Does your tracker or sports watch have a heart-rate function? Or have you used cardio equipment with built-in HR measurement? If so, you may be aware of the so-called “Fat Burn” heart-rate zone.
I advise you to “mostly beware” of falling for the allure of this name, man. It sounds too good to be true. “Burn more fat with lower exertion than you would with higher exertion!” And it is too good to be true. However, low-intensity workouts in HR ranges labeled “Fat Burn” do have occasional purposes. Hence my “mostly beware” admonition.
Let’s walk through the facts and logic here, and you can make your own call.
“FAT BURN ZONE” – CONTEXT
Most HR monitoring approaches divide pulse rates into five zones. Though they go by different names from different fitness-tech companies or advice sources, the zones are similar no matter who’s describing them. And the so-called Fat Burn zone is the second lowest.
HR Zones & Percentage of Max HR*
- 50-60% (very light or warm-up)
- Called “easy” by Garmin and “light” by Polar — and “Fat Burn” in many other instances
- For an average 50 year-old guy, this would be HR = 102-119
- Sometimes you’ll see Fat Burn described as being 55-65% of Max HR (straddling Zone 1 + 2)
- In FitBit’s case with just three simplified zones, Fat Burn is 50-69% of Max HR
- 70-80% (aerobic or moderate)
- 80-90% (more-intense aerobic and reaching anaerobic threshold)
- 90-100% (peak effort — anaerobic and unsustainable)
* Note on Max HR: there are more-sophisticated methods you can use, but [Max HR = 220-age] is the simplest rule-of-thumb. Additionally, due to fitness level and genetics, an individual’s Max HR can be as much as +/- 20 beats-per-minute vs. averages. So an average 50 year-old guy’s Max HR would theoretically be 170. But for an individual it could be anywhere in the 150-190 range. The highest HR I’ve seen for myself in recent times, during really hard HIIT workouts, is 181. So I guess that’s my Max or close to it.
“FAT BURN ZONE” – THE MYTH, AND WHY IT’S MOSTLY ILLOGICAL
Man, Zone 2 must have a creative marketing department! It’s called the Fat Burn zone NOT because it burns more fat calories than do higher HR zones. Rather, it’s referred to this way because it burns a higher proportion of total calories from fat (vs. carb sources) than do higher HR zones.
But in an absolute sense, higher HR zones burn more fat calories (and a lot more carb calories). This data from BuiltLean shows what I mean. Admittedly, they compare the very low end of the Fat Burn zone to the middle of the Aerobic zone to make their point. But the conclusion would be the same even with less cherry-picked numbers.
For 30 minutes of exercise:
- At 50% of Max HR: 120 calories from fat + 80 calories from glycogen (carbs) = 200 calories total
- At 75% of Max HR: 140 calories from fat + 260 calories from glycogen = 400 calories total
Yes, the easy Fat Burn zone workout gets 60% of its calorie burn from fat while the more-intense Zone 3 workout gets only 35% of its calories from fat. But in terms of actual calories, the Zone 3 workout burns 20 more fat calories and 180 more carb calories – for a grand total twice as many calories as the easier workout.
If you think about it, this is only logical. Example: If I run for 30 minutes at a higher exertion level, I travel farther than I do at lower exertion. Invoking the memory of high school physics class, the “work” being done is higher. I’ve moved the same weight (me) over a greater distance. So, of course I’m going to burn more calories.
And this benefit is just during the workout. It’s not even including the sustained increase in metabolism (“after-burn”) that higher exertion levels bring. And research suggests this after-burn takes place almost entirely from fat calories, by the way.
So this idea, that I can work out less hard and lose more fat than if I work out harder, just doesn’t stand up to scrutiny. And limiting yourself to Zone 2 workouts won’t improve your cardiovascular health as much, either. It’s instructive that Garmin, with respect to cardio benefits, calls Zone 2 “maintenance” while calling Zones 3 and 4 “improving” and “highly improving.”
I’ve got nothing against maintenance – it’s part of practical life as an OlderBeast. But why give up on “improving” when it’s available to you, brother?
ONE POINT OF POSSIBLE CONFUSION
Despite everything we’ve reviewed so far, there are still a few reasons we might want to – sometimes – do easier workouts in the Fat Burn HR zone. Before we get to those, though, let’s address this point of confusion that might come up at this point.
It’s about training your body to burn fat instead of carbs. There’s a physical stated called ketosis, in which your body can burn fat calories as the main fuel source even at much higher heart rates. That’s one of the objectives of the so-called ketogenic diet (which is ultra-low carb and pretty controversial) – to “train” your body to be able to do this.
There are other techniques via which ultra-endurance athletes also seek this shift in how the body works. The idea is to lower your dependency on carbs as a fuel source, since during competition you can’t replace them as fast as your body uses them. Whereas even lean people have many thousands of calories of body fat available as fuel.
I’m not trying to get into these advanced topics in this discussion! What we’re talking about here is whether, regardless of diet, non-extreme athletes should stick to the lower heart-rate zone called Fat Burn.
BUT STILL, OCCASIONAL REASONS TO STAY IN FAT-BURN HR ZONE
OK, given all this, why wouldn’t you always want to take endurance workouts into the aerobic HR ranges (Zone 3 and sometimes 4)? They burn more calories during the workout (including more fat calories), and have a fat-focused metabolic after-burn effect. And they contribute more to your cardiovascular health.
Yeah. These are the reasons why, most of the time, you should seek higher HR zones. But here are two reasons to sometimes “stay low” on heart rate.
First, when you go at it harder, you’re more prone to “worn down” type injuries. You’re also more likely to interfere with recovery from recent hard workouts you’ve had. These are particularly reasons to not always be in high aerobic HR ranges (Zone 4) and especially to not always go peak (Zone 5), as HIIT will have you do. But while “mellow” aerobic exercise low in Zone 3 helps with injury avoidance and letting your body recover – still while burning some calories and getting some de-stressing benefits of exertion – workouts in the Fat Burn Zone 2 do, too.
Second, having a light workout in the Fat Burn zone is a good thing to do when you can’t decide between a day-off or a workout. It’s kind of like splitting the difference. For example, if you’re interested in moving from five to six days per week, this is a big jump. You’re literally cutting your weekly recovery time in half (from two days off to just one).
Why not try a half-way step as a transition, doing an easy Fat Burn workout for a while on Day 6 (and maybe combine that with some light bodyweight strength work or a short yoga routine)?
Finally, more-intense workouts can spur your body to create cortisol, the stress hormone. Too often, too hard will put stress on your system. So this is a reason to sometimes keep HR a bit lower (at least in the lower-aerobic ranges, if not necessarily all the way down in Fat Burn).
For endurance workouts, then, you really want most of them to get you into HR Zone 3 and — depending on your goals — Zone 4 (even Zone 5 if you’re doing intervals). Don’t fall for the siren song of the Fat Burn zone, man. You’ll burn many fewer calories overall, including fewer fat calories as a component of that. And you won’t be improving your cardio health as much.
But sometimes, with variety and sensitivity to your body’s needs being the hallmark of smart OlderBeasthood, throw in a Fat Burn workout. It will still burn some calories and help you de-stress, and you’ll be even readier to go with some intensity the next day.
“I have only one itching desire. Let me stand next to your fire.” (Jimi Hendrix, Fire — click to listen)