Why your heart rate "fat burn zone" is a big fat lie!

I hate being the bearer of bad news, but as a fitness trainer that actually understands how the body’s metabolic biology works, I am used to it by now. Let’s just say that my, and 99.9% of the Certified Personal trainers out there learned zero about actual cellular metabolism yet, right now in any gym, in Anytown, USA a Certified Personal Trainer is giving a person advice about their metabolism and they think the Krebs Cycle is a cool band name. I am allowed to pick on my own. Most Trainers are totally ignorant about this stuff. Thus, coming back to me being the bearer of bad news.

I got a very good question about heart rate zones today and here is the very nerdy answer if you are into this stuff. I find it fascinating and it's one of the reasons why I got into HIIT training because of the benefits. Here is the bottom line up front (BLUF), the “fat burn zone” your cardio machine tells you that you are in is most likely lying to you. I will explain.

I have to start by saying this, everybody is different and on a different level of cardiovascular fitness. For me, having a heart rate of 170-180 isn't a big deal. I am very fit. Sure, I am sucking wind, but it's something I do often, I am fit enough and my body is conditioned for this. If a not-cardiovascularly fit person reached this heart rate and stayed there for a few minutes, they would feel terrible, lightheaded (possibly even pass out) get nauseous, maybe even throw up. It would feel like you are dying and could trigger panic. How do I know this, because I used to be that not cardiovascularly fit person! I remember doing my very first HIIT workout in the form of Shaun T’s Insanity program and I almost threw up and passed out within the first 5 minutes. My heart rate was insanely high and I felt like doggie doo-doo. Needless to say, I did not make it through an entire Insanity workout for quite some time.

So, when I talk about heart rate zones, they are in very general terms and are contingent upon one's cardiovascular fitness level. Your what I call "cardio zone" and what most cardio machines call "fat burn zone" (which I highly contest and is not actually based on how the body works) is around 100-120 bpm. Mainly you are giving your heart muscle a nice workout and burning a little glucose, but your body is able to keep up with the oxygen and glycogen requirements of the exercise. It depends on how much glycogen you have already in your bloodstream, stored in your cells and in your liver. For about 90% of people, if you stay in this zone, unless it is for a very long period of time, this is not a fat burn zone (although many heart rate monitors will tell you this is the fat burn zone, this is a total myth not based on any science).

In order to burn fat, your body has to get extremely low on its stored glycogen. For each of us, our bodies are different for how much that is. It only taps into fat cells for energy after the glycogen becomes too low to keep up with demand. This is why spending some time in an anaerobic zone is helpful. Depending on your fitness level, this zone is 120-140 is where your body will start to struggle to keep up with the oxygen and glycogen demand on the body. It will start pulling oxygen from muscle tissue and breaking down fat cells to convert into glycogen to try and keep up. This is why this high of a heart rate is so hard to spend too much time in, because your body can't work fast enough to keep up, so you should only be able to keep up these heart rates for just a few minutes at a time (if you are very fit) and maybe only a few seconds if you are not very fit.

It is also very hard on the body and triggers a stress response in the form of adrenaline, cortisol, and other endorphins. This is what you hear of called a "runners high" this is the body's stress response at being taxed too hard and not being able to keep up with the oxygen and glucose demand. These hormones help give the body a very temporary boost to keep going and to move those oxygen and fat breakdown responses along faster.

What is very cool about what I will just call the "high-intensity zone", is if you only spend a few minutes there during your workout, that Excess Post Oxygen Consumption (EPOC) effect of the body having to replenish the muscle, cells, and liver of oxygen and glycogen that it borrowed in a hurry, the body has to work hard for up to 48 hours replenishing those stores it used, thus actually burning more energy to do this! That is the advantage of spending some time in the high-intensity zone because you actually burn more calories for a shorter effort than you do if you did a longer workout only in the cardio zone.

Both zones technically could be a fat burn zone, but what most heart rate monitors call the "fat burn zone" (less than 120bpm) is most likely just a sugar and oxygen burn zone, sorry to be the bearer of bad news. I guess that term isn't very motivating to plaster all over cardio equipment, so I guess they thought "fat burn zone" was much more appealing (even though for most people, technically this isn't correct).

Now, of course, if you already eat a very low carbohydrate diet and you have a healthy metabolism, you burn fat at a faster rate and sooner than someone who does not. So, you feasibly be in a real fat burn zone at just 120 bpm. But, going off of national statistics for cardiovascular fitness and metabolic health, (it's in the dumpster and the dumpster is on fire, folks) I am going to assume that the average person jumping on a treadmill in any gym in Anytown, USA is not going to get to a fat burn rate very easily and will definitely benefit from adding some high-intensity intervals into their cardio routine.

And thank goodness, guess what? All of the Fire Team Whiskey Programming is based on these HIIT principles! So, you can work out for a shorter interval (yes, the suck factor is higher) but you burn more energy and more fat with a shorter effort. Who doesn't love saving time and burning fat? Nobody!