Nutrition Science and Everyday Application - beta v 0.1

FUEL SOURCES FOR EXERCISE 553
stored ATP is just about used up, the body resorts to another high-energy molecule
known ascreatine phosphate to convert ADP (adenosine diphosphate) to ATP. After
about 10 seconds, the stored creatine phosphate in the muscle cells is also
depleted as well.
• About 15 seconds into exercise, the stored ATP and creatine phosphate are used
up in the muscles. The heart and lungs have still not adapted to the increased
oxygen need, so the muscles must begin to produce ATP by anaerobic metabolism
(without oxygen). Anaerobic metabolism can produce ATP at a rapid pace but only
uses glucose as its fuel source. The glucose is obtained from muscle glycogen. At
around 30 seconds, anaerobic pathways are operating at their full capacity, but
because the availability of glucose is limited, it cannot continue for a long period of
time.
• As your exercise reaches two to three minutes, your heart rate and breathing
rate have increased to supply more oxygen to your muscles. Aerobic metabolism is
the most efficient way of producing ATP; it produces significantly more ATP for
each molecule of glucose than anaerobic metabolism. Although the primary source
of ATP in aerobic metabolism is carbohydrates, fatty acids and protein can also be
used as fuel to generate ATP.
Figure 10.2. Energy systems used to fuel exercise change with duration of exercise. The ATPcreatine
phosphate system is used up within seconds. The short-term and long-term systems kick
in and provide energy for exercise as the duration of the workout goes on.
The fuel sources for anaerobic and aerobic metabolism will change depending on the
amount of nutrients available and the type of metabolism.
• Glucose may come from blood glucose (which is from dietary carbohydrates, liver