I heard that the noontime strength-training class here is excellent,” one female client remarks to another. “But when I asked the instructor if it was a fat-burning workout, she didn’t understand my question. She just shrugged and told me not to worry about bulking up. I really need a workout that gets me into my fat-burning zone.”
Many clients exercise in order to lose weight or, more specifically, to reduce body fat stores. They look to their instructors and trainers for guidance. What is the best way to exercise in order to achieve this goal? Unfortunately, many clients seem to get the idea that there are two types of exercise — that which burns fat and that which does not. It’s true that the physiology of substrate utilization (use of fat, protein and carbohydrate for energy production) is fairly complex, and not something we would want to explain to a client in 25 words or less. But perhaps in our attempts to simplify these concepts, something occasionally gets lost in translation.
The effect of exercise intensity
In order to release energy, your body breaks down certain molecules that are stored specifically to provide energy when needed. The two primary sources of energy used by the body are carbohydrate (in the form of glycogen, stored in the liver and in skeletal muscles) and fats (primarily triglycerides, stored in adipose tissue). Your body does not like to use much protein for energy, except during exhaustive exercise.
How does your body decide which to use during exercise? Fuel utilization is regulated by an interplay of many factors. One of these is exercise intensity. During high-intensity exercise, your body prefers to metabolize carbohydrates, because carbohydrate metabolism is more efficient and yields more energy per unit of oxygen.
Does low intensity mean high fat loss?
The proportion of energy expenditure due to fat combustion is linearly related to exercise intensity. At very low intensities, your body relies predominantly on fat stores. At moderate intensities, you use some of each. And at very high intensities, carbohydrate provides the sole source of energy.
This has led some people to conclude that to maximize the fat-burning properties of physical activity, low-intensity exercise is preferable.
This conclusion is misleading because the proportion of calories that come from fat represents only one small part of the energy balance picture. Since the greatest fat burning (as a percentage of calories expended) occurs at the lowest exercise intensities, we might recommend that clients stay in bed all day. In fact, calories do count. In his article “Fat-Burning Exercise: Fit or Farce?” (Fitness Management, July 1994), John P. Porcari points out that if you burn 240 calories exercising for a half hour at a low intensity, 41 percent of those calories will come from fat, giving you a fat loss of 96 calories. Exercise for the same amount of time at a high intensity but burn 450 calories, and you will burn 108 fat calories, even though the percentage of energy supplied by fat is 24 percent.
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