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Choosing an Exercise Intensity for Fat Burning

Swimming One of the most common questions asked by people who want to lose weight is: "What intensity should I exercise at in order to burn the most fat?"

In order to answer this question, a little background information may be useful. The body has two major fuel sources:

  1. Fat stored in fat cells.

  2. Carbohydrate stored in muscles and the liver in the form of glycogen.

The body uses fat as its preferred fuel source at rest, and at low levels of exercise intensity (usually about 40% of maximum aerobic capacity). So at rest the body is using almost 100% of its energy requirements (your basal metabolic rate, or BMR) from stored fat sources.

As you start exercising, your body begins to switch from using fat to using carbohydrate. The greater the exercise intensity, the greater the proportion of carbohydrate it uses, until at maximum aerobic capacity almost all the energy required for exercise comes from carbohydrate.

Given this information, people get confused and think that the best way to lose weight is to exercise at a low intensity.

This is misleading for a number of reasons.

First, no matter what fuel source you are using, your body burns more calories exercising at a high intensity than it does at a low intensity. Furthermore, you may actually burn more fat in absolute terms exercising at a high intensity than at a low intensity. This may sound like a contradiction, but the following example may help you to understand a little better.

Let's say you were exercising at a slow walk and burning 100 calories in 30 minutes. In this situation, you may be using 80% fat as a fuel, and 20% carbohydrate. Therefore, in 30 minutes, you have burned 80 calories of fat, and 20 calories of carbohydrate.

Now, assume that you started running at a fairly fast pace, burning 400 calories in 30 minutes. You may be using 25% fat, and 75% carbohydrate. So, at the end of 30 minutes you would have burnt 100 calories of fat, and 300 calories of carbohydrate.

Your body has the ability to store the food that you eat in the form of carbohydrate and/or fat. In general, carbohydrate stores (in your muscles and liver) are replenished first, then the remainder gets stored as fat.

So let's assume that after you have finished exercising you have a meal of 500 calories.

In the first scenario, when you were walking at a slow pace, you had burnt a total of 100 calories (80 of fat, and 20 of carbohydrate). When you eat your meal, your body would first restore the 20 calories of carbohydrate that you burnt in your liver and muscles, and the remaining 480 calories would be stored as fat. Since you had burnt 80 calories of fat during exercise, the net amount of fat that would be stored after exercising would be 400 calories.

In the second scenario, when you were running, you had burnt a total of 400 calories (100 of fat, and 300 of carbohydrate). Your body would first restore the 300 calories of carbohydrate that you burn first, leaving only 200 calories to be stored as fat. Since you had already burnt 100 calories of fat during exercise, the net amount of fat that would be stored after exercising would only be 100 calories.

So the main thing to consider when you are exercising to lose weight is how many calories you burn in total when you exercise. In general you burn more calories when you exercise at a higher intensity, and when you exercise for a longer duration, and when you exercise more regularly, over a longer period of time.

But please keep things in perspective – don't go crazy! Usually 30-60 minutes of moderate intensity exercise – swimming, fast walking, jogging or cycling – four to six days per week is sufficient for general health and well being and to lose weight in a healthy way. Supplementing your programme with light to moderate weight training may help you to lose weight and tone up faster if you so desire.

Please note that the concepts in this article have been simplified, and the figures used in this example are only very rough and are used to illustrate certain principles. They should not be used for any other purpose.


Last updated: May 3rd, 2004

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