I want to talk about glucose a bit. People with diabetes (and our caregivers) think about it all the time, but we’re usually just thinking about blood glucose. That makes sense, because that’s the part we’re most actively trying to manage. But what about the other kinds of glucose in the body? What is it there for? How does it affect blood glucose?
As I wrote last month, a body stores energy in many forms, and blood glucose is dwarfed by muscle and liver glycogen and fat. We’ll leave stored fat out of the picture for now and just focus on glucose and glycogen.
When you eat, much of the energy content of the food—especially the carbohydrate part—is broken down into glucose, which enters the blood stream as—you guessed it—blood glucose. Insulin (either from a working pancreas, an insulin pump bolus, or an injection) reduces the amount of glucose in the blood, but have you ever wondered where it goes?
Insulin, our favorite hormone, acts as the doorman for glucose at two main kinds of cells: muscle cells and the liver. If your muscle glycogen is low because you’ve been exercising, glucose will enter the muscles, getting them ready for the next time you’re active. It can take upwards of 24 hours after a multi-hour endurance activity to restock your muscle and liver glycogen stores. Also, people who exercise more often can store more glycogen in their muscles, making them capable of working longer. This is a key part of the so-called “training effect,” and it’s also why people with diabetes tend to be more insulin-sensitive when starting a new kind of activity: New muscle groups are getting their own training effect and storing more glycogen.
At the same time, insulin is opening doors (big, BIG doors) in your liver. In people without diabetes, insulin flows directly from the pancreas to the liver, causing much of the glucose from food to be stored as glycogen there, too. (This is one of the reasons to bolus before eating; it gives subcutaneously dosed insulin a chance to get into the bloodstream, preparing the liver to receive glucose and prevent post-meal spikes.)
When your muscles and your liver are full, the extra glucose gets converted to fat and stored away for winter . . . or whenever. This is normal, even desirable. Almost 100% of the energy stored in a body is in the form of fat. Glucose burns efficiently and right next to where it’s needed. We need glucose, but it takes up a lot of space. Fat is more energy dense and still convertible to a form that cells can use. Think of it as the difference between your car’s gas tank and the underground tanks at a filling station.
“What purpose,” you might ask, “does liver glycogen serve? If glucose is there for cells to use, why put it in the liver?” That’s a great question, my dear readers.
Most of the time even the most active of us aren’t doing much. We’re sitting. We’re sleeping. We’re digesting. We’re thinking. Maybe we’re doing a low-level of activity: cooking, typing, standing, walking from one room to another, talking, etc. All of these activities use glucose. Muscles have stored most of what they need internally as glycogen, and when they use some of it, they dip into the life-sustaining river of blood glucose and basal insulin flowing past to replenish it. Other cells, such as nerves, don’t need insulin at all and just suck glucose in via osmosis. 
As the cells slurp up glucose from the blood, insulin levels drop and glucagon production increases—at least in people without diabetes. This stimulates the liver to release more glucose into the bloodstream. Voilà! Homeostais. Basically, the liver is (among other things) a giant blood sugar battery. It gets charged when you eat, and it slowly discharges as your cells do something. Blood glucose is the internal wiring, continuously moving small amounts of energy around to wherever it might be used.
That’s probably enough of the story for now. I foresee several more posts demystifying how the body makes and uses energy, all part of helping us athletes (with or without diabetes) understand how to optimize our training and racing. Feel free to ask me your questions, and I’ll do my best to answer them.
1 — This is the main reason why diabetes complications affect nerves so much. Cells that get their glucose from the blood by osmosis without insulin’s help can’t rely on low insulin levels to keep it out. When BG levels go up, more glucose enters those cells and changes their internal chemistry, modifying their proteins (called glycosolation). These protein changes damage the cells and make cellular repair more difficult. It also causes changes to blood flow, further damaging the tissues.