What the FBZ is
The FBZ is the idea that at a lower percent of your maximum heart rate you will, as a fuel source, use a greater percentage of calories from fat than from carbohydrates. This is true. At low intensity efforts, we may use fat for nearly 50% of our fuel needs and only 35% at higher intensities. It sounds like a no-brainer, until you do the math. Arithmophobes, fear not, it’s fairly simple and I’m going to do it for you.
Let’s say you’re a 35 year old, 5’4”, 140 lbs woman. Let’s say you walk at 3 mph for 60-minutes, working in the “fat burning zone” at 65% of your maximum heart rate. You burn 280 calories total. Fifty percent of those are from fat so that’s 140 calories, and the other 140 come from carbohydrates. (Your body can use protein for fuel, by the way, but that’s not its preferred source.) Not bad! And I’m proud of you for getting in an hour long workout! And you kept your heart rate low during this slow, steady-state cardio session.
Now, you’re that same woman who decides to kick it up a notch. Instead of walking to keep your heart rate in the fat burning zone, you get on the elliptical and really push yourself for 60 minutes. Your heart rate gets into the 75-80% range. Your overall calorie burn is 600, which is a lot more than your previous workout, and even though you were well out of the fat burning zone, you still used at least 210 calories from fat to complete this workout. So you burned more calories overall AND used more fat energy. In fact, if you’d only stayed on the elliptical for 40 minutes, you would have used the same number of calories from fat as the walking example and still have burned over 100 calories more! That’s a win.
I understand that there are a lot of reasons to do long, slow steady-state cardio, and if your goals align with those reasons that’s great! And if you’re just getting started with exercise, keeping the intensity low can help with adherence, prevent injury and muscle soreness, and help you avoid that ‘I’m going to die on this treadmill’ feeling.
The point is not what exercise you do, or how fast you do it. The idea is that working harder during your cardio sessions and not trying to stay in the mythical fat burning zone will help you lose weight, if that’s your goal. Questions? Comments? Let us know below or email email@example.com!