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Breakthroughs in genetics and technology are creating new knowledge in physiology in medicine at breakneck speed. In the October 2006 issue of the National Strength and Conditioning Association Journal, Dr. Bruce Craig wrote an interesting summary of the physiology of fat burning.
In the first paragraph, he repeated the longstanding belief that spot reducing exercising specific regions to lose body fat doesn't work. Last month, we reported the results of an elegant, sophisticated study from Denmark (American Journal Physiology Endocrinology Metabolism, in press, published online Sept. 19, 2006) showing that active muscle uses nearby fat stores as fuel during exercise.
It's too early to tell if this means that doing thousands of sit-ups will produce sixpack abs, but the results certainly went against conventional wisdom.
Dr. Craig's advice about fat loss was sound: the ability to burn fat during exercise depends on exercise intensity and the length of the workout. It takes time to mobilize fat in the fat cells, break them down into glycerol and fatty acids, and deliver them to active muscles where they can be used as fuel. This process takes 20 to 30 minutes. Once the fatty acids reach the muscle cells, it takes more time for them to cross the cell membranes, enter the cell energy centers and process them to form usable energy for exercise. The take-home message for bodybuilders wanting to cut fat is to exercise at least 20 to 30 minutes at 60 percent to 70 percent of maximum effort.