Daniel4

I took a 1.5hr, 26.6km ride today. When I got home I looked at my app and noticed it said the ride was about 322kcal.

I really don't think that ride was enough to burn 571 Big Macs at 563 calories each.

What is the conversion from exercise kcal to food calories and why do they sound like they use the same unit of measure? If everything was standardized to Joules, it would all make more sense.

(322 kcal over 1.5 hr, 26.6 km is 12 cal per metre, 3578 cal/min)

chaadster

1kcal is the same as 1 Calorie (aka “food calorie”). There are also calories, lower case (aka energy), 1,000 of which get shorthanded to 1kcal.

I don’t know why the “food calorie” thing got going, but I’ve just always learned it’s like a lazy (or layperson’s) way of saying kilocalorie.

So unfortunately, your ride was not even enough to negate a BigMac!

Daniel4

Thanks. Sounds like the same kind of laziness that drops the suffix "gram" from kilogram.

Polaris OBark

It is a peculiarity of American packaged food.

The same box of sugar-coated Capt'n Krunch says kcal in UK, for example.

Darth Lefty 

In my job, we use a lot of values that are in calories because a lot of the test data was made in the 1950s using bomb calorimeters. But a calorie is not an SI unit. It’s OK though because we wind up converting it to BTUs.

Daniel4

I just Googled " definition of calorie" and got this:
"

• 1.
  a unit of energy equivalent to the heat energy needed to raise the temperature of 1 gram of water by 1 °C (now often defined as equal to 4.1868 joules).
• 2.
  a unit of energy, often used to express the nutritional value of foods, equivalent to the heat energy needed to raise the temperature of 1 kilogram of water by 1 °C, and equal to one thousand small calories; a kilocalorie."

It may not be an SI unit but it's based on SI units, gms and degC.

79pmooney

The Calories and kcals had a battle to the death. A draw. Final score - 1:1.

Now, for real fun you have to go to England and watch the BTUs battle it out with the horsepower-hours (refereed by the Joules).

Polaris OBark

Fun fact: Joule is pronounced "Jowel", according to Joule, who claimed "Jewel" was the French bastardization of a perfectly good English name.

Iride01 

In the USA for some reason we started using Calorie with a capital "C" to denote kilocalories for figuring in nutrition related things. Most of the rest of the world uses kilocalories for nutritional type information.

A calorie with a small 'c' is just a typical calorie used in science things. Not everyone uses the big "C" though so it's always something that might need to be ask about to make sure what kind of calorie is being discussed.

MinnMan

BTUs? You jest? The only place I've seen BTUs in the last half-century is in the US HVAC world.
I do remember my Jr. High School intro to physics, where we did everything in foot-pounds. And Dynes. Sheesh.

Let's go further back then. Pictured here is the Nilometer- a 9th century well in Cairo used to measure the flood level of the Nile. Those markings on the column are in cubits.

flangehead

The US-based petroleum industry uses BTU. And pound-moles.

MinnMan

srsly?

And somehow they still manage to make billions of dollars.

Darth Lefty 

I wish. FORTRAN 4 too

pdlamb

As one grizzled old veteran told me, a good FORTRAN program can program in any language!

mr_pedro

These are just 2 units to measure energy and 1 calorie is about 4.2 Joule like you found above.
With the added note as mentioned in above posts, that food labels and when we normally talking about food calories, refer to k calories, which is 1,000 calories.

Now here are a few other interesting things to know:

• The Watt is a unit of power and is equal to Joule per second. This allows you to translate workouts to calories burned.
• If you do a 1 hour bike ride (3600 s) at 100 W average power, that means you did 360,000 J of work. Or in other words, 360 kJ of work.
• To get you to calories burned you would divide by 4.2, but you also need to consider that your body is ~ 20%-25% efficient. Meaning that you need to eat 4-5 x more energy than the work that is produced. Putting this together means that you can take the work done in kJ as the amount of food calories that you need to eat to compensate.
• So 1 hour at 100W gives 360 kJ of work and also about 360 Calories that you would need to eat to compensate.

Also, when doing these calculations to compute how much you can/should eat, you should be aware of possible large uncertainties in:
- Average power
- Efficiency of body
- Actual Calories in food (accuracy food labels)
- Amount of Calories that digestion is able to extract from food

This means that you are better off using the calculations as a starter and then use your appetite as a guide to feel if you are at maintenance, above or below and adjust if necessary.

flangehead

Yes. Dollars, not bitcoin.

himespau 

This whole having to say "food calories" every time I talk about energy storage with my students (for example comparing /g the energy stored in carbohydrates vs. lipids) is really annoying, but it's the only way to get it across to them. When I talk kcal or joules, it just doesn't sink in the way talking in terms of things they think about when dieting does.

MinnMan

Yeah. And when I teach Intro Earth Science and talk about temperature (for example, the putative 1.5 °C "tipping point"), I always have to point out the Fahrenheit equivalent.

himespau 

And then there's the bit about having to talk about Kelvin so they don't think the temperature (well thermal energy anyway) doubles when you go from 20C to 40C (I still have to talk to relatives about how grilling doesn't take any longer/more energy when you do it in the winter than the summer because a change of 10K is not a big deal when the inside is 700K.

pdlamb

True, but I think there's some benefit to using "human-size" units. You know, like a man's knuckle is about 1", my shoe is about a foot long, 60 mph is a decent highway speed that takes you a mile to go a minute. Let's be honest, we weigh things instead of measuring their mass, even if our measuring devices translate weight into mass. And a pint's a pound, etc.

Of course, I make sure everything at work is metric.

MinnMan

You mean, 10,000 km defined (originally) as the distance from the equator to the pole doesn't quite resonate for you?

pdlamb

Too far to count the clicks on my mechanical odometer!

Daniel4

I thought my original question had been settled.

MinnMan

Welcome to BF