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.