Nutrition Science and Everyday Application - beta v 0.1

556 ALICE CALLAHAN, PHD, HEATHER LEONARD, MED, RDN, AND TAMBERLY POWELL, MS, RDN
Figure 10.4. The effect of exercise intensity on fuel sources. Anaerobic exercise utilizes only
glucose for fuel. As activities become more aerobic, the body can utilize fatty acids and, to a small
extent, amino acids, for energy production.
One important clarification about exercise intensity and fuel sources is the concept of the
fat-burning zone. Many people think that in order to lose body fat, they should exercise at
a lower intensity so that fat is the primary fuel source. The fat-burning zone is typically
referred to as a low-intensity aerobic activity that keeps your heart rate between
60 and 69 percent of maximum heart rate. The cardio zone, on the other hand, is
a high-intensity aerobic activity that keeps the heart rate between about 70 and 85
percent of maximum heart rate. So which zone do you burn the most fat in? Technically,
your body burns a higher percentage of calories from fat during a low-intensity aerobic
activity. When you begin a low-intensity activity, about 50% of the calories burned come
from fat, whereas in the cardio zone only 40% come from fat. However, this isn’t the whole
story. High-intensity activity burns more total calories per minute. At this higher rate
of energy expenditure, you can burn just as much or more total fat and more total
calories as during a lower intensity activity. If weight loss is one of your goals, highintensity
activities will burn more total calories, helping to shift to negative energy balance,
and will promote a greater level of fitness. However, the best exercise program is one that is