Paul, how come if I walk 14 miles with my friend my calorie burn is 1,455 whilst hers is 749? I am 9st 4lb (59.1kg) and she is 7st 12 (50kg). Our height is similar - and, yes, I need to shift 5lbs!
Well, there are a few parts to this.
Essentially calorie burn is a measure of how much energy is spent performing an activity. In the same way it takes more work to move a 20kg dumbbell than a 10kg dumbbell, calorie expenditure is proportional to body weight.
So, over the same 14-mile distance, the heavier of the two people would theoretically use more energy to move their body and therefore burn more calories.
Estimating calorie burn
There’s a protocol for estimating calorie burn based on activity that uses METs (Metabolic Equivalents).
Using this method, each activity is given a factor based on its intensity compared to sitting resting. For example, running might have a factor of 12.9. Essentially, this means you burn 12.9 times more energy running than you do sitting still.
An important step with using METs is that you multiply this factor by your body weight. Again, larger bodies require more energy.
However, we haven't taken fitness levels into account.
Do fitter people burn more or fewer calories?
Calorie burn is also proportional to heart rate. Higher intensity activities burn more calories.
Fitter people can achieve the same work with less effort.
Their heart rate won’t go as high. They’ll use less energy (burning fewer calories) performing the activity.
Essentially, that’s what fitness is. Making your body more efficient at performing tasks.
The more efficient (i.e. fitter) your body becomes, the fewer calories it burns completing the same task.
There’s also the problem of measuring calorie expenditure and accuracy.
Measuring calorie burn accurately isn't simple and straightforward
The only truly accurate way to know exactly how many calories you burn is to spend some time inside a sealed metabolic chamber that measures gas exchange.
Outside of the lab, you can only really estimate calories burnt of you know your heart rate. The more accurate ways of measuring heart rate are with chest straps and heart rate monitors.
Activity trackers like Fitbit etc. that don’t have the chest strap to draw data from aren’t as accurate. They only give rough guides based on the information they’re given and how they are set up.
So there are a few factors at play.
It’s likely if you and your friend weren’t wearing chest straps and monitoring your heart rates then there’s some inaccuracy in the reporting.
Also, you may have been working at a higher intensity level (your friend may be fitter, or more accustomed to the route, etc).
As in the first point, for the same activity, heavier bodies generally burn more calories.
Hope that helps give a clearer idea of what’s going on.