If you wear a fitness monitor like a Microsoft Band 2, a Fitbit HR or a Jawbone UP3, then one of the key metrics that you’ll be monitoring is your heart rate.
The belief is that by monitoring our heart rate, we can measure our exertion and therefore identify whether you’re training in the ‘fat burning zone’ or whether we’re ‘anaerobic’. At the same time, seeing how exercise affects our heart rate can help us to roughly estimate our fitness levels and our aerobic health.
Most importantly though, many of us are interested in our heart rate as an indicator of our calorie burn. And this is something that is very much encouraged by these fitness trackers. When wearing a fitness tracker, you’ll be told that you burned X amount of calories over a certain amount of time and this number will be the calculated largely based on your heart rate throughout the day – along with a few other details like your weight and your activity levels.
But is this view a little simplistic? Can you really say that heart rate = calories burned?
What’s the Connection?
There is obviously a fair amount of truth in this connection and if there wasn’t, then it wouldn’t be such a widely held belief (actually scratch that – plenty of widely held beliefs are entirely based on misinformation!).
The reason for the correlation between heart rate and fat burn is that the body requires oxygen to burn fat. Oxygen is highly reactive, hence why it is necessary to start fires. And it’s this reactivity that the body relies on in order to be able to burn fat. You breathe in, the oxygen enters your blood and then it travels to your fat stores. Once there, the oxygen then reacts with said fat and burns it to retrieve usable energy to power the muscles (this only occurs once the body has used up the glycogen stores in the muscle and blood sugar).
Of course oxygen is transported around the body via the red blood cells through the blood vessels and this transport is driven by the heart.
The faster the heart is beating, the faster it is transporting oxygen around the body. And the faster that your heart is transporting oxygen around the body, the more fat you are likely to be burning for energy and the more glucose you are likely to be burning from your blood stream.
This is why it’s so useful to calculate a caloric intake and your daily calories burned – because that way you know how much energy there is to burn in your blood before you will start burning fat stores. As long as you maintain a caloric deficit, any aerobic exercise will increase your fat loss.
Why It’s an Imperfect Connection
But just because your heart rate is a good predictor of your fat loss, you shouldn’t get too carried away and assume that it’s a perfect indicator of your fat loss.
Why? Because your heart rate can increase for a number of reasons and it won’t always lead to more oxygen being delivered to your muscles and your fat stores.
A good example is the use of an ‘oxygen restriction mask’. This is a type of mask that you can wear in order to limit the amount of oxygen you take in. Unfortunately, this is something of a fad that doesn’t quite do what it promises on the packet. That is to say, that it won’t actually make you more athletic or help you to burn more fat. It will increase your heart rate but that’s only because you have less oxygen in your blood and therefore you need to pump it around your body faster in order to get the same benefits.
Similarly, the strength of your heart as it pumps can also impact on heart rate without it impacting on the amount of fat you burn. As you get fitter, you can deliver more oxygen around your body while actually using fewer heartbeats – that’s because your heart has become stronger! This doesn’t then mean that you are burning fewer calories though.
Likewise, the air pressure, your blood pressure, your size and the way you’re breathing can all have a big impact on your heart rate without necessarily altering the amount of calories you’re burning. If you have a panic attack, then your breathing will become very rapid and shallow and your heart rate will increase at the same time. But that doesn’t mean that you can burn more calories by having anxiety attacks!
To conclude then, it’s certainly true that there is a strong correlation between your heart rate and the amount of calories you’re burning. If you train harder and this results in your heart rate becoming faster, then as a general rule that is likely to indicate that you’re burning more calories as well.
BUT this doesn’t necessarily always follow exactly and there are other factors to keep in mind. And that means that your fitness tracker isn’t 100% accurate (and especially seeing as the heart rate monitors tend not to be particularly efficient themselves!). Keep this in mind when training and make sure to pay attention to other indicators as well – such as your waistline!