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Can you trust the calorie counts on exercise machines?

August 23rd, 2011

After an interesting e-mail conversation with Dr. Yoni Freedhoff of the pull-no-punches Weighty Matters blog, I ended up writing a guest post about some of the factors you need to think about if you ever look at the calorie number that your treadmill (or elliptical or exercise bike or whatever) spits out. Without giving away too much, I’ll say this: if you’re not considering the difference between gross and net calorie burn, you’re kidding yourself! READ THE FULL POST HERE.

  1. Brian
    August 23rd, 2011 at 13:25 | #1

    I’m a bit confused by the study posted in connection with the idea that people with a higher body fat burn less calories overall. (http://www.ncbi.nlm.nih.gov/pubmed/19086187) The results and conclusion from the study seems to indicate the opposite (according to my interpretation) – that heavier individuals (higher body fat individuals) expend more energy in all weight bearing activities.

  2. alex
    August 24th, 2011 at 11:50 | #2

    @Brian
    Heavier people definitely burn more total calories than lighter people doing the same task — but we’re interested in how their calorie burn compares to the prediction from exercise machines, not how they compare to each other. For two people of equal weight, the person with more body fat will typically burn fewer calories while doing the same activity. In the abstract you link to, it concludes that oxygen cost (and thus energy expenditure) is roughly the same when expressed relative to “fat-free mass”. In other words, calorie burn is basically a function of how much muscle you have.

    I realize this is a little convoluted — maybe an example will make thing more clear. Consider someone who is 6 feet tall and weighs 70 kg pounds. They’re running on the treadmill at 4 mph, which — according to the published data used by treadmill makers — burns about 6 calories per kg of body weight per hour for the average person. That means that after an hour, they’ve burned 6 x 75 = 450 calories.

    Now consider another person who is 5 feet tall and also weighs 70 kg, and also jogs on the treadmill at 4 mph. According to the treadmill, they also burn 450 calories in an hour.

    Here’s the problem: the short 70 kg person has far more body fat than the tall 70 kg person (that’s why they’re the same weight). That means that the tall person has more fat-free mass. But the study you linked to tells us energy expenditure is roughly the same for the people with the same fat-free mass, so the short and tall person can’t have burned the same number of calories. What actually happens is that, since the short person has more body fat (and thus less fat-free mass) than the “average” 70kg person, he or she burns fewer calories than the “average” value displayed by the treadmill, while the tall thin person actually burns more calories than the treadmill value. Hope that makes a bit of sense — but if not, please feel free to ask for more clarification!

  3. Brian
    August 24th, 2011 at 18:52 | #3

    Alex, thanks for the clarification. It does make more sense to me now!

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