Insulin. Pretty, isn't it?

What, exactly, is a calorie?

You eat a chocolate bar, it has 325Kcal. You bike for half and hour, you burn 325kcal. Great!

So if we chop up a chocolate bar in smaller and smaller pieces, at some point will we find tiny little pellets of calorie? No. Well, yes. Sort of. What we will find, eventually, are little crystals. Molecules of sugar. Calories are sugar. A whole lot of sugar crammed together is fat. Or protein.

Obviously, I’m greatly simplifying the process. Let’s look at what actually happens when you “burn” a calorie, and what happens to the “calories” you eat.

Our muscles are loaded with a thing called ATP. Adenosine Triphosphate. If you follow the common advice and don’t eat anything you can’t pronounce, you could avoid eating this. You won’t live very long though. ATP is the molecule in our muscles that allows them to move. Nothing else is used, just ATP. Every muscle in the body has a store of ATP. It’s only enough for a few seconds of activity. It’s just enough to kick-start the muscle and start a sustaining reaction that steadily produces more ATP. The fuel for this reaction is Glycogen. Each muscle has a nice store of glycogen…enough for a moment or two of activity, or just enough to kick-start a sustaining reaction utilizing lactic acid. Once we have enough lactic acid in the muscle, the muscle can start using it to produce ATP.

You can actually feel your body switching from mechanism to mechanism by running as fast as you can. The first second or two feels effortless (ATP stores), and then you start to pant and your heart races…it feels hard and you slow down ever so slightly (glycogen storage.) You can keep that up for bit, but then you start to feel that burn, and you have to slow down and breath a lot harder (lactic cycle!) but as long as you slow down enough, you can keep going for a long time.

A muscle at rest has ATP and glycogen stored in it. Lactic acid is generated as a byproduct of burning glycogen. We use up the ATP right away, and use up glycogen to replace the ATP, and lactic acid comes from glycogen…hey, where do we get glycogen from???

Insulin! You hear a lot about “blood sugar” and we tend to think of it as some sort of bad thing. What it actually is, is the available pool of glycogen for the use of all the muscles. Once glycogen is in a muscle, it doesn’t go back out. Our strategic reserve of glycogen lies in the blood, in the form of glucose…sugar. Insulin stuffs it into the muscles that need it, in the form of glycogen. Some of the free sugar in the bloodstream comes from immediate digestion, but most of it is released from the liver, where it is stored as Glycogen.

When you burn a calorie exercising, you are using up ATP. (Science fact! if you weigh 200lbs, you probably “burn” 200lbs of ATP in a day. The body is so insanely good at recycling that the unusable waste of that consumption only amounts to a few thousand calories. Neat huh?) That ATP has to be replaced, which is done from sugar…glycogen. The glycogen in the muscle is replaced from the blood pool, which is replaced from the storage in the liver, which takes its share directly from digestion…the chocolate bar you swallowed a while ago.

You swallow that chocolate bar, and the digestive system immediately tries to break the entire thing down into glucose (it’s actually a little more complex than this, but go ask a science person for more details. You’ll make them happy and learn a lot.) A chocolate bar is almost all glucose, so the digestive system doesn’t have much to do but transportation. Fat and protein digestion and synthesis I’m not gonna talk about today, but it’s very nifty. The glucose temporarily enters the bloodstream, but only via a single vein that leads directly to the liver. The liver then decided how much sugar has been used up out of the blood pool, and shoots out new replacement sugar. Excess is stored within the liver. If the liver is full, it converts the sugar into muscle…ha ha! Just kidding, it converts it into fat, which gets shoved out in the blood to be eventually collected in your love handles.

If you exercise a lot, and don’t eat enough, the liver will get the fat back from the love handles and turn it back into liver glycogen, and restock the blood pool. That process can take time, so if there is more immediate need for sugar, the body will claim it from another source: muscle protein. Protein can be broken down into sugar, just like fat. Which is why when you look at the calorie count on a chocolate bar, it lists calories for the fat and protein as well as the sugar content of the chocolate bar.

So calories equals sugar. Got it? Any questions?