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Where I try to explain that “calorie” readout on cardio machines

by John on April 30th, 2013

I’ve done quite well, with regards to my outward appearance, since the new year. And with this comes a lot of questions about working out & what people should do. Once we get past the obligatory “how did you do it?” question, I seem to field a lot of “what machine will let me burn the most calories?” questions. Simply, the harder you work, the more calories you burn . . . the number that a given machine might display doesn’t really mean a whole lot. But the geek in me wants to explain just what’s going on.

So, first off, what does it mean to burn a calorie (for my own sake, I’m going to use kcal for “calorie,” just because it’s less typing – but what you see as “1 calorie” in your Diet Coke is 1kcal for these mathematical formulas)? When we refer to “burning a calorie,” we are forcing our body to consume oxygen in order to allow the body to do whatever the heck it was trying to do. Even a patient in a coma is going to “burn calories,” because it takes energy to keep your lungs breathing and your heart pumping. The bigger you are, the more energy (consumed in the form of food) it takes to just keep your body going.

If you want to gain weight, you need to take in more kcals than you expend. If you want to lose weight, you need to expend more kcals than you take in, as you need your body to use its energy stores to convert that oxygen.

It gets tricky, though, in determining just how many calories you need. If you log into a site like Fooducate or My Fitness Pal, they’ll ask about your current weight, and your current height, and your activity level (working out should never factor into that activity level if you’re logging workouts — if you’re like me where you work out more than most people, but have a job where you find yourself still most all of the time, choose the option that has the least activity associated with it) and come up with some number. This is a rough-approximation of the number of calories needed to keep you right where you are. Why is this a rough approximation? Because the fitter you are, the more calories it takes to keep you running. Easily, this is the biggest benefit to why you should be working out. It’s why you should take up running, or cycling, or tennis. It’s why you should be lifting weights. The more muscle you have, the more your body needs just to maintain its current state.

So, why does that calorie readout mean nothing? Well, it has to make a lot of assumptions about you. The only way to, truly & accurately, measure how many calories you burn, during a given workout, is to work out, every time, with complex machinery to determine just how much oxygen you’re consuming. And that’s not really feasible (unless your name is Ivan Drago). But, we can try to use our heart rate to determine just what is going on (you’ll have to forgive me for using my own metrics for these variables in what follows, but those are the numbers that are most familiar to me).

The first thing we need to determine is our VO2 max – or the maximum amount of oxygen that our body can consume in time. The Uth-Sørensen-Overgaard-Pedersen formula tells us that we can approximate VO2 max by taking our maximum heart rate (mine is 186) divided by our resting heart rate (mine was 45, taken when I last donated blood), times 15. For me: (186/45)*15 = 62mL/kg/min. So, for every kilogram of mass I have, I consume 62 milliliters of oxygen per minute, when I, simply, can’t go any harder. And, the reason I’m writing this post now is, well, I weigh, almost precisely, 100kg, so it makes the math a bit easier. I consume 6.2 liters of oxygen, per minute, when I risk losing control of my body functions by going any harder.

Obviously, when we work out, we don’t work out “as hard as we can go,” but we can use our heart rate to determine just what percentage of that “balls to the wall” effort we’re using. If I’m running 9 minute miles, I’ve measured my heart rate around 135 beats per minute, which is approximately 73% of my maximum heart rate . . . does this mean that I’m consuming 73% of the oxygen that I’d be consuming at my maximum effort? No. First off, it’s not a linear formula, but that’s missing a bigger, but easily corrected factor. We need to take my resting heart rate into account – my heart is going to beat, whether I’m running or not, so we need to take the change in what my heart is used to doing. (135-45)/(186-45) = 63%. Still, this formula isn’t linear, but this is close enough to work with, because this will always be an approximation if I’m not working out in the lab. If my heart rate is running at 135 beats per minute, I can say that I’m consuming 63% of the maximum of the 6.2 liters of oxygen per minute that I can consume, or 3.95 liters of oxygen per minute.

If we accept that, for every liter of oxygen consumed, you “burn” 5kcals1, I can say that I’m burning 19.75 kcal per minute when I run at nine-minutes per mile.

If my heart rate goes faster? I burn more calories per minute. If my heart rate drops? I burn fewer calories per minute. And this is just as true for me as it is for anyone else. If you get on the elliptical machine but your heart rate doesn’t really elevate, it doesn’t matter what that readout says. You know how hard you’re working. Traditionally, I “trust” that calorie readout only if I’m very used to that specific machine, and I was too busy catching up with Words-With-Friends to keep careful measure of my heart rate as I went — if the calorie readout tells me more than I was expecting, I throw the number away. There is no “special machine” that is better, or worse for you. If your heart gets pumping, you’re good.

On this whole topic, though, I’d like to point out the “fat burn zone.” Many machines list this — if you’re serious about your health, and you’re used to working out, ignore that zone. Obviously, if you’re just starting, you need to be careful, and whatever you can do to keep yourself from passing-out/puking/peeing-yourself (if you’re new to working out, you may have a very limited VO2 max and your body, simply, doesn’t know how to respond to your requests for the intake of a huge amount of oxygen, so it will push back by doing things that will make you stop; but, if you’re new to working out, be prepared for significant increases in fitness, quickly). The “fat burn zone” is a guess of the heart-rate where the maximum number of calories are pulled from fat stores, at that time, in order to keep the body going. The problem with staying in that zone, just because it’s “the fat burn zone,” is because, when you’re done with your workout, your body balances itself out. At the end of the day, the only numbers that matter, for weight loss/gain, are the number of calories you ingest and the number of calories you burn. Ignore the “Fat Burn Zone” and go above it, if you’re serious about burning fat.

Of course, to further complicate things, as your general fitness level improves, your maximum heart rate may increase, and your resting heart rate will certainly fall . . . a measure of VO2 max, to be truly accurate, would need to be calibrated for every work out . . . and, well, after writing all of this out, I’m exhausted — and that hasn’t even taken a workout into consideration. I’m not about to do that. I just do a sanity check whenever I see a “calories burned” number, and remember that it’s all an approximation.


From → Keeping Active

2 Comments
  1. Hubman permalink

    You’re close, but the estimates are likely high. Not bad for a computer whiz/musician though!

    I’ve had my VO2max tested in the lab on many occasions over the years and it’s NEVER been higher than 50 ml/kg/min. You and I have ridden together, I have to think that your estimate of 62 is too high, I can’t imagine that you’re that much more fit than I am.

    Rather than using resting heartrate as part of the estimate, it’s simpler to use the anaerobic threshold as an estimate of the highest sustainable rate of energy expenditure. For most individuals, it’s somewhere around ~70% of VO2max. So for my max of 50, my max sustainable workrate is 35 ml/kg/min. I weight about 90kg, so a little over 3 L O2/min. That’s just over 15 kcals per minute.

    For you and I, I think that’s a much more reasonable estimate of max energy expenditure during exercise. Anything higher than that just isn’t that plausible.

    The danger with those kcal estimates is when people place too much faith in them and think they can eat that much more. I’ve learned to only allow myself to replace ~70% of the kcals that MyFitnessPal says I burn during exercise.

    Random trivia- do you know the technical definition of a calorie? It’s actually a unit of heat, specifically the amount of heat required to raise the temperature of 1 liter of water by 1 degree C.

  2. I have a very hard time believing that I’m anywhere close to as fit as you – but, I did have my VO2max measured in a lab when I was nowhere near as fit as I am now (this was about a year before I was married, when I had been working out for about 3 months, but was a good 60 pounds heavier than I am right now), and I rated 42 ml/kg/min. From the time we rode together, my resting heart rate has dropped from 62 to 45, however. 62 might be too high, but I have to think that 50 is too low.

    My issue with the anaerobic threshold is the length of time that I’m out. If I run 8:00-miles, I can keep myself going for about an hour, but I need a few minutes, when I’m done, before I can get myself going again. If I run 9:00-miles, I can keep myself going for much longer (2+ hours) with no recovery time . . . so that’s typically where I plan to go. But my heart rate is markedly different between those efforts – 135-140 for a 9:00/mile run, 160-172 for an 8:00/mile run. I’d argue that the latter is pretty close to the 70% threshold – perhaps just a tiny bit above it.

    And I did know the definition of a calorie – though I didn’t include it here because, well, the concept of energy brings me into my theoretical physics class from college, and there are just demonstrations that I don’t want to re-live 🙂

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