Understanding Blood Sugar Control: The Basics

In the first post about understanding blood sugar control, I addressed why blood sugar control is important.

So, now let’s take a step back and talk about where the glucose in your blood comes from and what your body does with it.

How does glucose get in the blood?

There are three main ways that glucose gets into the blood…

1) The primary source is dietary monosaccharides. As mentioned in the post about carbohydrates, the body digests carbohydrates into monosaccharides before being absorbed into the epithelial cells and blood stream. (If you want more details about how this works, check out my video about leaky gut, which explains how a healthy intestine functions.)

The blood that contains the absorbed monosaccharides travels directly to the liver via the portal vein. (As an interesting side note, the portal vein is the only vein in the body whose destination is not the heart. Oh wait… is that only interesting to me?!)

Although some glucose will pass through the liver and enter the body’s main circulation, most of the glucose and almost all of the fructose and galactose are absorbed by liver cells. The incredibly powerful liver will use some of it and release the rest into the blood stream.

So, the first source is consumed carbohydrates that went through the liver on their way to the rest of the body.

2) If blood glucose levels drop too low (below ~80 or 90 mg/dL), the pancreas will tell the liver to release some of the stored glucose (called glycogen). In this situation, the liver breaks the glycogen chain into individual glucose molecules and releases them into the blood stream for use by cells throughout the body. (Note: your muscles also store glycogen, but they lack the mechanism to release the newly freed glucose into the bloodstream.)

3) If the liver runs out of glycogen and the blood sugar level is still too low, the liver (and the kidneys in cases of starvation) can convert other things into glucose and release the newly formed glucose into the blood stream. These “other things” are from non-carbohydrate sources and include lactate, pyruvate, glycerol and some amino acids. These by-products may already be in the liver from dietary fats and proteins, or they may be delivered to the liver from muscles and fat cells.

What does the body do with blood glucose?

Beyond what was described previously for the liver’s use of glucose, certain cells can pull glucose right out of the blood. These cells are found in the central nervous system (including the brain), placenta, fetal tissues, kidneys and pancreas.

When the glucose enters the pancreas at a high rate (as would happen after eating), the pancreas releases the hormone insulin. Just like other hormones in the body, insulin is a messenger and its job is to tell the cells, “Hey, I got glucose here, open up and let it in!” Think of insulin like the key that opens up cell doors for glucose to enter.

As you may have guessed from the list above, only certain cells have a “lock” that responds to insulin. These cells are found in the muscles, heart and fat cells. This fact is important for two main reasons:

  1. The cells that don’t need insulin (like your brain) cannot protect themselves from a glucose onslaught as easily because there is no lock. (Remember in the first post about blood sugar control when I said that blood sugar spikes can cause neurological and other problems? This is part of that mechanism.)
  2. When insulin is pumping and your fat cells are responding to it by letting glucose in, they cannot release fat. It’s like a one way door exists on your fat cells… either glucose is coming in or fat is going out but not both. (For more on this, read my post about fasting.)

What is the body’s goal as it relates to blood glucose?

Because the cells need a constant supply of energy, the body works to keep blood glucose within a fairly consistent range… not so low that cells risk starving and not so high as to cause problems.

Keeping blood sugar in that ideal range of 80 – 90 mg/dL involves the interworking of all types of cells from the liver to the pancreas to fat cells.

The good news is that our bodies were made to do this. The bad news is that what we choose to put in our mouths can make that job overwhelmingly difficult (refer to the problems that can be caused by blood sugar spikes).

In the next post, I talk about how the body eventually protects itself when we frequently choose to eat food that causes blood sugar spikes. And then finally, we’ll discuss how to keep your blood sugar levels under control.

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Footnote:

I think it’s important to mention that insulin release is not solely dependent on carbohydrate consumption.  Insulin release is associated with the consumption of all macronutrients. As a storage hormone, insulin is responsible for:

  1. Ensuring that the nutrients from just eaten food (amino acids, fatty acids, glucose) get out of the blood and into cells where it can be used or stored, and
  2. That previously eaten food that is already stored in cells doesn’t get out.  This prevents the blood from being flooded with extra nutrients when the nutrients from the food we’ve just eaten are already there.

That being said, the biggest surge of insulin release is related to the consumption of carbohydrates.

 

Sources:
— Gropper SS, Smith JL. Advanced Nutrition and Human Metabolism. 6th ed. Belmont, CA: Wadsworth Cengage Learning; 2013.
— Hall J. Guyton and Hall Textbook of Medical Physiology. 12th ed. Philadelphia, PA: Saunders Elsevier; 2011.

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