convert nutrients (carbohydrates, fats, and protein) to ATP. Aerobic
metabolism is used primarily during endurance activities.
Anaerobic metabolism (glycolysis), creates ATP exclusively from carbohydrates, with lactic acid being a byproduct.
As lactic acid builds up in the muscle it causes physical discomfort and pain that limits performance.
Anaerobic metabolism produces energy for short, high-intensity bursts of activity lasting no more than several
minutes before the lactic acid build-up reaches a threshold (the lactate threshold) and muscle pain, burning and
fatigue make it impossible to maintain that intensity.
During exercise an athlete will move through these metabolic pathways. As exercise begins, ATP is produced
via anaerobic metabolism. With an increase in breathing and heart rate there is more oxygen available and
aerobic metabolism begins and continues until VO2max is reached. If VO2Max it surpassed, the body can not
deliver oxygen quickly enough to generate ATP and anaerobic metabolism kicks in again. Since this system is
short-lived and lactic acid levels rise, the intensity can not be sustained and the athlete will need to decrease
intensity to remove lactic acid build-up.
http://sportsmedicine.about.com/cs/nutrition/a/aa080803a.htm
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