Monday, January 31, 2011

Kilojoules vs. calories for dietary intake?

Lots happening here: new bikes being built, new routes being ridden, massive snowfalls accumulating, etc. More on all that later.

Here's my immediate debate: caloric expenditure.

I'm using to count calories. I won't entertain arguments about that. It works for me. So that's my major premise: counting calories using an online food diary is a good way to slowly lose bodyfat.

Here's the minor premise: recording calories burned helps me determine how many to consume each day.

I have a good sense of my resting metabolic weight. But using my Garmin Edge 500 with my new Powertap is yielding some conflicting data. So, powertap-addicted numbers junkies, here's my question:

After a 75-minute easy spin (zones 1-2), the Garmin tells me that I've burned 647 calories-- but expended 755 kilojoules.  Which number should I use to calculate the ideal caloric consumption for that day? Sometimes, the calories burned is higher, especially if I'm really rested, fed, and hydrated. On the other hand, if I'm fatigued or slightly dehydrated, my heart rate is lower for the same effort, and hence the calories burned number is lower. But power is power, no? Shouldn't I record kilojoules expended, regardless of heart rate?

Thoughts from all are welcome: but don't tell me to stop doing the math or that powermeters are dumb.


  1. Here is my understanding, which is based on 1) reading a couple of exercise physiology and exercise nutrition textbooks and being friends with a guy who has a Phd in Exercise Physiology: 4 kilojoules is equal to 1 kilocalorie, which is what we commonly refer to as a "calorie." So 1 kilojoule is a quarter of a calorie. However, your body is only 20 to 25% efficient at converting energy into work, which means it takes 4 or 5 kilojoules of energy to produce 1 kilojoule of work. That means that to do 755 kilojoules of work on the bike, as recorded by your powertap, you have to burn 3,020 to 3,775 kilojoules. Divide that number by 4 (since 4 kilojoules are equal to one calorie) and your 755 kiljoule workout is between 755 to 943 calories.

    Ignore what the Garmin tells you about calories burned. It's junk information. Focus on the kilojoules from the Powertap. I usually multiply my kilojoules by 1.125 to figure calories. Or you can just use the straight kilojoules number.

  2. From Judd Van Sickle, my coach at UC Davis and Biomechanical Engineer at the UC Davis Sports Performance lab:

    "It's been my experience that the caloric estimation on the Garmin can be off as much as 75%. Typically, it overestimates, but apparently your Garmin is special. KJs is a measurement of actual work performed. And a precise measure of just However, in order to get to Calories, you do need to know your efficiency (and that is not even constant at different training loads) to accurately measure caloric expenditure. You can use VO2 data to estimate your efficiency, and then convert KJs to kcals with decent accuracy. Of course, you need to remember that a VO2 measurement takes into account basal metabolism and internal load, so you need to subtract out the baseline value, or use delta efficiency. Short answer: use KJs, but with a conversion factor to adjust for a generic efficiency and you will be much more accurate than the Garmin calorie counter."

  3. Yes, use Kilojoules. Like you pointed out, calories based off heart rate is dependent on to many factors. Even the equation that is used to figure out Calories burned can be different between manufactures. Check out the cycleops site here for more detail
    If you have any questions email me at

  4. EOB, I'm lazy. I just use the kj and mark that as my calories burned. Yeah, I can do math but its simpler just to look at it say it says 754 kj and say '750 cal' and move on.

  5. there are many tools to calculate calories and kilojoules. you can search it in google or just visit here