Just about every treadmill or exercise bike you've ever seen has a chart on it showing users how hard to push themselves to achieve their chosen fitness goals. According to charts like the one pictured below (which are often programmed into the electronic guts of the cardio equipment), you're in the "fat burning zone" if you're exercising at 50 to 60 percent of your maximum heart rate, and you're in the "cardio training zone" if you're at 75 to 85 percent of your maximum heart rate.
This is bullshit.
For weight loss purposes, the theory goes, it's counterproductive to work too hard; instead, you'll optimize your fat loss if you take it easy and work out less intensely. According to this argument, low-intensity aerobic training forces your body to burn more fat as an energy source, while high-intensity cardio work burns more sugar and less fat. As New York Times reporter Gina Kolata summarized in her excellent 2003 book, "Ultimate Fitness: The Quest for Truth about Exercise and Health":
The idea is that if you want to get rid of fat, you should force your body to burn it. Muscles prefer to use sugar, in the form of glycogen, for energy, because it is so easy to metabolize. but they also can use fat. The fat-burning-zone hypothesis says that if you don't push yourself too hard, your muscles will burn fat and you will lose weight. Extreme exertion, however, will force your muscles to burn glycogen, leaving your body's fat intact.Sound too good to be true? It is.
Here's the real deal: While it's true that a higher proportion of calories burned during low-intensity exercise comes from your flab (approximately 60 percent versus 35 percent from high-intensity workouts), higher intensity exercise still burns more calories from fat overall. Kolata writes:
It turns out that the myth of low-intensity-exercise fat burning is a misunderstanding of a basic relationship, best seen on a graph. One line shows muscles' use of fat as an energy source as a function of exercise intensity, and another line depicts muscles' use of carbohydrates as a function of exercise intensity. the crossover point, where equal amounts of fat and carbohydrate are being burned, comes at about 60 percent of your maximum effort, or a heart rate that is about 70 percent of maximum. After that, the amount of carbohydrate burned exceeds the amount of fat, and this imbalance increases as exercise intensity increases. If you guet to your maximum heart rate, less than ten percent of the calories you burn will come from fat.In other words, who cares if a greater percentage of your calories are burned from fat than from sugar/carbs? What really matters is the total number of calories you burn from fat. And the more intense the exercise, the more total calories (from both fat and carbs) you'll burn.
That led to the argument that as long as you keep your heart rate low enough, you will burn more fat than carbohydrates and you will lose more weight. The problem is that the argument is neglecting a crucial component: the number of calories burned. The harder you work, the more energy you expend, and the more calories you will need.
So suck it, "Fat Burning Zone" charts.