Know Better, Do Better: The Myth of the Fat Burning Zone

In the spirit of paying it forward, I am planning on sharing interesting and helpful information I have learned through my preparation for my Certified Personal Training exam. A Weight Watchers leader once said in a meeting, “when you know better, you do better.” Wise words that can be applied to exercise, diet choices and many other health related issues. I hope you will find the information in the “Know Better, Do Better” series as fascinating as I do! As always, consult your doctor before beginning any exercise program.

If you are a gym-goer who does cardio on a machine, whether it be a treadmill, elliptical or stationary bike, you’ve likely seen a heart rate zone labeled the “fat burning zone.” You likely also noticed that to work within that heart rate zone – 50-60% of your max heart rate (220-your age) – means you are barely moving. Seems a bit counterintuitive, doesn’t it? Move less to burn more? That can’t be right.

I think we’ve all heard that this is a myth, but I’ve always wondered why exactly. Much has been written on this topic, and we’ve always been told, “don’t be tempted by this promise of maximum fat burn.”

Well, here’s why.

The science that has perpetuated the fat burning zone is centered around the source of energy your body uses while working out. Your body will use different fuel types (fat, carbohydrates, protein) during your workout depending on the amount of oxygen exchange going on, i.e. how hard your heart is working. At a lower heart rate (and lower oxygen exchange) your body will use fat as the primary energy source, therefore burning a larger overall percentage of fat.

Burning more fat, we like that. This sounds great so far, right?

The downside is that while the amount of fat being burned might be high percentage wise, when you operate at a such a low heart rate, your overall calorie burn is very low. The Law of Thermodynamics teaches us that the only way to lose weight, or reduce body fat, is by burning more calories than you consume. What that means is that operating at a low heart rate (i.e. fewer calories burned, even though fat is a primary energy source) will not lead to a reduction in body fat.

So the short version is:

  • The type of fuel burned (fat, carbohydrates, protein) has no bearing on weight loss.
  • Burning calories is required to lose weight (Law of Thermodynamics).
  • The way you burn calories is to get your heart rate up.

Make sense?

The fat burning zone is certainly misleading, but we all see why it can be appealing. Work out easier, burn more fat! Sounds too good to be true, and it turns out, it is. Don’t fall victim to myths and gimmicks!

My cardio recommendation is to do interval training, or high intensity interval training (HIIT). This consists of 20-30 minutes of interval cardio with your heart rate moving around in the 70-85% of your max heart rate range. I typically aims for  1-2 minutes at the higher intensity, followed by 2-5 min at a lower intensity, repeated. Make sure you hold the lower intensity long enough to get your heart back back into the lower heart rate range and maintain that for at least a minute.

HIIT cardio, done with sprints, adding incline or increasing intensity, is a quick hit tough workout that will burn! This is great when you are pressed for time and is also a serious calorie killer.

If you are a new exerciser, I would limit the duration and intensity of your cardio sessions and work up to these levels. Maybe even start with simple steady state cardio (maintaining your heart rate in the 65-70% of max heart rate range) as way to build up endurance.

Any path you choose for cardio, it’s just important to get your heart rate up. That’s it. Anything that accomplishes that goal, and is something you can sustain, is the best cardio for you.

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4 Comments to Know Better, Do Better: The Myth of the Fat Burning Zone
    • Megan
    • I guess the question I would have with that would then be what about when you exercise lots in a calorie deficit? At that point I would think the fat-burning zone could be beneficial because otherwise you may end up using muscle as your body’s fuel essentially slowing your metabolism.

      • Mary
      • Hi Megan, first, I would be worried about what you mean by “exercise lots in a calorie deficit.” You should be very cautious will calorie restriction during weight loss. My own personal experience is that if you cut back too much, it will have adverse effects on your rate of weight loss because it slows your metabolism, thereby lowering your basal metabolic rate (BMR), which means your body needs fewer and fewer calories to function. That makes ongoing calorie restriction that much harder. You should have a calorie deficit of no more than 500 calories a day below your BMR. (Google how to calculate.) Even in a calorie restricted environment, the same principles hold true regarding the fat burning zone. Here’s an example. Say you worked out for 30 minutes in the fat burning zone (50% of your max heart rate.) You would burn around 200 calories, of which about 60% could be from fat (120 calories.) If you exercised that same 30 minutes at a higher intensity, say 75% of your max heart rate, you could burn around 400 calories. While only 35% of those calories comes from fat, it still translates into more fat calories burned – 140 calories. It is a rare and extreme case where your body has no other fuel source but muscle to burn. I would make sure to focus on eating enough of the right fuel (carbs are great fuel!) to burn during exercise. Thanks for reading!