Calorie Counting with Perfesser Friar

Now that New Years is here, and the big Holiday pig-out is over, lots of us are making resolutions to lose weight.  And we’ll say it’s time to start  “counting calories”.    But how many of us really understand what a calorie actually is?

A calorie is  a unit of energy.   Energy is the ability to do work (applying a  force to move something).  Or the ability to generate heat.    With the exception of nuclear reactions (which I wont’ get into here) that’s pretty much how all the energy is used in this Universe.  Energy goes into work, or is dissipated as heat.

By the official definition (in the thermodynamic sense):    

1 calorie = the energy required to raise one gram of water by 1 degree Celsius.

And that’s actually NOT a lot of energy.   One gram of water is one cubic centimeter.  That’s smaller than the dice you play Monopoly with.   And a change of one degree Celsius is barely something you’d be able to notice by touch.

In fact, if you rubbed your hands together, there you go!  You probably burned several calories’ worth.  The warmth your palms generated could easily heat a dice-sized cube of water by several degrees.

Now, what about FOOD calories?  (The numbers you see in cookbooks or on the food packages).    Those calories are same thing, right?

Well, er…not exactly.

You see, a FOOD calorie is a thousand times larger than a thermodynamic calorie.

(Yeah, I know it’s confusing.)  But don’t blame me.  (Go yell at the science-geeks who made up these names!)

To get back to that definition:

1 food calorie = 1000 thermodynamic calories = 1 kilo-calorie (or 1 kcal).

In the more recent literature, they often talk about food in kilo-calories (kcals) to try to avoid this confusion.

Depending on their size,  a typical adult requires approximately 2000-2500 food calories (or kilo-calories) every day.   And of course, we get this energy from food, that our body burns and metabolizes.   But exactly how does this work?

Think of a log burning in the fireplace.    The log is plant material.  The complex molecules in the wood (cellulose, lignin, resins) react with the oxygen in the air and break down into simpler compounds (carbon dioxide, water, and waste (i.e. ashes).   This combustion process release energy.

The exact same thing happens when you eat an apple, or a piece of bread, or a Big Mac.   Our body converts the complex molecules of proteins, sugars and fats, and breaks them down into simpler molecules  (carbon dioxide, water and waste).  This is also a combustion process, and it also  that releases energy.

Except the food doesn’t burn quickly all at once in a burst of flames.   The process happens happens more slowly, through the biochemical reactions within our cells, spread out over several hours.   But it’s a similar reaction.  Food (i.e. plant and animal material) plus oxygen go in.    Heat, water, carbon dioxide and waste goes out.

In fact, that’s how they measure the caloric value of food. They actually BURN samples of food in specially-devised instruments called “calorimeters”  and measure the heat given off.   The laws of physics don’t care whether the food is burned in a fire place, a calorimeter, or in the mitochondria of our cells.    For a given chemical reactions, the net release of energy remains the same.   And our body takes this energy, and converts it into heat and work.

The heat part, we can easily relate to.   Hold your hand in front of your mouth when you exhale, and feel the warmth of each breath…that’s the heat of your body burning your food.  Anything alive has a metabolic rate and generates heat.

But what do we mean by”work”?

Work, from a physical sense, is defined as (Force x Distance).

Basically, work means pushing something (with a force) to make it move.   We do that all the time with our muscles, when we walk, run, open doors, play with our kids, open the cork on wine bottles.   In fact, right now, I’m using a bit of force with my fingers to make the keyboard move, so I’m doing “work”, in a physical sense.

Anyway, getting back to the 2500-odd kilo-calories you use up every day.   That’s equal to 2.5 million thermodynamic calories, which is quite an impressive number, when you come to think of it.   I wont’ bore you with the calculations, but if you do the math, that’s  enough energy to bring 4 liters of water (just under a gallon) from room temperature to the boiling point.

No way!”, some of you might say.   “Boiling a gallon of water?  That’s way too much!  Surely I don’t get all that energy, just from the food I eat?

Well,  “Yes way!”   You DO burn off that much.   There’s a lot of energy packed into the food you eat. (Have you ever seen a bacon grease fire?) .   Imagine half a pound of buring bacon grease…that’s approximately 2500 calories.  You could probably heat a lot of water with that.

But thankfully, the energy you burn is spread out over the whole day.   Lucky it doesn’t burn all at once, like the grease fire, our you’d have steam coming out of your ears.

Just think how much energy it takes to keep that big hunk of meat you call your body at a  temperature 98.6 F.   In fact, if you do nothing else but breathe and EXIST, it takes a minimum of about ~ 1200 kilo-calories to sustain life.   The remaining 800-1300 kilo-calories are used up moving around, and doing the things you call living.

Now, here’s an interesting fact:  each person gives off about as much energy as a 100-watt light bulb.

Makes sense, if you do that math.    2500 kilo-calories a day = 2,500,000 thermodynamic calories.

Divide this by the 86400 seconds there are in a 24-hour day, and you get (2,500,000) / (86400) =  29 calories per second.

Again, I won’t bore you with the unit conversion.  But 29 calories per second  works out to about 120 watts of power.   Which is close enough to a 100-watts.

So next time your’e in a crowded auditorium wih no air conditioning, you’ll know why to room seems so stuffy and hot.   It’s because each person is giving off as much heat as  a bright lamp.  Ten people give off as much heat as one blow-dryer on “high”.

Now, how does this tie into the FOOD we eat?  Howcome we can scarf down a Big Mac in 2 minutes, but it takes hours to burn it off?

Good question.   That’s because of the way the laws of physics are designed.   Physical work (moving things) dosen’t burn off that much energy.   Not when compared to all the chemical energy stored in food.

But that’s a topic that will be covered in the next Perfesser Friar Science post.

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26 Comments on “Calorie Counting with Perfesser Friar”

  1. Mike Goad Says:

    What? No conversion of matter into energy? mass x speed of something or other squared?

    Good discussion on “work” in the scientific and physical activity sense of the word, which is different from “work” for employment or “work” as defined under copyright law and international treaties.

  2. You spelt Professor wrong.

  3. Friar Says:


    Yes, Energy = (m* c^^2), where m = mass.

    But you only find that with nuclear reactions, where atomic particles are being converted to mass. Like inside a nuclear reactor or atom bomb.

    But this dosen’t apply to everyday chemical reactions, which just involve the re-arrangment of the atomic bonds between individual atoms. So for simplicity sakes, I didnt’ bring it up here. That could be for another post.

    But good point, though.


    Well, I’m just a dumb illiterit enjineer. I know the math and science, but I can’t spell rite.

  4. Mike Goad Says:

    Having had my hand on the controls of a nuclear reactor, I was being a little facetious. 😉 and a little groggy at that time in the morning.

    and, of course, besides reactor “theory,” we had to know a bit about work, kinetic energy, heat transfer, fluid flow, and more, so when I say that this was a good post, I was trying to say that it was a good, accurate discussion.

  5. Friar Says:


    Yeah, I realize you had Nukular experience. Didnt’ mean to sound so long-winded. But I figured this answer would probably benefit other readers, who I’m sure will probably ask the same question.

    I’m groggy too right now (and I’ve HAD my coffee!) 😉

  6. Raises hand……So if we do yoga in those hot rooms that are so trendy right now , does it really make our muscles more fluid? Seems like that heat would decrease the work we have to do (less resistance) so that actually a 30 minute yoga session would not burn as many calories as we think. Huh, perfesser?

  7. Frank Says:

    Opps, I’m sorry I was looking for a witty blog – this seems more like a science class – brings back nightmares of my university days – Now I am beginning to be afraid of numbers – I wonder how many calories I burned up thinking that – guess I’ll have to reread your blog to find out.

  8. Friar Says:


    I’ve been googling the subject. Some claim that hot yoga burns more calories. But its’ controversial. Others say these claims are dubious.

    Sure, you sweat a lot….but that’s just your body’s response to trying to cool yourself off. You lose weight when you sweat and you get dehydrated. But you regain that weight back once you drink back the water you lost.

    For example, if you stood in a sauna or outside in the Gulf Coast in the peak of summer, you’d also sweat like a pig, but you woudn’t necessary be burning that many calories just standing there.

    I’m a skeptical Friar. Not that I’m saying yoga dosn’t have great health benefits. Just that if it were me, I’d read up more on hot yoga, and I’d take some of the claims with a grain of salt.


    Awww. 99% of the time, I’m witty and smart-ass. You just happened to pick a day when I wrote about science.

    Check out my older posts, or come back later. I promise to be funny again! 🙂

  9. Thanks Friar,
    Hm, it’s 38 degrees outside here so I don’t have to worry about that at the moment. I know scuba divers burn huge calories because their bodies are having to work harder to keep warm. It would seem that cooling a room would make more sense than heating up a room.

  10. Friar Says:


    Fundamental law of heat transfer. The rate of heat dissipation is proportional to the temperature difference. So if it’s really cold, your warm body will lose heat more quickly. Compared to a warm room, where the temperature gradient isn’t as pronouced.

    Scuba. BRRRR. Been there, done that. I once dove in Lake Huron in May..the water temp. was 38F. 😦

  11. Brett Legree Says:

    All this talk about calories is making me hungry. We should go for a piggie burger tomorrow, at that truck stop with the 20-foot naked gorilla on the roof.

  12. Friar Says:


    I wonder if their burgers have a lot of trans-fats?

    God, I certainly HOPE so!

  13. Friar-Are you crazy!!!??? HYPOTHERMIA.

    Brett-A naked gorilla?

  14. Kelly Says:


    Just visit him at my blog today. Evidently, he had to have someplace to expend a few extra kCals.


    Have I said before that you totally rock?

    Don’t answer that. I know I have. Probably an embarrassing number of times. I love your sense of humor.


    Is this because I wanted to eat fudge last week?

    Which I did.

    & it was darned good.

    & the scale didn’t move, thank goodness.

    I searched all through here for the answers to my caloric qu.s, but didn’t find them.

    So, okay. I can only gain the weight of what I eat and no more. Then other than nutrition, which I do get, why do we worry more about chocolate or butter or potato chips than we do about icky rabbit food? Is it just because we can eat a much larger volume of (stuff I hate) than of delectable things I love, so we end up eating more if we eat things with lots of kCals in a small volume?

    What I mean is, are there bad calories and good calories? Do butter calories take longer for me to get rid of? They don’t, I assume.

    And why is it that I thought I understood all of this once, in science classes, but I still don’t get it? (That’s Inside Kelly’s Brain, which is a topic for another post, eh?)



  15. Friar Says:


    Regarding the naked gorilla. Oh…….Just go check out Kelly’s blog. 😀

    I actually was writing this a while ago….but last weeks discussion on fudge got me to finish it off,

    As for calories…it’s a unit of energy. A butter calorie shouldn’t take any longer to burn off than a broccoli calorie.

    But it’s a matter of nutrition. If you ate nothign but sugar and butter every day, that would meet your ENERGY needs. But it certainly wouldn’t meet your nutritional needs. You’d lack vitamins and minerals, and you’d probalby end up with scurvy or something.

    As for burning calories off…It also depends on where the energy is stored in your body. When you eat, the first place energy is stored is in the glycogen (your blood sugar). That’s for short-term energy storage.

    Any extra calories above that get get stored as fat, for longer term storage.

    So, when your body needs energy, the first thing it does is burn the short-term glycogen in your bloodstream. Once that’s gone..then it starts burning your fat. (Which is what you want).

    Once the fat is gone, that’s when your body starts burning up your protein (i.e. your muscles fibers and tissue). That’s when you start starving to death.

    But lucky, we eat enough every day that we usually only have to rely on the glycogen in our bloodstream. (Or UN-lucky, depending how you look at it, beacuse that means we hardly ever burn off our fat!)

  16. Karen Swim Says:

    Friar, as a closet science geek (uh oh did I type that out loud) I love these posts! This was a great discussion of calories and I know enlightening to many. Our bodies are a fascinating machine and if we took time to understand even a little we would not obsess over some of the craziness and misinformation.

  17. LOL….I am just shaking my head… it’s not even Thursday night… okay, I am seeing the truck stop gorilla in your cartoon form… catchy , you two are contagious…

  18. Friar Says:


    Glad you like it. Wait till next Science post, when I write about mechanical work and how we can calculate the calories we burn off!

    Brett and I are thinking of going into business for ourselvles…Hijacking blogs and increasing comment traffic. 😉

  19. That could be all kinds of funny.

  20. Kelly Says:


    See, you do know how to stay on-topic… just not at MCE!

    Cool stuff, though. Really, once I knew this. I hate that my brain can’t store everything I’ve learned.

    Then again, there’s probably plenty of stuff I’ve learned that it’s good to forget. So strike that.

    Makes sense for now. Let’s see how long I can hold onto this understanding.

    I think I’d better go burn off some of last week’s fudge.

    Until later,


  21. Friar Says:

    Ohh, and I can’t WAIT till tomorrow, it’s Quotable Wednesdays at Kelly’s blog. That’s always good for some off-topic discussion.


    Oh, I’m sure the comments would go off-topic here too, if I had the equivalent of myself to pop in and visit!

    PS. Don’t read the previous comment. 😉

  22. Quotable quotes at Kelly’s is like a Rorschach ink blot for you and Brett. 😉

  23. Friar Says:

    @Janice and Kelly

    That’s what happens, when you have a random thought-generator in your head. 🙂

  24. steph Says:

    Kelly: what you want to be concerned about is 1) not consuming far more calories than you burn and 2) the the nutritional value of the calories. For those who are relativly active or have high metabolisms, this is the greater concern rather than how many calories they consume. While all calories are energy, what percentage of the calories is fat, for example? All that info is normally provided.

    Friar: I really love you perfesser persona! As long as it was, this post kept my interest. You have such an awesome knack for this! Still think you should produce your own set of science encyclopedias for kids! You’re so good at explaining things and keeping things simple.

  25. Friar Says:


    Thanks…I think a science book is a great idea. It’s on the back-burner….on my short list of things to work on (once I finish Basil!)

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