Nutrition Science and Everyday Application - beta v 0.1

554 ALICE CALLAHAN, PHD, HEATHER LEONARD, MED, RDN, AND TAMBERLY POWELL, MS, RDN
glycogen, and glucose synthesis) or muscle glycogen. Glucose is the primary energy
source for both anaerobic and aerobic metabolism.
• Fatty acids are stored as triglycerides in muscles, but about 90 percent of stored
energy is found in adipose tissue. As low- to moderate-intensity exercise continues
using aerobic metabolism, fatty acids become the predominant fuel source for
exercising muscles.
• Although protein is not considered a major energy source, small amounts of
amino acids are used while resting or doing an activity. The amount of amino acids
used for energy metabolism increases if the total energy intake from your diet
does not meet your nutrient needs or if you are involved in long endurance
exercise. When amino acids are broken down and the nitrogen-containing amine
group is removed, the remaining carbon molecule can be broken down into ATP
via aerobic metabolism, or it can be used to make glucose. When exercise
continues for many hours, amino acid use will increase as an energy source and
for glucose synthesis.
Figure 10.3. Fuel sources for anaerobic and aerobic metabolism. Both dietary sources and body
storage of carbohydrates, fat, and protein can all be used to fuel activity. Amount varies depending
on duration and intensity of the activity.