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Burned by Calories Burned?

by Brad Hefta-Gaub, April 27, 2009

Tracking Calories

One reader writes…

Quick question……as I obsessively record numbers (time training, distances, weight, calories) how did you find out your actual calorie burn? Did you have some kind of testing done? I mean, my Polar gives me an estimate based on my settings, but, well, it’s never been validated. Any ideas?

Lots of people like to track distance, time, calories consumed, and all kinds of other data when they exercise. If you’re focused on maintaining, losing, or even gaining weight, it’s important to know how much energy you’re using during your exercise. Treadmills, elliptical trainers, and stationary bikes at the gym often have features that report how many calories are “burned” during a workout. But how accurate are these machines? Can a machine really tell you how many calories you burned?

Heart Rate Monitors

While heart rate monitors can give you really great information, they have their limits. Some models may have a fitness test that will estimate VO2 Max. You can then set your own personal zones based on this test. You also enter your age, and gender. However, devices like this cannot take into consideration the gas exchange or body composition. i.e. someone with more lean body mass will burn more calories than someone who weighs the same, but has more body fat. Muscle is metabolically active, fat is not.

Metabolic Testing

Metabolic Testing is the one precise way of determining your daily caloric rate. There are a couple local sports rehab/training clinic near where I live that do this test, along with a couple other tests like measuring your Lactate Threshold and even your VO2max for around $100-$200 depending on how many tests you want done.

These test will tell you how many calories a day you need at rest (no activity-base metabolic rate BMR), and working out at various levels. The tests, use equipment like EKGs, a Respiratory gas mask, power meters, etc; and measure the gas that you exhale in order to determine what chemical reactions are happening inside your body at certain activity levels. From this they can calculate how your body is actually consuming fuel and oxygen, etc. Very exact, very scientific, IMO, very cool!

Should I Get Tested?

If you are a serious competitive athlete, I would recommend doing these tests. I would like to do it at some point. It’s probably the most accurate way of measuring your metabolic rate. And understanding your precise metabolic rate would be a useful tool in better understanding your fueling needs for training and racing.

Can I estimate my BMR?

Yes, there are several formulas designed to estimate your base metabolic rate. Here is one formula for estimating BMR:

Harris-Benedict formula:

  • Men: BMR = 66 + (6.23 x “typical” pounds) + (12.7 x inches) - (6.8 x age)
  • Women: BMR = 655 + (4.35 x “typical” pounds) + (4.7 x inches) - (4.7 x age)

Note: The BMR for women is less dependent on height and weight, which is why the formula for women has a higher constant and which is why BMR is never less than about 1100 calories a day.

Take me as an example:

  • BMR for male, 37, 5′4″, 140lbs…
  • BMR = 66 + (6.23 x 140) + (12.7 x 64) - (6.8 x 37)
  • BMR = 66 + 872.2 + 812.8 - 251.6
  • BMR = 1499.4

There are other formulas for this estimate, that take different parameters like body fat, etc, and so various formulas come back with a base metabolic rate between 1450 and 1750 for me. In theory, this is how many calories my body needs to simply stay alive and maintain my body structure as is.

How does BMR Effect Calories Burned in a Workout?

These algorithms estimate your base metabolic rate, which is certainly useful in determining your daily caloric needs. However, most HRMs and Fitness Equipment will use the concept of METs on a per exercise basis to calculate how many calories you are burning while doing that exercise. METs are a ratio of calories used during an activity to your base metabolic rate.

METs

What are METs? According to Wikipedia:

A unit of metabolic equivalent, or MET, is defined as the number of calories consumed by an organism per minute in an activity relative to the Basal metabolic rate (BMR/RMR). A single unit (1 MET) is the caloric consumption of that organism, or individual, while at complete rest. For example, one might consider the restful state following a quiet night’s sleep as a good example of a single MET. This is a base-line unit for that one individual, and since each individual has a varying BMR, a MET is, therefore, variable from one person to the next. One might consider a single unit the energy required to just stay alive without doing anything more.

Researchers have created catalogs of METs for different activities like walking, running, swimming, cycling, playing soccer, ironing clothes, having sex, you name it. These catalogs are the subject of endless research papers. But they basically boil down to “activity, MET ratio”… so here are some examples I’ve found in some recent published papers…

  • bicycling-BMX/Mountain; 8.5 MET
  • bicycling->20mph, racing, not-drafting; 16.0 MET
  • stretching, hatha yoga; 2.5 MET
  • running 8mph(7:30min/mile); 13.5 MET
  • home activity, ironing; 2.3 MET

Calculating Calories from METs

In theory, the METs for an activity come from the concept of measuring the chemical reactions from those activities… and so you probably should use your BMR and the METs to determine your actual calories burned during an activity… right? Well, it sounds like most research just says to use the following formula for calories burned during an exercise.

Calories = METS x weight (kg) x time (hours)

Why? Well, I guess researches agree that since the MET tables were built using a constant that only included weight of the subjects they were measuring, and the BMR of those subjects may or may not have been utilized in creating the MET tables, you really aren’t getting any more accurate by using your BMR. Or are you? I guess this is the secret sauce that explains why Polar and Garmin and the treadmill at the gym are different. Maybe that’s the technology behind Polars “OwnCal” feature.

Theory vs. Practice

Have I lost you yet? Well, if you’re still reading, here’s what I’ve found in practice. I once heard a great quote, often attributed to Computer Scientist Randal L. Schwartz -

The difference between theory and practice in theory is much less than the difference between theory and practice in practice.

If I go out and ride my bike for 1:15 hours, let’s say I ride at 19.5mph, total distance 24.4 miles.

  • According to the MET calculations, I should burned: cals = 16.0 * 63.5kg * 1.25 = 1270 calories.
  • According to my Garmin, I burned: 1333 calories
  • According to my Polar, I burned: 1250 calories

Ok, so these are all way off from each other… where did the come from? How close to reality are they? Which is most accurate?

Based on my observations, I believe that Garmin uses a table close to METs. I am sure it doesn’t use Heart Rate at all, because I’ve used it in spin class, where my heart rate is high, but I’m not moving (according to the GPS) and it will read 0 calories burned.

I am pretty sure Polar uses Heart Rate and some proprietary algorithms based on heart rate. I say this because Polar definitely shows lower calories for me if I have a lower heart rate.

Using Calculated Calories as a Ticket for More Food

The example above was a typical one way commute for me when I would ride my bike to work in the spring/summer… and if I went out and ate an extra 2600 calories per day (assuming doing this commute twice a day), I know I would gain weight… but supposedly that’s what even the most conservative of these estimates claimed I was burning doing this exercise.

When I want to watch my caloric intake carefully, here’s what I do. I use these calorie calculations as an estimate. I take the most conservative one and then take off another 15-25%. Then I watch the trend over the long run (say 1-2weeks) and I see if the calorie deficit I am supposedly creating is really translating into lost weight. A 3500 calorie deficit should translate to 1 lbs of body weight. As I watched this over the long run, I would “adjust” my BMR and my fudge factor.

Now you can see why I’m interested in doing the metabolic rate test… it sounds like it would actually be easier.

Sources:

 

 

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2 Comments: :

Burned by Calories Burned?

April 27th, 2009 3:58 pm

Forrest Lumpkin says:

Your MET calculation seems to have an error. The “time” mulitiplier should be 1.25 hours not 2 hours. That would give 1270 calories NOT 2032 calories. The corrected value is between the Garmin and Polar values giving a spread over all three values of about 6% - not too bad in my opinion.

Now, just a few thoughts….
I would suspect that an MET of 16 is too high for you and this activity. Remember your body may be more efficient (requiring fewer calories of “fuel” to do the same amount of “work”) than the statistical averages upon which the MET estimate of 16 was based. (The testing you discuss would give some data to help solve the riddle especially if the metabolic rate is measured for an activity that is as close to what you plan to do daily.) Also, it is not clear upon what assumptions the MET of 16 is based. For example, does it assume a level or hilly terrain? Or really super hi tech bike or a bottom of the line Schwinn ten speed which is heavy and has lots of rolling resistance?? All of these variables would affect the MET measured for even the SAME individual.

The above explains some of the fact that you can’t ingest an “extra” 2600 calories per day w/o gaining weight. Also, my guess is that when you or not doing this activity, you are replacing it with some other physical activity which is burning substantial calories (remember even resting for that 1:15 hours would “burn” 1/16 of those calories — 80 calories). For example, you won’t be as “tired” if you drive to/from work instead of bike. So, you may go out and play tennis with the “extra” time (if any — depends on your local traffic of course) and “extra” energy.

Bottom line - thinking you have a “pass” to eat more based on estimates of calories burned during exercise (and they are estimates - usually optimistic on exercise equipment) misses the facts that they are estimates and you would be still be burning calories doing something else other than that exercise. Such thinking can fool you and can actually lead to weight gain which you do point out. One needs to keep monitoring one’s weight and adjust activity and caloric intake as needed in order to achieve or maintain a desired weight. With that in mind, the calories burned numbers are interesting and “possibly” useful if used with other information. This is what you are doing when “adjusting” the BMR and “fudge factors” based upon your actual weight lost over a 1-2 week time period. You are getting an idea of how much to adjust the activity level and caloric intake to achieve your weight goals. However, since everybody is different, the formulas that you derive would only apply to you and your circumstances (your age, the ratio of protein/fat/carbohydrates in those consumed calories, your current fitness level, the activity itself, etc.) For example, if you changed from biking to jogging at roughly the same perceived intensity, I would imagine you would need to come up with a new set of fudge factors since some different muscles are being used. Or, as you age, improve your fitness, or change your diet, the formulas would need to be adjusted as well.

Finally, a healthy diet depends upon more than just the number of calories consumed, of course. The “types” of calories are important. Man does not live by Snickers alone. ;-)

April 27th, 2009 4:03 pm

Brad Hefta-Gaub says:

Forrest, thanks for finding the error in the calculations. You are correct.

And, your comments about eating healthy and not considering exercise to be a ticket to eat more is great advice that everyone should understand.

Thanks for the feedback, we’ve corrected the math.

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