the deception of “calories burned”

This isn’t a new topic. It’s tangential to my article titled “you’re running too fast to burn fat”. Often I hear friends or clients saying “why should I walk if I can burn more calories doing a HIIT workout?” Well let me explain that to you.

If you’ve read “you’re running too fast to burn fat”, you’d know that as exercise intensity increases, what we burn changes. At low intensities we burn fat, and at medium to higher intensities we burn carbohydrates and creatine phosphate.

This is important if you want to lose weight, and that looking at “calories burned” after a hard workout isn’t telling you the whole story.

Here’s an example:

I could do a 30 min HIIT class and burn 400 calories. Sounds great, right? Until you realize that 90% of those calories would be from carbohydrates - meaning, we didn’t burn 400 calories of the fat around our belly, we burned mostly muscle glycogen, aka carbohydrates, stored in your muscle tissue.

Now, let’s compare this to slowing down. I can walk at a fast pace, 3.5-4.0 mph, for an hour and burn ~400 calories. Double the time, sounds inefficient right? Not when 90% of those calories would be from fat, meaning we actually used calories from the cookie pouch around our waist.

For you math wizards, in the HIIT example I would burn at most around 40 calories from fat. In the walking example, I’m burning about 9x that amount (360 calories from fat).

To make this a fair comparison, let’s look at both intensities for an hour long duration.

HIIT vs Walk

800 cal/hour > 400 cal/hour

80 calories from fat < 360 calories from fat

4.5X more fat walking

The worst part is when I burn 400 calories from carbohydrates, I have to replenish that at some point otherwise my body would have a hard time doing literally anything. That means eating carbs, which is totally fine but doesn’t make staying on a diet easy.

*Yes it is true that the point of HIIT is that you’ll burn more calories AFTER the workout, but that effect is overstated and is assuming you won’t f*ck up your diet.

The best part is that slowing down is easy, and you don’t ever have to replenish your fat stores, making staying on a diet much easier.

Now does HIIT have a spot in a well-balanced program? Absolutely. I usually recommend 1x a week. But just as I’ve said before, the bulk of every non-athlete’s cardio time should be spent in your fat burn zone.

What is the fat burn zone?

That differs from person to person, and is hard to pinpoint without a direct VO2 Max test (meaning with a gas mask - see mine below). However, it’s pretty easy to estimate. Anything, I mean literally anything, above sitting on your ass is going to start burning more fat. That only switches if you cross an intensity that is hard enough where your body needs to use carbs.

That’s a pretty wide zone, from as easy as walking to the bathroom all the way up to an easy jog most people are burning fat. The real take away here is if your goal is to burn fat, it’s safer to be on the slower side.

My VO2 Max test

Time is along the bottom, the speed increases from left to right as the test duration increases. Speed started at 2.5 mph and peaked at 10 mph. Orange line is fat burn throughout the test, blue is carb burn.

Let’s analyze my results above. In between the purple brackets is my optimal fat burn zone, at a heart rate range of 119-132 bpm. Does that mean I train at 132 bpm? Hell no, look at what happens to the chart if I just go a little too fast. Fat burn plummets to ZERO and carb burn spikes.

On the other hand, what happens if I’m a little to the left of the purple bracket (meaning slower, and below 119 bpm)? I still burn WAY more fat than carbs - and it’s easy AF.

So, without a VO2 max test, would I want to tip-toe the line that I think is optimal for fat burn or just stay safe an go a little slower? The choice is pretty easy.

For those interested in my speeds, at 6.12 mph I broke through my fat burn zone. I’ve been working at this for 2 years and am now at the 90th percentile for fat burn, and I stop burning fat at a 10 MINUTE MILE. THAT’S VERY SLOW. When I started, it was 2.8 mph and a heart rate zone of 99 bpm.

So, it’s safe to assume if you aren’t a featherweight cross country runner, walking fast or walking with an incline is where your zone is. And after you’ve worked at it for long enough, a light jog would work.

Just remember, it’s safer to be slower and crossing the threshold from a fast incline walk to a jog takes time - as the impact of a jog requires more energy.

A more granular analysis for staying below the threshold:

If you look at my peak lipid metabolism (32-39 grams per hour) and do some math (fat has 9 calories per gram) it shows that at my best fat burn, I’m burning 39*9 = 351 calories from fat per hour.

At that zone, I’m also burning about 60 grams of carbs per hour. At 4 calories per gram, that’s 240 calories from carbs per hour. Put that together, and I’d be burning a TOTAL of 591 calories per hour in my PEAK FAT BURN ZONE.

Meaning, if I care about burning fat I shouldn’t be aiming to burn higher than 591 calories per hour, as that would mean I’ve crossed my threshold - and remember, I’ve been working at this for 2 years.

More importantly, shortly after crossing my threshold I eventually get 100% from carbs, 0% from fat. The speed at which that happens for me is only about 7.8 mph, or a 7:42 mile - not a record setting speed. So for anyone saying “if I go a little faster its ok, cause I’ll still burn SOME fat”, eh not really.