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If I want to loose weight should I be exercising in the “Fat Burning Zone”?

A patient asked me this question. The treadmill she uses has a “fat burning zone” setting, which is, relatively speaking, a slower speed. If find this setting misleads to people to think they must exercise slower to ensure they are only burning fat at this level and therefore optimize weight loss. Here's why it is misleading...

We use primarily fat and glucose (a sugar) as the main fuels. This energy is measured in calories, which is useful because we can measure exercise and the energy in food with the same units. At rest and with lesser intensity (slower) exercise the fuel primarily used for energy is fat. (That's true.) There is glucose being used as well but less than the amount of fat. If you wre to sprint as fast as you could, after about 10-15 seconds the primary source of energy is glucose. The science behind it is too complicated to explain here, but basically the body prefers fat as a fuel, however it takes longer to break down and there is only so much that it can metabolize all at once. When activity is more intense, it still burns fat but the extra energy comes from the addition of more glucose being burned.

To illustrate this, consider a hypothetical example. Let's say a person who is 145 pounds exercises three times a week for 30 minutes on a treadmill. If that person walks three miles per hour as exercise, then exercise science tells us that that person will burn about 80 kilocalories per session walking 1.5 miles each time. The exercise is easy so the vast majority of those calories will come from fat as a fuel source. Now consider, if that person were able to jog at five miles per hour instead; about 265 kilocalories would be burned during the 2.5 miles covered in each 30 minute session. Interestingly, the distance covered is less than half, but the amount of calories is over three times as much. With the jog, the percentage of calories coming from burning glucose will be much higher than the walking.

Now if this person wants to lose weight (fat), the fact is the faster, more intense exercise will be more useful. Because at the end of the day the biggest influence on weight loss is the balance between the number of calories consumed (food eaten) versus the number of calories spent (exercise and physical activity). Ultimately it does not matter which fuel source was used. Both are always used, just in different relative amounts. If the body runs low on one fuel source it replaces it from the food eaten. For our hypothetical person to walk at 1.5 miles, three days a week, for 30 minutes a session, it would take 15 weeks to expend the equivalent number of calories in one pound of fat. That's almost four months. If that person spent the same amount of time jogging as described above, it would only take about one month (90 minutes a week is not that long).

Now, these numbers are real but it is a simplified hypothetical example. A sedentary person who starts walking 30 minutes on a treadmill several times a week might begin to change their metabolism, and also begin loosing water weight. They might very well see a change in their weight very quickly. This is great and encouraging, but it is not a loss of fat stores. In the long run fat loss with walking will be slower than with a more intense exercise. Other factors include eating habits. Some people start exercising and erroneously think it means they can eat anything they want and GAIN weight. Others are opposite; they who feel better about themselves because they are exercising and start making healthier food choices which has a further benefit to losing fat.

So how intense should someone be exercising to loose weight? The general answer is if you are not already exercising it is best to get clearance and/or advice from your doctor or a clinical exercise physiologist. Basically though, start slow but gradually build. Keep it comfortable. Usually you want to build distance (and thus increase duration) over time, then later work on speed/intensity. In the beginning you will be burning both fat and glucose. Later when you are fit enough to exercise at a higher level and longer duration you will still be burning fat and glucose but more of both. And this increase in caloric expenditure will give you a better chance of loosing weight and keeping it off. The precise answers of which exercise, how long and how fast are dependent on your fitness level, health status, medical conditions you may have, medications you are taking, any injuries you have had in the past, your lifestyle and your fitness goals.

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