You've all seen the setting on the treadmill or elliptical machine for fat burning or the weight loss zone. You've also heard that higher intensity exercise burns more calories and helps you lose more weight, particularly due to the after-burn effect.

Which is right? Well, both.

The fat burning zone is low to moderate intensity exercise specifically performed at less than 65 percent of your maximum heart rate, or MHR. You know you are in this zone when you can pass the talk test, which means being able to hold a conversation while working at this intensity.

When you exercise in the fat burning zone, up to 75 percent of the calories you burn come from fat rather than glucose in your blood stream. Your intensity has to stay low because the body needs more oxygen to burn fat as fuel as opposed to glucose - hence the term aerobic training. The downside to staying at this lower intensity level is that you don't burn as many total calories in the same given time as a higher intensity workout.

High intensity or high intensity interval training, or HIIT, is where you are exercising at more than 85 percent of your maximum heart rate. When you are exercising at this intensity, you can barely talk and your uptake of oxygen is severely limited. This forces your body to utilize a higher percentage of blood glucose for fuel.

This also is known as anaerobic training. Additional benefits of high intensity training include the concept of the “after-burn,” which researchers have not been able to quantify.

A typical 130-pound woman exercising for 30 minutes at 60 to 65 percent maximum heart rate will expend 146 total calories of which 50 percent are fat calories. At 80 percent to 85 percent maximum heart rate, she will burn 206 total calories of which 40 percent are fat calories.

One might ask why anyone would work out at low intensity given how superior high intensity workouts are for burning calories and ultimately total fat. Low intensity exercise promotes physiological adaptations such as improved mitochondrial energy production, increased oxidative capacity of skeletal muscle, higher blood volume and will help your body better utilize fat for fuel.

First, you can’t work out at a high intensity as long as you can at a low intensity. And if you are not properly conditioned, it simply isn’t safe for your heart, joints and connective tissue to work out at high intensity. Furthermore, a high intensity workout could also sabotage your efforts to stay on an exercise program because you become too sore.

Low intensity work is necessary for anyone trying to complete a sporting event lasting longer than two hours.

The best approach is to mix intensities. Everyone can benefit from at least one day of low intensity work for an extended period of time. For optimal health, most of us should exercise for two or more hours one day a week. You should also try incorporating two to three days a week of shorter high intensity workouts once your body is adapted to being able to handle this type of work.