Calorie Conversion

[mammoth76]

My physics book says that one Kcal is equal to 4.187 Kjoule, but in the iBike program one Kcal equals .956 Kjoule. What is the reason for the apparent discrepancy?

[coachboyd]

The average body runs at about 25% efficiency. So, for tracking calories, it's approximately a 1.1 to 1 conversion.

Kilojoules is a measurement of energy (what you expended)
Calories if a measurement of fuel (amount you need to replace what you expended)

[mammoth76]

Thanks for the explanation. So kcal tells me how many Big Macs I need to eat to avoid losing weight and is just kjoules adjusted by an effeciency factor which is assumed to be 25%. That made me curious since it would seem that different activities would vary widely in effeciency, so I used Google and found that effeciency varies from a low of 3% for shoveling up to 30% for walking up a 5% grade. Bicycling was indeed listed at 25% though I imagine it varies depending on posture, cadence, temperature and other factors. Also, I found a paper that compared the effeciency in running of trained runners to that of inactive people and, counter to what I would have thought, the inactive people were 27.6% effecient while the runners were only 20.7% effecient. Which means that a trained runner will consume 33% more calories than the same weight couch potato who runs the same distance. So I would assume that the kcal figure iBike gives me is just a rough guideline and I better not depend on it for planning the amount of food I need for a long ride. Sorry if I am rambling, but your answer peaked my interest.

[coachboyd]

The only way to truly know what YOUR body burns at exercise is to do a VO2 max test. This will give you calories per minute in each zone along with how many of those calories come from glycogen or from stored fat. VO2 max tests are great for building a training plan.

If you are looking to count calories, and get a good idea how much food you need to replace depending on your workouts, the Kcal number should be close enough for that. It's not going to be exact down to the calorie, but you really don't need to be. It's not like any of us are individually weighing each ingredient of all our food every day. The more trained you become the more efficient you become, but the 25% is still a good general guideline to follow.

Tracking calories based on watts becomes a lot more accurate because it's based on the workload that you are actually doing. There is a math formula for converting watts over time to Kilojoules.