The human body relies primarily on fat and carbohydrates to meet its energy needs. Both sources provide good sources of fuel, but how the body uses them varies. Part of the answer lies in the chemical structure of the two types of nutrients.
Carbohydrates can be either simple or complex, depending upon number of sugar molecular chains a particular food has. Generally, carbs are relatively easy for the body to break down and digest. Complex carbohydrates like whole grains take longer than simple carbs. Fats are composed of an organic alcohol called glycerol and fatty acids. Metabolizing fats is considerably more complicated from a biological perspective.
Fat Burn Zone
While the body will use carbohydrates and fat for energy at any given time, it may use one type over another to optimize energy use. The body considers low to moderate intensity exercise as a less stressful experience than high intensity workouts.
During these times, the body will rely primarily on fat to meet its energy needs. A 2009 study by the University of St Thomas in Minnesota researched the effects of intensity on fat burn. The study found that maximum fat burn occurred at 54.2 percent of maximum oxygen uptake or VO2max. The study concluded that without precision testing, an individual can assume maximum fat loss between 60 to 80 percent of maximum heart rate or 220 minus age.
Intense Exercise
Intense exercise elicits as stress response by the body. The need for energy is peak during these times. To meet the demands, the body must use the most efficient means it has to keep up with these conditions. As blood sugar levels drop, hormone release signals the body to release stored glucose in the form of glycogen.
As a carbohydrate, it is easy for the body to break the chemical bonds of glycogen and produce more energy. It can do this quickly and efficiently so that the cells have a ready source of fuel available to them. Their simpler chemical structure allows the body to do so. The result is a higher calorie burn.
Putting It All Together
This is not to say that intense activity won't burn fat. Meeting energy needs is the body's primary focus. Going to carbohydrates first provides the most efficient way to accomplish this purpose. Yet, if needed, the body will turn to fat burn.
A 1995 study published in the Journal of the American Dietetic Association found that fat burn occurred regardless of exercise intensity. Fat burn is merely a reflection of calorie expenditure. The study suggested that if time for exercise is limited, focus on high intensity exercise will yield quick results.
The human body has adapted and evolved to maximize its resources. Fat burn provides the best way for the body to produce energy when activity intensity is low without depleting carbohydrates stores needed for quick flight or fight responses. In this way, the body is always prepared to act.
Sources:
- D. Carey. Quantifying Differences in the "Fat Burning" Zone and the Aerobic Zone: Implications for Training. Journal of Strength and Conditioning Research, October 2009; 23(7):2090-2095.
- M. Grediagin, et al. Exercise Intensity Does Not Effect Body Composition Change in Untrained, Moderately Overfat Women. Journal of the American Dietetic Association, June 1995; 95(6): 661-665.
- Sports Fitness Advisor: VO2 Max, Aerobic Power& Maximal Oxygen Uptake sports-fitness-advisor.com
- G. Tortora et al. Principles of Anatomy and Physiology. 2005.
