Kitchen Chemistry: What are Calories, and Why Do We Eat Them?

At one time or another, we’ve all looked at a food package to determine whether a food is “healthy” or not, according to the nutritional doctrines of the day.  Often, the first (and perhaps only) thing that someone looks at on such a label is how many calories a food product contains per serving.  But what are calories?  Are they good or bad for us?

A calorie is a scientific unit of energy

What food packages label as “calories” are actually “kilocalories”, or a unit that equals 1,000 calories.  In chemistry (and other sciences), a single calorie is the amount of energy it takes to heat one milliliter of water one degree Celsius.  Wait…huh?

Momentarily setting aside the discrepancy in naming conventions, let’s paint a mental picture of what this “calorie” actually does.

Think of a centimeter.  A centimeter is about 1/3 of an inch.  Now, think of a square that’s 1cm on each side.  That’s a square centimeter because each side is equal, and all four sides are 1cm long.  To get the area of a square, you multiply the length–1cm–by the width–1cm–and get…you guessed it: 1 square centimeter (a unit of area).  Now, think of that same square and make it three-dimensional.  It’s now 1cm wide, by 1cm long, by 1cm tall.  That’s a cubic centimeter or “1cc”.  1cc equals 1 milliliter (1ml).

So, let’s take that 1ml (1cc) cube and fill it up with pure water.  Next, we’ll put a tiny amount of fuel under it and light it on fire, then wait for the temperature of the water to go up by one degree Celsius.  The moment it has done so, we put out the fire.  How much of that heat energy just went into the water?  Exactly one calorie.  How do we know?  Because by definition, 1cal is how much heat energy it takes to raise the temperature of 1ml of water by exactly 1 degree Celsius.  See?  We’ve just used a single calorie to heat water.  How scientific!

As it so happens, 1cc [1ml] of water weighs exactly one gram.  Isn’t the metric system neat?

Alright, I see that you’re wanting to know how this has anything to do with food.  The neat thing about a unit of energy (a calorie, for example) is that it doesn’t just measure heat energy.  It also measures kinetic energy, positional energy, nuclear energy, and–what we care about, right now–chemical energy.  The amount of chemical energy that our bodies can extract from a morsel of food is what is being measured and written down on the food label.  More properly, this is chemical potential energy: energy that’s stored in chemical bonds that can be re-arranged to make heat, movement, and other fun stuff happen.

Why, then, does the label use kilocalories instead of calories as its unit of measurement?  Creatures with over 15 trillion cells in our bodies, each of which need energy to survive, we need a great deal of energy to keep living.  So, any meaningful measure of nutritional energy will have to be in the thousands.  1,000 calories (“small calories”) = 1 “large calorie” or kilocalorie, which means that we don’t have to put a whole bunch of zeroes at the end of every “calories per serving” number on a cereal box.  That saves space and is easier to read.

Why do we need chemical energy to live?  Because without it, our cells would be rendered immobile–unable to respirate, unable to repair themselves, unable to move oxygen and water around, etc.  When our cells stop moving, we’re dead.  The tricky thing is that we can’t just pump heat into our bodies and have our cells magically turn that into energy; our cells are combustion reactors, not very unlike the engine of a car.

Wait, what?

Yes, you read that right.  An automobile takes chemical energy from molecules called hydrocarbons and breaks the chemical bonds to release energy.  How does a car do that?  First, it takes a bit of energy to get the process started.  This is called activation energy.  The battery sends a jolt through the system (with the help of the starter, spark plugs, etc.) that lights vaporized gasoline on fire.  That little explosion makes the pistons move, which cause the wheels to spin, and also gives parts of the engine enough kinetic energy to fill the reaction chambers with gasoline and light it on fire.  This produces a chain reaction, because each little reaction makes another reaction happen (until you break the “chain” by cutting power to the ignition process–A.K.A. “turning it off”).

Our cells do basically the same thing.  We have a chain reaction already happening inside each and every one of our cells since the moment of conception.  Our mother’s womb feeds those cells chemical energy in the form of sugar and other things–all of which are hydrocarbons–and that lets our cells keep going while having enough energy left over to make more cells.  Eventually, we get big enough to survive on our own, and voila! we are born!  Every day, we put more food into our bodies because that food is made up of hydrocarbons that our body knows how to break down.  We have enzymes, symbiotic microbes, digestive juices, and other things that let our bodies disassemble a wide variety of chemicals and turn them into the stuff that our cells run off of.

In fact, the chain reaction that keeps us alive is part of the same chain reaction that started life on this planet!  Think about that for a moment.  If, at any point between the creation of the first primitive lifeform and when we were born, that chain reaction had completely stopped, our mothers would have passed away before giving birth to us.  Isn’t that remarkable?

If cars use gasoline for fuel, what do our cells use?

Short answer: a simple sugar called glucose.  Most single-celled organisms love sugar because it’s the easiest thing to light on fire and get energy from.  In human cells, we have a little cell-within-a-cell called a mitochondria that does the hard work of lighting stuff on fire without making us explode and die.  Our cells take in glucose, burn it with oxygen, and use the energy that produces to turn a low-energy chemical (adenosine diphosphate) into a high-energy chemical (adenosine triphosphate), which then goes around and deals smaller, safer amounts of chemical energy to its “customers” in other parts of the cell.  That lets the cell move around and do its job for the rest of the body–whether that be passing around oxygen (red blood cells), killing invaders (white blood cells), contributing to larger movement (cells that are part of muscle tissue), storing energy for later (fat cells), and whatever else our bodies need to do.

The average adult requires 2,000 kilocalories of digestible chemical energy per day to avoid cell death

It’s true: if we don’t get enough calories, our cells die.  We need about 2,000,000 calories (2,000 kilocalories) per day to make sure that, at the end of the day, we still have the same number of cells that we started with.  Of course, if we spend a lot of energy on exercise, we need more than that to maintain the same number of cells; and if we don’t get much exercise, we’re spending less energy, and don’t need to eat as much chemical energy to keep us going.  If we want to lose weight, that means that we’re actively trying to make some of our excess cells die by not feeding them enough.  When cells are in distress, they release lots of chemicals that tell our brains (comprised of nerve cells) and other parts of our bodies that something is very wrong.  In other words, it hurts.  This can manifest in tiredness, moodiness, etc.  Our bodies are built to gather more energy and make more cells, not to lose energy and have cells die.

Up until the last 100 years or so, this wasn’t an issue because we didn’t have reliable food supplies, and therefore had very little capacity to overeat.  Food was too scarce, too expensive, and required us to spend a lot of energy to get it.  That kept us skinny.  Now, our food supplies are pretty awesome, and starvation is basically gone in the USA.  (Yes, it still happens, but almost never on the grand scale, which may be the most remarkable achievement in our species’ survivability, ever.)  We can now sit at a desk all day, spending almost none of our stores of chemical energy, and still have cupboards and refrigerators stocked full of food!  We eat because our bodies tell us it’s “time”, and if we get a little too much, our bodies say, “That’s great!  We won’t starve, now!”

Evolution is a little behind the times–and that’s why we get fat: our bodies are telling us to eat as much as we can so we don’t die of hunger, but we have so much food available that we can literally kill ourselves by eating too much.  From a biological standpoint, that’s a very good thing…mostly.

So, to finally answer the question in the title: we will die if we don’t get enough calories, and that’s why we eat them.  We read labels and diet because, for the first time in human history, literally billions of people have the unique problem of being so wealthy, in terms of food availability, that we can eat ourselves to death.  How many calories are too many?  That depends on your body’s size (how many cells you’re currently maintaining); the amount of chemical energy you’re spending on a daily basis (A.K.A. “exercise”); whether you want to gain weight, lose weight, or stay the same weight; and the unique quirks of your particular body’s metabolic process.  (Each person manages their chemical energy slightly differently, and as a result, some people can seem to “eat anything and stay slim”, while others can’t.)

Fun fact:The kinds of cells that die first are somewhat dependent on what you’re (not) eating.  Your brain and nerves love fat.  Your muscles love protein.  Everything loves carbohydrates (simple and complex sugars like glucose, fructose, sucrose, and starch–which is made of glucose and/or fructose).  Every diet has a trade-off.  Don’t believe anyone who tells you that a diet is risk-free.

The important thing to remember is this: (calories in) – (calories out) = (net gain or loss).  If the net gain or loss is 0, then you’ll stay the same weight.  More means you’ll gain weight, and less means you’ll lose weight.

Finally, please be aware that our bodies need things that aren’t caloric (don’t contain chemical energy that our bodies can burn) like minerals (iron, magnesium, etc.), vitamins, amino acids (what proteins are made of), and so on.  The only way you’re going to get everything you need is by consuming the right amount of calories (not too little or too much) from a wide variety of sources that also contain other stuff that you need.  If you eat nothing but starch, fat, and sugar, you’re going to get very sick, indeed.  Follow the age-old wisdom of eating a little bit of everything in moderation and not being too picky.