The human body has evolved a sturdy, complex, and ingenious solution to what is perhaps the most fundamental problem that any life form has to solve—acquiring and managing energy. In this series of three Insights, we’re describing some key aspects of that solution by following several fuel molecules as they move through the body.
In this Insight we’ll track three glucose molecules.
The first one, which we’ll call “Glucose #1”, enters the body as part of a cluster of long chains of glucose molecules that were part of, let’s say, a piece of bread eaten near the beginning of a meal. Some of that cluster is broken apart by saliva when the bread was chewed, and that’s when our glucose molecule comes loose.
Glucose #1 travels into the stomach and small intestine, where it is moved into the bloodstream. It is then transported by blood through the liver, and then to a location just outside a cell in a leg muscle.
The pathway of a glucose molecule from mouth to leg muscle.
Glucose #1 is escorted into the cell through the action of various other molecules, including insulin.
Once inside the cell, Glucose #1 is broken up into pieces in a series of chemical reactions also involving oxygen. Those reactions release energy which is then used in the cell to power other chemical reactions that enable the cell to survive, reproduce, and carry out its functions in the body. The remnants of the Glucose #1 are expelled from the cell as carbon dioxide and water.
Now let’s follow another glucose molecule, Glucose #2, as it travels a different path. As part of the starch in a potato, it remains rather tightly bound to many other glucose molecules until it reaches the small intestine, but then, as with the first molecule, Glucose #2 is separated from its fellow travelers and moves into the bloodstream.
That bit of potato was consumed in the latter part of the evening meal, so the blood already has plenty of glucose. Consequently, when #2 arrives at the liver it is pulled out of the blood and joined to a tiny granule of a very large number of loosely bound glucose molecules in the liver, called glycogen.
During the night, when no more food is being consumed, that granule of glycogen, along with many others, is gradually broken up into separate glucose molecules to replenish the level in the blood. Glucose #2 is detached from its granule around 3:00 am. It is then transported to a blood vessel that feeds the brain. It meets its destiny (breaking up to provide energy) inside a nerve cell involved in the regulation of the body’s breathing action.
We’ll track one more glucose molecule. Glucose #3 starts out as a part of a very tiny grain of flour in the crust of a piece of pie, a late-night snack. At that point in time the blood already has plenty of glucose, so glucose #3 wasn’t needed in the blood. Therefore, like Glucose #2, Glucose #3 is pulled out of the blood supply in the liver. But now there isn’t room to make any more glycogen, which is rather bulky, so the liver converts Glucose #3 into a component of fat called a fatty acid.
With plenty of glucose and fatty acids already in the blood, that particular fatty acid is not needed for energy. Under some circumstances it might become stored in a fat cell in your body, but in this particular case there is so much glucose being delivered to the liver that it can’t get rid of all of the fatty acid molecules it is making, so this one becomes part of a fat molecule that ends up getting stuck in the liver itself. Of the three glucose molecules we’ve tracked, the fate of this one may well have the greatest impact on health. (See the eSavvyHealth Insight, How Does My Liver Get Fat?)
What’s most important to know about all this? What happens to glucose once it gets into your body depends on the circumstances at the time that it enters the bloodstream. And when you flood your bloodstream with glucose, a substantial amount of it inevitably turns to fat. The question then becomes what’s the fate of fat, which we’ll take up in the second article of this series.
