The goal of many athletes and exercisers is to “get cut” or to lose excess fat while at the same time retaining muscle mass and power. Unfortunately, it is not possible to lose copious amounts of fat without incurring some loss of muscle mass. The reason for this is because at any given moment during a day and even during exercise, the human body does not exclusively make use of a single substrate or energy-producing substance such as carbohydrate, protein or fat. As mentioned in previous articles, substrate utilization is exercise intensity dependent ? that is higher intensity exercise will rely more heavily on carbohydrate but will still metabolize small quantities of protein and fat. Likewise, during low intensity, extended duration exercise the body will rely more heavily on fat utilization yet still receive a percentage of the energy yield from protein and carbohydrate.
The key to fat loss that spares muscle mass and power is to shift energy balance in the favor of increased expenditure. In my opinion, the source of this energy expenditure is irrelevant – as long as a negative caloric intake is achieved. Here’s why: long periods of extended cardiovascular exercise tends to metabolize more fat but a major drawback is that it has a muscle wasting effect (look at the build of any marathoner). This is due to the fact that cortisol (a protein catabolizing hormone) levels are increased for long periods of time. Also, during endurance type exercise, amino acids from muscle tissues are required to keep blood glucose levels normal should liver glycogen (stored carbohydrate) levels deplete. Lastly, during endurance-type exercise, only the low threshold Type I muscle fibers will be activated, resulting in little to no stimulation/training of the larger and more powerful Type IIA and B fibers.
Therefore, the best advice for individuals wanting to “cut” while preserving muscle mass and power is to perform high intensity interval training (HIIT). Interval training is a unique form of training that utilizes short periods (3-5 min.) of intense running (close to maximal capacity/VO2max) separated by rest periods of the 2-3 times the same duration. The intensity of interval training allows for a greater power output (greater energy expenditure in less time) and the activation of the large Type II muscles, thus preserving power and strength gains. Also, since Type IIA fibers exhibit some of the qualities of Type I muscle fibers (endurance muscle), performing interval training will actually lead to greater increases in fitness, since a larger quantity of muscle will be stimulated by the increased power output.