Originally Posted by ofajen

As a reminder, dietary calories are measured in units of energy. The dietary “calorie” is actually a kilocalorie for use in comparison with SI units of work, such as joules or kilojoules. one calorie = 4.184 Joules so one dietary calorie = one kilocalorie = 4.184 kJ.

As you will see in examples below, riders tend to be about 25% efficient at turning dietary calories consumed into useful pedaling work which would be measured again in units of energy. Most of the rest is ultimately dissipated as heat though there is also internal work done just moving the mass of legs, pedals and cranks at a particular cadence.

Power meters measure the instantaneous rate at which external work is being done on the pedals and is measured in Watts= Joules/sec.

For those interested, that internal power load and resulting internal work is strongly dependent on cadence, so at a given speed on the road (and external power load) the total load your body must power will be a higher level that is an increasing function of cadence.



So I presume that’s looking at pedaling at 200W for one hour. Total external work would be 200 J/ sec times 3600 sec = 720,000 J or 720 kJ. At 25% efficiency, that requires 4 x 720 kJ = 2,880 kJ of dietary energy consumed which is 2.880 kJ/ 4.184 kJ/kcal = 688 kcal = 688 dietary calories. The 25% figure is rough and varies a bit with individuals so don’t read more than two figures at most. Figure 700 dietary calories roughly.



Power is actually the rate at which work is being done, measured in watts or kW. Coincidentally, the factor of 4 from the 25% metabolic efficiency roughly cancels the 4.184 conversion factor of dietary calories or kcal to kJ. So, yes, one kJ of cycling work is roughly one dietary calorie consumed.

Probably more than folks wanted to see but there it is for folks who might wonder.

Otto