kJ vs Calories

I was looking at some of my activities on Strava and Gamin Connect and noticed something strange, they both seem to equate work done (kJ) to Calories.

In Gamin Connect for a 105km road ride, it has Work = 2343kJ, and Calories = 2342 C.
In Strava it has Total Work = 2346kJ. Their glossary says Total Work is:

Total Work, expressed in kilojoules (kJ), is simply the sum of the watts generated during your ride. There is a close 1–to–1 ratio with Total Work and Calories expended during a ride.

But, if you take a look at any online calculator, they say 2343kJ = 559.6 Calories, not a 1-1 relationship, and all food packaging shows the same, a difference in kJ and kCal.

Plus, there is no way I could have and extra 2300Calories after that ride, that does not work.
Any ideas why they are showing as the same on Garmin and Strava?

I’m sure someone will be along shortly with a full and proper explanation but I seem to remember it being slightly different to calories of food.

It’s to do with energy expenditure rather than burning calories or something like that! I THINK it actually works out as 1kj = 3 calories when it comes to food calories whereas energy is energy!

I could be spouting a load of old bollox though

It’s because your body is pretty ineficient at actually doing work. 1 Calorie of food produces about 1kJ of work at the rear wheel.

Or if you measured food in kJ’s, you’d need to eat more kJ’s than your body actually put out.

The rest is wasted as heat (unless you need heat, in which case the body can convert it directly to heat, or make you shiver which is just wasting energy to generate heat).

1Kcal = 4.2 kJ.

Note that Kcal and Cal (capital C) are used interchangeably, mostly because no-one other than nutritionists uses calories anymore so the unit calorie (230,000th of a mars bar) is redundant.

There’s a difference between calories and Calories, not just the capitalisation. The energy in food is measured in Kilo-calories but everyone just said “calories” so “Calories” began to be used as a means of shorthand for Kcal.

Joules are not the same as calories but there is a linear relationship: 1Kcal (or 1 Calorie) is 4.2Kj (approx). Now since the human body is very roughly 25% efficient in converting energy in (Calories) to work done (KiloJoules) then you can divide the Calories by 4 to get a rough idea of the work done in kJ

so when myfitnesspal guesstimates my hours exercise as 500 cal.
does that mean i can eat an extra 2000 cal and ineffectively process 500 cals out of it
or
does it actually mean that i have done 125 extra cals of effort and can therefore eat a further 500 cals of average food.

No it means you’ve burnt 500calories of food.

The only things that report energy output (AFAIK) in kJ are power meters (and devices that are trying to infer power in some way, the garmin probably uses an equation based on speed x estimated drag coefficient + energy used gaining elevation to work out kJ’s, then uses the 1:1 conversion to Cal’s.

Rule of thumb would be ~100 calories/hour – sedate stuff, 500+ is exercise. And that’s food calories.

Also you’ve got to subtract the calories you would have burnt anyway from the 500 if you’re going to treat it as extra. So 500-100 = 400 extra calories, and your ‘normal’ would have to be adjusted to about 1400/day (the amount you’d burn staying in bed, doing a desk job, watching TV etc).

I would say that it’s the latter: 125 kJ of work equating to 500 Calories food. Most people would be hard pushed to do 500 kJ/kCal of work in an hour, 6kCal/min is hard effort.

You have to take your base metabolic rate off the figure – you didn’t stop breathing, thinking, etc for the hour’s exercise!

I would say that it’s the latter: 125 kJ of work equating to 500 Calories food.

No 500kJ of work = 500 Calories of food (but 125kcal of work).

There’s me getting c & C mixed up 😳

^^^^^^ this….

I kind of knew what I wanted to say but whitestone and thisisnotaspoon have pretty much summed up exactly what I’ve been told by those that know!

I took it all in, absorbed it all then forgot about it haha

Ok so 1kCal = 4.2kJ

Strava will be either taking your kJ from a power meter (1 watt = 1 Joule per Sec) or their power estimation algorithms (no idea how these work). The kJ is the amount of energy that you put into your bike. Say you did 300W for 100s – you’ve done 30 000 joules of work – 30kJ.

Most good estimates put the human body at about 25% efficient so you had to actually burn 120 kJ of energy to do that 30kJ work. Handily, since 1kJ is 25% of 1kCal, you can just approximate energy burned (kCal) = work done (kJ).

Okay, understood.

Unfortunately that means my diet is worse than I thought and now need to eat even less, or just ride more, which is difficult at the moment with the bad weather.

TBH, I wouldn’t worry too much, if you keep yourself hydrated and weigh yourself regularly and see that you neither lose nor gain weight then your dietry input is about right in terms of energy.

It might be that you are eating cr*p but that’s another matter!