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Heart Rate and Calories Burnt - How Do They Relate?

2010-05-24 6 min read marco

You’ve heard me chat about calorie counting, and you’ve heard that I use a heart rate monitor to figure out how many calories I burn on my workouts. You’ve heard my surprise at finding out that “starvation mode” is measurable in the heart rate – reducing caloric output during the day, forcing lower calorie consumption and the usual “plateau” effect in many starvation diets.

But why is there such a strong correlation between the heart rate and the calories consumed? I mean, sure: if your heart beats faster, it must mean that more energy is spent. But why is that the only variable that matters? Why doesn’t body temperature figure into this equation? Why not the air temperature, or the types of food you eat, or the kind of exercise?

On the other hand, why do you have to specify sex, age, and body weight when you set up your HRM? Why do you tell your Stairmaster or elliptical the same thing? How does the machine know what that means?

Let’s start with the most important part. Your heart pumps blood through your body for many reasons: it moves nutrients around, cleans up waste, pushes the immune system throughout. Imagine it like it’s the only gas station in town, and the blood vessels are the highways. UPS trucks, passenger cars, motorcycles, minivans: they all drive on the highway, but they need to stop at the gas station once in a while.

Now the heart is smart (or at least the thing that controls it): whenever more of something is required, but the concentration in the blood can’t be changed, the heart pumps faster. In most cases, the concentration can change; this is the case with most hormones, for instance, that are released into the blood at their own rate with very quick variations.

The one thing whose concentration can’t exceed a maximum is oxygen. Your red blood cells transport oxygen in the molecule called hemoglobin. Each hemoglobin molecule holds exactly one unit of oxygen bound, and if you need more oxygen, you have to bring more red blood cells to the destination. The only way to do that is to increase the flow of blood, which means the heart must pump faster.

Now the main reason that the body needs oxygen is to burn “stuff” with it. The bond between oxygen molecules is very strong, and if you break it up and tie the oxygen to something else, the extra bond energy is released to the body. The body uses this extra energy to cause other chemical reactions that are used to make you contract muscles, think, fall in love, and the like.

The thing is, oxygen is not only one ingredient of energy for the body; it’s pretty much the only one. When your body decides to burn carbohydrates or fats, it uses oxygen. When your body decides to build proteins and bones, it need energy to connect the long chains of aminoacids or to bind the calcium carbonates, and that energy is bought with oxygen. Sugar won’t make you build muscle, but without additional energy, the body is unable to take the protein you eat and make it into abs and biceps.

Now you see the logic: everything depends on how much oxygen you burn. The more oxygen you use, the more calories you burn – you could easily think of oxygen as the real nutrient, only that we get that for free and don’t have a good way to track oxygen consumption. At the same time, the more oxygen you need, the more your heart needs to pump, because you have only this many red blood cells, which means only this many oxygen molecules in your blood at any given time.

So what happens when it’s cold around you? You need to burn more, but how does that have anything to do with oxygen? Well, your body needs to keep a constant temperature. To do so, it needs to burn calories, just like an electric furnace eats up current (and quite rapidly so). The only way for the body to burn calories is to consume oxygen, so it’s the same cycle again.

Ok, that’s all great, but why does the elliptical want to know your age, weight, and sex? Well, you see, the first point comes up here again: the heart needs to pump blood for a variety of reasons other than exercise. When it does so, it creates a foundation level of activity that can’t go away no matter how still you are. To that you have to add the amount of energy the body needs to keep constant temperature. Add to that the amount of energy the body needs to build, replace, and repair new cells. That adds up to quite a lot, and it depends quite a lot on your (you guessed it) age, sex, and weight.

Surely, there are other factors that are more important than the three mentioned. For instance, muscle burns a lot more calories than fat tissue, while bones really don’t burn much. If you knew how much of your body is made of muscle, how much of fat, and how much of bone and other inert “stuff,” you’d get a much better idea. But while everybody knows their age, sex, and approximate weight, not a lot of people know their body composition. So we use equivalence tables that are not quite 100% accurate, but good enough for the casual user. For instance, women have in average higher body fat percentages than men, which translates to lower calorie consumption at equal weight. Older people don’t need as much energy to build tissue, so they use less energy.

In the end, the logic works pretty well. The age, weight, and sex questions tell the machine where your burn rates go. That means the machine can tell how many extra calories you burn if you go, say, from 90 bpm to 120 bpm. For instance, I burn about 10 calories per minute at a heart rate of 140. That’s of course about 600 calories an hour.

Notice that those machines that don’t monitor your heart rate still tell you a number of calories burned. They assume you are the average person, however that average person is defined for that machine. Different machine assume different average people, as you can easily tell when you actually use a heart rate monitor and check how many calories you burned per calorie declared by the machine.

When you do so, you’ll notice something odd: virtually all machines show more calories burned than you actually did according to your expensive HRM. That’s because the machines want to be sold, and the machine that makes it seems easier to burn a lot of calories is the one that gets chosen the most. You see that in those informercials that promise you will be able to burn twice the calories with a particular product than with all other exercise forms.

So, the machine that says you burn 2 calories when you really burned 1 makes it seem really easy to burn calories, and people will flock to it. Of course, the machine isn’t really cheating: it’s just assuming you are a person that burns really fast, which means it will assume you are a really young and heavy man, say an NCAA line backer.

Trying out a bunch of machines, I ended up with a declared to burned calorie ratio of between 1.5 and 1.2 – that is, for every calorie I actually burned, the machines told me I had burned somewhere between 1.5 and 1.2 calories. Remember that when you work out without an HRM!