Easily the question I get most: "What heart rate do I have to stay at to burn fat?" Everyone's been sold the same idea - there's a magic zone, and if you go too hard, you "stop burning fat." The zone is real. What you've been told it does is not.
From the Bench
Your body runs on two main fuels: fat and carbs (sugar). At an easy effort, most of what you burn is fat. Push harder, and your body shifts toward sugar, because sugar burns faster. That's where the five heart-rate zones come from:
Zones 1–2 (easy, you can hold a conversation): mostly fat
Zone 3 (moderate, starting to get uncomfortable): the in-between, no man's land
Zones 4–5 (hard to all-out): mostly sugar
Here's the part the treadmill chart leaves out. When researchers actually measured this, they found your body burns the most fat at a moderate effort - not the easiest one - and that the exact effort is different for every single person. There is no universal fat-burning number. Yours is yours. This is Zone 2.
The Take
So "stay in the fat-burning zone to lose fat" - yes, easy effort burns a higher share of fat. Here's the catch nobody mentions: burning a higher percentage of fat is not the same as burning more fat, and the fuel you burn during a workout is not what makes you lose body fat.
You lose body fat by burning more energy than you eat, day after day. Burn whatever fuel you want during the session - if the math over the week is off, the fat stays. It ultimately comes down to calories in vs. calories out. HOWEVER…
The type of calories in vs. the type of calories burned is what drives body composition change: this reduces fat while maintaining muscle. These truths factor in for the type of fuel choices:
If you don’t have enough protein, your muscle protein will eat itself, and you won’t see positive body composition changes.
If you don’t have enough carbs, your muscle protein will eat itself, and you won’t see positive body composition changes.
Too many carbs will not yield enough muscle protein to change composition in a positive way.
Easy effort still earns its place. It builds your engine, helps you recover, and makes you harder to break. It's just not a weight-loss cheat code, and going harder doesn't cancel out fat loss either. Both belong in your week.
And those zones on the treadmill chart? They're a guess stacked on another guess - the old "220 minus your age" math. If you want your real zones and your real fat-burning effort, it's a test, not a wristband. A finger-prick lactate threshold test finds the exact points at which your body changes gears. A breathing-mask VO2 max test shows precisely when you flip from burning fat to burning sugar.
The watch estimates. The lab measures.
On the Desk
Creating the HTK Method App and building functional screens for in-person locally in NYC Metro Area
Exploring new movement meditations like this one through my PhD summer course (Mind-Body Medicine Integration Practices)
Reading: how diet affects health at the blood biomarker level but psychosocial health as well - excited to share that with you once the research is digested
You just got two cents. If you want the whole dollar - your real zones, your real numbers, and a plan built on them - book your alignment call.
- Coach Cash
References
Achten J, Jeukendrup AE. Maximal fat oxidation during exercise in trained men. Int J Sports Med. 2003;24(8):603–608. doi:10.1055/s-2003-43265.
Achten J, Gleeson M, Jeukendrup AE. Determination of the exercise intensity that elicits maximal fat oxidation. Med Sci Sports Exerc. 2002;34(1):92–97.