Will I lose body fat more efficiently by performing my aerobic workouts at a low, rather than a high, intensity?

One other consideration is post-exercise energy expenditure, often referred to as excess postexercise oxygen consumption or EPOC. Research has demonstrated that EPOC is higher after high-intensity exercise, when taking into account duration and mode of the exercise performed. Basically, you continue to burn a slightly higher number of calories following an intense exercise session than you would after a lower or moderate intensity session.

The effects of increased EPOC are cumulative, meaning that the more times you exercise, the more time is spent returning your body to equilibrium and overall calorie expenditure is higher. From simply this standpoint, I'd argue that it's more "efficient" to spend an exercise session at a higher intensity, simply to burn more calories from fat both during the exercise and during the period of higher EPOC, rather than drag out longer durations at a lower intensity.

A: Many aerobic exercise programs and videos feature low-intensity workouts which purport to maximize fat burning. The argument behind such an alleged theory is that low-intensity aerobic training will allow your body to use more fat as an energy source, thereby accelerating the loss of body fat.

While it is true that a higher proportion of calories burned during low-intensity exercise come from fat (about 60 percent as opposed to approximately 35 percent from high-intensity programs), high-intensity exercise still burns more calories from fat in the final analysis.

For example, if you perform 30 minutes of low-intensity aerobic exercise (i.e., at a level of 50 percent of maximal exercise capacity), you'll burn approximately 200 calories - about 120 of those come from fat (i.e., 60 percent).

However, exercising for the same amount of time at a high intensity (i.e., 75 percent of your maximal exercise capacity) will burn approximately 400 calories. Using a 35 percent fat utilization yardstick, 140 of the calories you've burned will have come from stored fat.

Although the more vigorous exercise burns both more total and more fat calories, the less intense form of exercise has its benefits as well. For example, because many overweight people tend to find that lower-intensity exercise is more comfortable, they may, therefore, be willing to engage in such workouts.

The point to remember is that low-intensity workouts do, in fact, promote weight and fat loss. You just have to do them for a longer period of time.

Low-intensity aerobic exercise, however, is not a better or more effective way to lose weight than more intense physical activity - the idea of a "fat-burning zone" is simply a myth.

Keep in mind that you lose weight and body fat when you expend more calories than you consume, not because you burn fat (or anything else) when you exercise.

Source: Bryant, Cedric X. 101 Frequently Asked Questions about "Health & Fitness" and "Nutrition & Weight Control". Sagamore Publishing, 1999.

You are so right Brenda. One is not using fat as a fuel source when training at high intensities. One is primarily using carbohydrates. The same is true as far as training at lower intensities. The only time we are using fat exclusively as a fuel source is at rest.

However, to respond to your question, whether you will burn fat more "efficiently" the answer is no. You will use more energy training at higher intensities which will result in weight loss. You will change the ratio of fat mass to fat free mass as a consequence of training. It's not because you are burning fat more efficiently, it is because you are working at a higher intensity.

Hope this helps.

You will "burn" body fat (i.e. use body fat as a fuel source) at ALL levels of exercise (low or high intensity). What varies is the RATIO of the total calories used or burned during an exercise bout. So, for example (this is strictly hypothetical and not intended to be accurate) let's say you burn 200 calories at a low-intensity exercise, of which 40 percent was by using fat as a fuel source. That means 80 calories are coming from fat. But let's say at a high-intensity exercise you burned 300 calories in the same amount of time as the low-intensity exercise. And let's say that 30 percent came from fat as a fuel source (that's 90 calories from fat). Even though the percentage of fat used for fuel is LOWER in the high-intensity exercise, the total calories used, and the total amount of fat calories used are higher in the same amount of time. So to answer your question, which is based on EFFICIENCY, I would say that the high-intensity exercise (given the same variable of time) is a more efficient way to "burn body fat." Here I'm equating "efficiency" with the MOST in the same or lesser amount of time (using time as my measure of efficiency).

I hope this helps.

LaRue, CSCS
www.lecfitness.com
lecfitness@yahoo.com

All of the answers are right on.

While the biochemistry of fat metabolism is a fascinating and complex process the answer to your question is actually quite simple. The more intense, the longer the duration and the more often you workout (coupled with proper nutrition, hydration and rest & recovery,) the greater your total kilocalorie expenditure.

It's not what chemisties are active during a given workout. At the end of the week if you've expended more calories than you've consumed (again, given proper nutrition & hydration) then you'll lose body fat, since fat is the body's ideal energy reserve.

If our goal is to lose body fat, it is best to perform Cardio exercise that is moderate and less strenuous. A good plan is to first bring down the levels of glucose for fuel and still meet our demand for oxygen, which is an aerobic state, so that there is a possibility to burn fat for fuel. But if we are working too hard we would no longer supply enough oxygen to meet the demands of the muscles and then we would require more fuel than just glucose and glycogen, and without adequate glucose for fuel our bodies would start breaking down muscle tissue to use for fuel, build lactate in the blood, and we would soon tire out and would not be able to continue much longer. So, working too hard during Cardio would cross us over into the Anaerobic Threshold, when the Cardio would no longer be aerobic but become anaerobic due to the body needing glucose for fuel. Since there is an inadequate supply of glucose our body will break down muscle for fuel and thus move us backwards and the muscle loss will weaken us, slow the metabolism down and with less muscle we would have less of a capacity in our bodies to burn body fat, as fat is burned in the muscles.
When to perform your cardio is an important consideration as well. Many have had good results immediately following an anaerobics workout, and also before breakfast of a meal when blood glucose levels are lower. However if a client is not ready for that type of cardio yet (example: older and weakened from sarcopenia) then it would be best to perform it with ideal blood glucose levels at first until the client is ready for that type of strenuous training.
It is best to always experiment to see what techniques and approaches work best for you and for your individual clients.
The subject can be explored more deeply, but to keep it relatively simple perform cardio with the objective of conditioning our heart and lungs (rather than thinking that we are burning fat when performing it), keep synergy with the four balanced quadrants of anaerobics, cardio, eating supportively and proper rest (along with ideal blood glucose levels throughout the day with caloric balance) - and we have put in place the conditions for fat loss.

Don’t just do cardio – do smart cardio. Smart cardio = mixing it up!

It is good to do interval training as well as HIIT....
AND also throw in one longer low intensity (in a given week).

Keep it fresh - keep it changing; otherwise, the body adapts...so no more "stress imposed".

And a reminder about "ongoing" HIIT; it is not just the amount of calories etc. burned during and after (EPOC), it is the physiological changes that occur when “consistently” doing it. It will leave our bodies working "more efficiently" all the time; therefore with the capability to rely on and use/release the free fatty acids. So bottom line: the more we use of this application, the better our bodies will "run" - liberating those FFAs.

So don't just do cardio. Do smart cardio.....and mix it up!

A great resource that I have recently stumbled across addressing this is a book entitled, Which Comes First, Cardio or Weights? Fitness Myths, Training Truths and Other Surprising Discoveries from the Science of Exercise, written by Alex Hutchinson. There is an excerpt, specifically about the myth of the fat burning zone, in the July/August issue of Canadian Running.

Both forms will help you burn fat and calories. However if you are looking for efficiency, you want to do more work in less time. Higher intensity workouts will allow you to burn more calories in less time and as one of my colleagues pointed out above they will also allow you to continue to burn more calories after your training session. But remember that if you are fueling your body with the right nutrition and incorporating both strength and cardiovascular training your body not only burns fat during exercise, but also at rest.

In most cases I encourage interval exercise. Alternate intervals of increasing intensity to the onset of hyperventilation, then slowing down to catch one's breath. When comfortable, speed up again. Sometimes increase intensity quickly, other times more slowly. The key is to slow down enough so the next effort interval can be maximally effective and prevent early fatigue. I recommend doing the interval workout for at least 30-45 minutes.

Gina's suggestion concerning the need for sensible nutrition and including strength training are right on the mark.

Take care.