Stepping onto a cardio machine and choosing the "fat burn" program likely decreases the amount of fat one burns. While the "fat burn" program encourages exercisers to burn higher percentages of fat, the problem is that it encourages slower speeds, fewer calories burned, and less fat burning overall.
Here's why one may want to avoid programs labeled as "fat-burning zone" found on today's treadmills, elliptical trainers, bikes, and other cardio machines.
For fuel, the body can burn fat, carbohydrate, or proteins. At rest, it is generally assumed the average body burns about 70 percent fat and 30 percent carbohydrate. Fats are stored and utilized from almost anywhere in the body while carbohydrates are readily floating in the blood stream, available in the muscles, or stored in the liver. Protein (obtained by breaking down muscles and tissues) is not generally used as a fuel source unless one begins to starve or diet.
As the body moves at higher speeds or simply exerts more effort during a workout, increasing energy expenditure, the percentage of fat burned actually decreases. For example, a person who burns a 70 percent fat-to-30 percent carbohydrate ratio at rest may drop to a 65 percent fat-to-35 percent carbohydrate ratio when beginning to power walk. Upon jogging, that ratio drops further, perhaps to 60 percent fat-to-40 percent carbohydrate, or 55 percent fat-to-45 percent carbohydrate; this is where the fat burn zone usually encourages the exerciser to remain. Increasing speed to a run then causes a further drop in percentage of fat burned, perhaps to 45 percent fat-to-55 percent carbohydrate. When any person performs a proper sprint (requiring maximum speed, generally lasting less than 10 seconds), he or she is burning all carbohydrate and zero percent fat. This seems to make a good argument for exercising in the fat burn zone until one examines further.
A person who trains in the fat burn zone will often burn less fat than if they had simply trained at higher intensity. Let's assume that a 30-minute "fat burn" cardio session burns 300 total calories, equating to a total of 180 fat calories burned and 120 carbohydrate calories burned (assuming a 60 percent fat-to-40 percent carbohydrate ratio). But what if that same person trained faster, forcing a lower percentage of fat fuel but more calories overall? In the same time, that person might burn 400 calories at a 50 percent fat-to-50 percent carbohydrate ratio. This means that 200 calories of fat and 200 calories of carbohydrate were burned. So in the same amount of time, ignoring the fat burning zone promoted by the cardio machine, more fat was burned.
It is a nice marketing move for cardio manufacturers to highlight a fat burn zone on their machine panels, but not one that helps the average exerciser. Instead of encouraging effort and improved intensity, it actually encourages many gym-goers to slow down and stay within the zone. Under the "fat burn" premise, one might as well spend workout time sleeping, where very few calories are burned but an 80 percent fat-to-20 percent carbohydrate ratio can be obtained. In effect, it encourages many to remain lazy and actually hinders their fat-burning goals. Worse, the effects are much more pronounced than just missing out on a few extra fat calories.