*Originally published in my first blog, Exercise Navigator.
I was recently asked about the levels, or zones, that you see on the machines at the gym. For example, if you’re at x heart rate, then you are in your “fat burning zone”. What does that mean?
First, let’s remember that heart rate is tied to oxygen consumption. We discussed that in the post about target heart rate. Oxygen consumption can be measured through spirometry, which is that apparatus that is used in experiments that measures your breathing and the mix of gases that are expired. From that, we can calculate how much of the fuel that the body burns is coming from different sources of energy because we know the chemical equations for the reactions that take place. We burn mostly carbohydrates and fats when we exercise. Sometimes we burn protein, but if we’re adequately fed we shouldn’t burn too much.
Scientists talk about a “crossover effect” where the majority of fuel being burned crosses over from the body preferentially burning fat at less than about 65% of maximum capacity to it preferring carbohydrate at above 65% of maximum capacity (Brooks and Mercier, 1994).
You may be inclined to look at this and think, “I want to burn fat; I’ll stay at less than 65% of capacity.” I would caution against this, however. The key here is that we are talking about percentages of fuel burned. So if you spend 60 minutes at 20% of your max and burn 100 calories, then 50% of those, or 50 calories, are from fat. You may think that’s great! However, if you spend 60 minutes at 70% of your max and you burn 500 calories, then 30% of those, or 150 calories, are from fat. So you are burning more actual fat if you are at the higher percent for your 60 minute workout. As we discussed last time, up to 82% of max is probably achievable for most healthy people with the talk test.
So, the bottom line is that the “zones” on the cardio machines are just a big trick! Keep your heart rate in that comfortable “talking zone” and use it to track your progress as mentioned last time and you will be off to a great start!
Here’s to happy and smart training!