The Fat-Burning Zone – Myth or Magic?

Fat: it’s on everyone’s minds right now. Holiday parties, cookies and milk, too many bottles of wine and big smorgasbord meals all combine this time of year to make it easy to put on the pounds. So people are thinking about A) gaining weight, B) being overweight, or C) that in the new year, they are going to lose weight.

Heart Rate ZonesCome the first week of January, gyms throughout the country will be packed with people on cardio equipment, staring at heart rate graphs and hanging onto the heart rate sensors to make sure they are staying in their “fat burning zone”. This zone – 60-70% of your maximum suggested heart rate (calculated as 220 minus your age) – will keep you doing low to moderate intensity exercise, and it’s true that more of the calories burned will come from fat stores. Will that help you drop the pounds? Let’s discuss.

From a cellular perspective, a lower intensity workout requires a slower supply of cellular energy – read: calories – which is perfectly matched to the slower process of breaking down fat stores. Even the most fit people have higher stores of fat than they do of carbohydrate (which provides cellular energy very quickly), and the high volume of fat stores let us keep moving at a slow-and-low intensity for a relatively long time. And so we loop around to the beginning of the paragraph: Lower intensity generally means slower movement, as well as lower (and therefore slower) energy demands. In plain english, lower heart rate and energy demands equal fewer calories burned over a given period of time. 

Let’s look at what happens with higher energy demands: Once your heart rate rate starts edging about that 70%, you’re getting into the “aerobic zone”, or if you’re really hustling, into your “anaerobic zone”. Since your heart rate is higher, your cells need more energy, pronto, to keep you moving. During periods of high energy demand, our bodies don’t have time to break down fat stores. Instead, they make quick energy by breaking down sugars (our stored carbohydrate) and other cellular molecules. But we only have these in limited supply, which is a big part of the reason that sprinting and other high-intensity activities can’t be sustained for more than a minute or two. The body is infinitely clever though. When we have to stop, gasping for breath, our bodies immediately go into recharge mode, creating more energy-providing molecules for our cells to use. In plain english, higher heart rate and energy demands equal more calories burned over a given period of time, but you won’t maintain the highest intensity for the entire period of time. 

So we have Option A – burning fewer calories, but primarily from fat stores, or Option B – burning more calories, but from non-fat sources, and only able to do it for a short time. Does it sound like either is a clear winner? Maybe not.

But here’s the magic: While we might love the little graphs that neatly divide our heart rate ranges into “fat burning” or “anaerobic” training zones, the body doesn’t work with such strict segments. In fact, the fuel you use during any given workout comes from a combination of stored cellular energy-providing molecules, stored carbohydrate, and stored fat. High intensity interval training is actually now the most recommended fat-loss workout. This method combines short bursts of high intensity exercise with short periods of recovery, and maximizes calories burned (and fat lost!) in three ways:

1) The high intensity periods (your aerobic and anaerobic zones) burn A LOT of calories for the duration of the interval.

2) The recovery periods drop your heart rate into your “fat burning zone”. You may burn fewer calories during this interval when compared to the high intensity, but you’re still burning them at a higher rate than if you’re sitting on the couch, and the recovery period allows the body to prep for another high intensity burst.

3) Possibly the most useful calorie burn is after your interval workout is over. After a high intensity workout, your body will continue to burn calories at a higher-than-normal rate as it repairs muscle, replenishes cellular energy stores, and makes you a little stronger, faster, and fitter for your next workout. Depending on the amount of time you spent at high intensities, your post-exercise calorie burn can remain elevated for up to three days. And because this is all happening in a “behind the scenes” sort of way, your fat stores are helping fuel this recovery as well.

So is the fat burning zone a myth? Definitely not – fat is your primary fuel source for any low to moderate intensity exercise. The real question is will it actually result in more weight lost overall, and that is a whole different story! While there are definite reasons to stay working in the low-moderate exercise intensity zone, like rehab situations or chronic health conditions, if you’re healthy and looking for fat loss, getting out of your fat burning zone will do you a lot more good.


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