The skinny on low-intensity exercise and burning fat!

Does exercising at a lower intensity burn more fat than at a higher intensity?  Yes.

Will you lose weight faster exercising at a lower intensity than at a higher intensity?  NOOO!

There’s been a long-standing myth in the fitness community that exercising at a lower heart rate (“the fat burning zone”) is the best way to burn fat vs. exercising at a higher heart rate. If that sounds too good to be true, that’s because it is not true!  It’s all about percentages and if you’re not clear on how the percentages of fat calories relate to overall calorie expenditure, you’re likely to believe the myth.

Around 2 decades ago, researchers found that fat is primarily used as the fuel source for light aerobic activity and that carbohydrates stored in the body (which convert to energy faster than fat), are primarily used as the fuel source for higher intensity exercise.

Percentage-wise, more fat calories are burned during low-intensity exercise.  For example, approximately 60% or more of total calories burned during light exercise may come from fat, whereas around 35% or possibly less of total calories may come from fat during vigorous workouts.  However, when you do the math, higher-intensity exercise still burns more calories from fat, and more calories overall, in the end.

Here’s a good hypothetical scenario: 

A 140-pound woman does a one-hour walk for exercise at a light to moderate intensity (60-70% max heart rate – no huffing and puffing), she may burn around 192 calories.  If 75% of those calories come from fat, then that equals 144 fat calories (144 calories came from fat out of the total 192).  Now, if she steps it up a couple of notches and jogs at a moderate to moderately high intensity (70-80% max heart rate – a little huffing and puffing, or heavy breathing), she’ll burn around 288 calories.  If only 50% of those calories come from fat, she’ll still burn the same number of fat calories (144), but she will have burned nearly 100 more calories in that same hour.  Remember, you have to burn 3,500 calories to lose a pound of fat (and those calories can come from carbohydrates or fat), so exercising at the higher intensity will get you there faster!

Low-intensity workouts can still help you lose weight, it just takes a whole lot longer.  Our hypothetical woman would have to walk for around 90 minutes to burn the same number of total calories as a one-hour jog.

On the other hand, one of the major benefits to higher intensity exercises (such as ones that build muscle strength like circuit training with weights, boot camps, cycling or running reasonably hard) is that these activities require your body to work at a much higher metabolic rate.  That means your metabolism stays up long after your workout is over.  This is what is referred to as “afterburn.”   At the end of the day, the more muscle you put on your body, the more calories you burn all day long.  For example, even at rest, each pound of muscle on your body can burn approximately 30 to 50 calories a day.  Muscle is what’s known as “metabolically active” tissue.  Fat on the other hand is lazy and only burns only around 3 calories a day! 

But back to the original point…. The possible upside to doing low-intensity exercise is that it’s less intimidating and easier to stick to for people who are just getting started on a fitness program.  Therefore, if you’re out of shape and need to start exercising regularly you may enjoy it more if it’s low-intensity.  Then, after you’ve been at it for a little while, you may get the urge and motivation to kick it up a few notches

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