I keep coming across wrong thinking about the fat burning zone in multiple areas so someone upstairs must be telling me something. Thank goodness that better info sources like NASM are out there. These better sources are like a ligthhouse on a stormy night — they help you navigate your way through all the ignorance.

So here’s the incorrect idea: because the aerobic pathway (think long slow distance exercise) uses primarily fat as fuel, then people should only do that kind of exercise to burn fat. The usual recommendation is to keep your heart rate really low like around 50-65% of your maximum heart rate and if you can still hold a conversation then you’re at the right level. If not, you need to back off. Exercising above that rate kicks you out of the fat burning zone into a zone that gets more anaerobic as it goes higher — you know, one that requires you to sweat and actually make an effort and goodness knows we can’t have that.

This is what it looks like:
The “maximum cardio benefit zone” above is the area where you’re burning less fat and more carb, which is what people want to avoid.

Now before I go any further, let’s point out something: if I can talk comfortably while exercising then I’m not exercising very hard, right? Which means I’m not burning many calories, right? This will become important later on.

So let’s look at a breakdown of how many calories from carbs or fat you’re burning while exercising in the different heart ranges: (Source: Indoor Rowing Training Guide, Version 2)
There is a sliding scale from 100% fat, which is being burned at the very low end of exertion off the chart above, all the way up to 100% carbs which are being burned at the top of the exertion chart in the anaerobic zone. At the one end you can think of walking or slow jogging and at the other end imagine full out sprinting. All the ranges in between show a split of fat and carbs being burned with the percentage of fat going down and the percentage of carbs being burned going up as the exertion increases.

So let’s take an individual who walks for about 20:00 on a treadmill at about a 3mph rate and that puts him at around 60% of his maximum heart rate, which is what the “fat burning zone” idea recommends. He’s burning around 4.8 calories a minute for a total of 96 calories in that 20:00. About 67% of those calories are from fat so that would be 64 calories of fat burned and 33% are from carbs, making that 32 calories from carbs burned. Since the percentage of fat is higher than the percentage of carbs, this looks like he’s burning crazy fat.

But let’s put this guy back on the treadmill and have him move twice as fast at around 6mph. He’s now burning around 9.75 calories a minute for a total of 194 calories and 54% of those are coming from carbs and only 46% are coming from fat. So the percentage of fat burned is lower, making this seem like a worse option than the fat burning zone.

Not so fast!

While the percentage of fat burned is lower, let’s look at the numbers: 54% of 194 is 104 calories from carbs and 46% of 194 is 90 calories from fat. So working out in the fat burning zone for 20:00 got us 64 calories of fat burned and working out above that burned 90 calories of fat — nearly 50% more fat calories burned!!

To top it all off, he burned twice as many calories as he did in the fat burning zone, which will help him lose fat faster. A workout in the fat burning zone would have to be twice as long to get this caloric expenditure. How’s that fat burning zone stuff sounding now?

An obvious objection to this is that most people can’t work at a higher pace every day, with the implication that they’d be better off working out at a lower rate every day than doing a harder workout 3x a week. From experience I can tell you that if people can work out 3x a week or 6x a week, they’re more likely to actually make 3x a week because of their busy schedules. And there’s always the option of working harder 3x a week and doing 2-3 recovery workouts at a lower heart rate each week. This last option maximizes your fat loss and is the most productive use of your time.

So the bottom line is, in general, ditch the idea of a fat burning zone and learn to work at higher intensities. You’ll burn even more fat and who doesn’t like that?