Ever wonder what’s with the heart rate charts on cardio machines? And why the heck, if you work in the “fat burning zone” all the darn time, you don’t feel as if you are actually burning fat?

A popular myth says you must exercise within a specific range of heart rates to burn fat. What happens if you venture out of that zone?

Jason R. Karp, PhD, a nationally-recognized speaker, writer and exercise physiologist, coaches recreational runners to Olympic hopefuls and sheds light on this issue:

Fuel Use During Exercise

You use both fat and carbohydrates for energy during exercise, with these two fuels providing that energy on a sliding scale.

During exercise at a very low-intensity (walking), fat accounts for most of the energy expenditure. As exercise intensity increases, so does the contribution from carbohydrates, while the contribution from fat decreases.

Once the intensity of exercise has risen above this threshold, carbohydrates become the only fuel source.

If you exercise long enough – one and a half to two hours in one session, but who has time for that? – your muscle carbohydrate (glycogen) content and blood glucose concentration become low. This presents a threat to the muscles’ survival, since carbohydrates are muscles’ preferred fuel.

When carbohydrates are not available, the muscles are forced to rely on fat as fuel. Not a bad, right?

Since more fat is used at low-intensity exercise, people often assume that low-intensity exercise is best for burning fat, hence the “fat-burning zone.”

However, while only a small amount of fat is used when exercising just below the threshold, the rate of caloric expenditure and the total number of calories expended are much greater than they are when exercising at a lower intensity, so the total amount of fat used is also greater.

The Bottom Line

For fat and weight loss, what matters most is the difference between the number of calories you expend and the number of calories you consume.

Notice I didn’t say fat you consume? Fat and weight loss is about burning lots of calories and the types of foods you ingest to fuel your body. For the purpose of losing weight, it matters little whether the calories burned during exercise come from fat or carbohydrates.

Go Hard

Those who have trained with me, or seen me train, know I love to get my workouts in quick and hard. It’s not about the amount of time you spend in the gym, it’s about what intensity you put into the workout.

A great way to perform high-intensity exercise and decrease your body fat percentage is through interval training, which breaks up the work with periods of rest.

Not only does interval training allow you to improve your fitness quickly; it is also more effective than continuous exercise for burning lots of calories during exercise and increasing your post-workout metabolic rate.

Try one or two of these workouts each week:

1. Five to six by three minutes at 95 percent to 100 percent maximum (max) heart rate (HR) with two-minute active recovery periods.

2. Four by four minutes at 95 percent to 100 percent max HR with three-minute active recovery periods

3. Eight to 12 by 30 seconds fast with one-minute active recovery periods.

Each of these interval workouts should include a warm-up and a cool-down.

Go Very Long

Every once in awhile, throw in a workout of long slow distance. Going for a nice long walk definitely counts! Throw this in at least once a week, when you have time.

Lift Weights

Weights are an extremely important part of a weight loss program. Increasing your metabolism and burning calories for hours after a training session, and increasing your resting metabolic rate will ensure you are burning more calories at rest.

I like to combine strength with my interval cardio sessions, getting more burn for my time!

This column is provided by Mrs. Lee Randell, independent fitness consultant, who is an ACE certified advanced health and fitness specialist and personal trainer. You can reach her at www.mrsleerandell.com.