"Calories burned" vs Work (kJ)

Trux, thanks for your replies…it’s all very interesting. So, are you saying Garmin’s new way of giving total activity calories (to include the ‘resting’ amount plus work done) correct or not. I’m confused (doesn’t take a lot). The total calories it quotes for an activity when using a power meter & heart rate monitor, should I ignore that & concentrate on the work equivalent of calories.

I’m interested because I have 2 x 280km rides to do in June & I’m keen to try & get my nutrition right. With 11+hrs in the saddle, 80 calories or so per hr could make a difference between bonking & getting it right. I get the cal/kcal bit, I think we’re all a bit lazy in that sense & always just say calories.

cheers 

dougiereid said:

So, are you saying Garmin’s new way of giving total activity calories (to include the ‘resting’ amount plus work done) correct or not.

It is not a new way. It was always like that. Calories burned during an Activity were always (or minimally all those years that I own a Garmin watch) equal to RMR * time + Active Calories. The principal factors determining the resulting Calories are your RMR, the duration of the activity, also the Activity Level (set in your User Profile), and then especially the Heart Rate and HR Variability during the given activity. The reading from the power meter has only marginal effect, serving more or less just to determine your lactate threshold, and hence the difference in the metabolism between the aerobic and anaerobic modes.

You should adjust your nutrition (the daily Calories ingestion) to the total Calories expenditure, it means the RMR + Active Calories. However, there is such a huge margin of error on both ends, that it is way too naive to think you can get it right, just reading the numbers. Use them rather just for the orientation, and for seeing the tendencies and evolution, as you progress, or when you change something in your training, or in your nutrition. And of course, listen more to your body, than to the watch or to nutrition charts.

dougiereid said:

should I ignore that & concentrate on the work equivalent of calories.

I would go with the mechanical work equivalent (that is, the kJ reading instead of kcal) if you have a power meter. 

trux said:

It is not a new way. It was always like that. Calories burned during an Activity were always (or minimally all those years that I own a Garmin watch) equal to RMR * time + Active Calories.

I don't think so, at least for cycling-specific Garmin Edge models. I've owned several Garmins and different power meters since 2012 and noticed this discrepancy for the first time only 9 months ago. Earlier the difference between kJ and kcal was on the order of a couple kcal. 

Yep, it was confirmed that Garmin only recently changed this. In fact my Fenix 5x still only converts the work done to calories & ignores the resting stuff. Every ride I’ve done with the 5x, calories = work. Now with the 530, it’s plus the extra ‘stuff’. I only questioned it on here because I noticed the discrepancy. 

Will def use the figures with caution but both triathlons I’ve done, the second one where I took in approx 450cals per hour, I felt much better. The 1st one I just cuffed & had a few bars etc. thanks all for the input

Leopejo said:

traditional 1 kcal = 1 kJ. 

what do you mean traditional ?

according to traditional physics it's : 1kcal=4.184kJ

your training work - in kJ from power meter is useful to track volume of your training, not nutritions. Probably you could correlate both and find your own formula, but most of the time, when you do harder ride, you are always in calories deficit (you burn more than you can chew)

I know my physics, thank you. But I know my physiology too. The human machine, at least when pedalling, requires to burn about 4-5 times as much stored energy as the mechanical work it produces. In other words, the human mechanical efficiency is around 20-25 %. Therefore since the invention of power meters, people have been using the 1 (burned) kcal = 1 kJ of mechanical work. 

It is nice to have an estimate of burned calories during the activity to compare with whole day calories. 

ok, it works accidentally like this for biking (1kcal of burned energy to 1kJ of output energy, but don't use = sign).