Heart Rate Zones - Fat Burn


First, I’ll preface that my heart rate has always been higher than normal. I’ve been tested, worn heart monitor halters, stress tested, etc. Nothing wrong functionally with my heart, hardware and software all look fine. Blood pressure, cholesterol, other measures all with normal range.

That said, with my Apple Watch I do get some feedback on my heart rate while moving about my day. So, after I finished moving landscaping rocks around my backyard this morning, I took a gander at one of my heart rate apps. My question is, with the heart rate zones, once I hit Fat Burn stage, do I continue burning fat at the same rate in higher, more intense zones? My understanding is that there is an “ideal” fat burn zone but logic tells me working harder, sweating more is better.

(Bob M) #2

Those fat burn versus heart rate guides are ludicrous. They really are useless. Just exercise.


@Emacfarland had some actual metabolic testing done to evaluate this during exercise. I know there’s a topic about it but I’ll have to dig up the link…

After fat adaptation, you will utilize a higher percentage of fat even in the more intense zones that cause primarily glucose burning in non fat adapted athletes. It’s a gradual progession from mostly fat to mostly glucose and lactate which is shifted upward with keto.

(Bob ) #5

In the old days of running, they’d say you have a “4 cylinder heart” or a “Toyota heart” - the small engines run at high RPMs. The opposite, naturally low rates were called a “diesel heart”. The big diesel engines run at lower RPMs for the same power. My wife has a normal heart rate that’s kind of high due to thyroid issues. She’ll see me take my pulse over morning coffee and marvel that it’s in the upper 50s - hers is around 100.

There are two things about those target “fat burning heart rate” ideas to bear in mind. First is that if you’re well fat adapted, you’re burning fat all the time. That means the higher your heart rate the more you’re burning all the time. Yeah, that’s diluted a little by using whatever glycogen you’ve packed into your muscles when you’re not exercising, but the metabolic advantage from being fat adapted is the big thing. The second to bear in mind is those are all from studying sugar burners, not fat adapted, keto folks (except for a few studies by Jeff Volek and Steven Phinney).

You asked:

I believe that you don’t really burn fat at the same rate in higher more intense zones, you burn at a higher rate by being fat adapted and working harder. I think your logic is quite right in saying “working harder, sweating more is better”.

(PJ) #6

I’m curious does she do lowcarb?

Because my heart rate is in the upper 80s-90s off ketogenic eating. As soon as I shift to keto – within a couple days – let alone fasting! – it drops at least 20 points, from 60s to 70s.

(Bob ) #7

Yes, she’s fully keto. Her heart rate drops to the 70s now and then but I sure don’t know why.

Her problem is from her thyroid. She’s been on Synthroid (synthetic thyroid hormone) for over 20 years and has had to reduce her dosage several times as she lost weight. He system seems to be extremely sensitive to dosage. She takes different dosages on alternate days to hit a long term average.


Jury’s really out on how (we) work at the higher rates. Just like any exercise / weight lifting, the lower zones allow us to run fine on fat while the higher intensity stuff like HIIT runs better on glucose, we can do it with fat but the performance isn’t the same. This is the general idea of the lower (fat burn) and higher (cardio/endurance) stuff, but it’s not perfect and everybody’s ranges are going to be different. Only way to know what’s best for you is V02 max testing, if you’re really into cardio based exercise or a running it’s very useful but for most… eh.


Thanks all for your thoughts. Sounds like I just need to ignore any of the pretty charts and graphs and just know that moving my a$$ is good no matter what my heart rate. :slight_smile:

(Wendy) #10

Hi Bob
I just went in for my thyroid bloodwork. This am. I currently take .75 I have been concerned cause I have noticed some hair loss. In the past that has been a good sign thyroid levels have changed. I have been on Keto since April. I was wondering if your wife’s levels changed after she started keto? Or if they have always been up and down? And how often does she have it checked?

(Wendy) #11

Since April I have lost 32# :grin:

(Bob ) #12

I think that’s the important part. I think my wife was adjusting her dosage as she lost weight and has been at goal for a while now (and on this alternating pill size protocol, too).

She gets it checked pretty much just at her annual checkups. Which makes me realize she has probably been on this alternate day dose for almost two years.

(Wendy) #13

Thank you for your reply. I usually go every 6 months. For thyroid. Will be anxious to see if the level has changed. Your wife is very lucky to know her dose. And when to change it. I have another 20# to go. Congratulations on her goal! Its definitely a work in progress.