Part 2 of a three-part series exploring muscle, metabolism, and what happens to blood sugar after it enters the bloodstream.
Last week, we met a muscle most people rarely think about.The soleus sits deep in the calf, quietly helping us stand, walk, and maintain our posture. But researchers have discovered something else about it: when activated in a particular way, this small muscle can become a surprisingly active user of fuel circulating in the bloodstream.
And that raises a much bigger question.
What if our conversations about blood sugar have been concentrating so heavily on what puts glucose into the bloodstream that we've overlooked what is supposed to take it back out?
Because glucose isn't simply something we need to keep from getting too high.
It's fuel. And it's supposed to go somewhere.
After You Eat, the Story Is Just Beginning
When we eat carbohydrates, digestion breaks many of them down into glucose.
That glucose enters the bloodstream, blood glucose rises, and the pancreas responds by releasing insulin.
Most of us are familiar with at least that much of the story.
Insulin helps signal cells that glucose is available. Some of that glucose can be used immediately for energy. Some can be stored as glycogen in the liver and muscles. If energy continues to be available beyond what the body needs and can readily store, other metabolic pathways come into play.
It's an extraordinarily coordinated system.
But there's something important hidden in that explanation.
The system doesn't depend only on supply. It also depends on demand.
Imagine deliveries arriving continuously at a warehouse.
If products are also leaving the warehouse, everything moves along nicely.
But what happens if deliveries continue while fewer products are going out?
Eventually, storage becomes a problem.
Glucose metabolism isn't exactly a warehouse, of course, but the analogy helps us see something we often miss.
We've spent an enormous amount of time talking about the deliveries.
Perhaps we should also be talking about the customers.
Muscle Is One of Your Biggest Glucose Customers
Skeletal muscle represents a substantial portion of the human body.
And muscle needs energy.
When you're walking up a hill, carrying groceries, gardening, getting out of a chair, climbing stairs, lifting weights — or even repeatedly contracting a small muscle in your calf — those muscles require fuel. Glucose is one of the fuels they can use.
Muscle is therefore an important destination for glucose after we eat, particularly under the influence of insulin.
But muscle contraction adds another fascinating piece to the story.
Contracting muscle can increase glucose uptake through mechanisms that aren't entirely dependent upon insulin.
In other words, movement doesn't simply "burn calories."
Contraction itself changes what muscle asks the body to provide.
The customer has walked into the store.
Insulin Is Part of the Conversation — But Not the Entire Conversation
Insulin is sometimes described as a key that unlocks the cell so glucose can enter.
It's a useful analogy, although human physiology is considerably more complicated than a lock and key.
When tissues become less responsive to insulin, the pancreas may compensate by producing more of it.
For a while, that can keep blood glucose looking relatively normal.
That point matters.
Because a glucose reading doesn't necessarily tell us how hard the body had to work to produce it.
Someone can have glucose numbers that don't look particularly alarming while insulin has already been climbing behind the scenes.
Eventually, if the system can no longer compensate adequately, glucose begins rising as well.
This is one reason metabolic changes can develop quietly for years before a laboratory value finally gets someone's attention.
The signal we notice may be relatively late in the conversation.
Now Add Muscle Back Into the Picture
Here's where things get interesting.
Muscle isn't merely waiting passively for insulin to deliver glucose.
When muscle contracts, it creates its own demand for energy.
That contraction activates cellular pathways that help move glucose transporters toward the surface of muscle cells, allowing more glucose to enter and be used.
One of the important players is a glucose transporter called GLUT4.
You don't need to remember the name.
What matters is the concept.
There are multiple ways the body can tell muscle:
Glucose is available. Bring it in.
Insulin provides one signal.
Muscle contraction provides another.
And that gives us a very different way of thinking about movement and metabolic health.
Exercise isn't merely something we do to compensate for calories we've eaten.
Movement changes the metabolic conversation itself.
This Is Why the Soleus Research Is So Interesting
Now we can return to the little muscle we discussed last week.
Researchers studying the soleus found that sustained activation of this highly oxidative muscle could dramatically increase its use of circulating fuels while using relatively little of its own stored glycogen.
In their laboratory experiments, this affected both glucose and insulin after participants consumed glucose.
The important lesson isn't that the soleus possesses some magical ability that other muscles don't.
It's that the research gives us a striking demonstration of a larger principle:
What happens to glucose depends partly on whether our tissues have a reason to use it.
We can change the supply.
But we can also change the demand.
Which Brings Us Back to Sitting
Consider a fairly ordinary modern day.
We wake up.
We eat breakfast.
We sit in the car.
We sit at a desk.
We eat lunch.
We sit some more.
We drive home.
We eat dinner.
Then we sit to watch television or spend time on a computer.
Glucose enters the bloodstream several times during that day.
But for long stretches, some of the largest potential users of that glucose — our skeletal muscles — aren't being asked to do very much.
That doesn't mean sitting causes diabetes.
Nor does it mean someone can simply move more and ignore nutrition, sleep, stress, genetics, body composition, medications, hormones, liver function or the many other factors involved in glucose regulation.
It means there is another piece of the metabolic picture worth considering.
How much demand for fuel are we creating?
Maybe We Need a Different Question
When someone's blood sugar begins creeping upward, one of the first questions is often:
"What should I stop eating?"
Sometimes that's an important question.
But perhaps it shouldn't be the only one.
We might also ask:
How often are my muscles being asked to use glucose?
Not just:
Did I exercise today?
But:
How much of my day did I spend moving?
Did I walk after eating?
Did I interrupt long periods of sitting?
Am I maintaining enough muscle as I get older?
Do my muscles regularly receive a reason to demand fuel?
Those questions don't replace the conversation about nutrition.
They complete it.
There's Another Reason This Matters as We Age
We naturally tend to lose muscle mass as we get older unless we give the body a reason to maintain it.
Usually we talk about that in terms of strength, independence, balance and fall prevention.
All very important.
But consider what we've just discussed.
Muscle is also metabolically active tissue.
So preserving muscle isn't only about being strong enough to carry groceries or get up from the floor.
We're preserving tissue that participates in glucose regulation.
We're preserving places for fuel to go.
We're maintaining part of the body's metabolic machinery.
Suddenly, the conversation about maintaining muscle as we age becomes much bigger than appearance or even physical strength.
Muscle is part of metabolic health.
The Body Is Always Responding to What We Ask of It
This is the part I keep coming back to.
The human body is extraordinarily adaptive.
If we repeatedly ask muscles to work, the body responds to that demand.
If we stop asking them to do very much, the body adapts to that, too.
Neither response is punishment.
It's physiology.
And that's why I think the soleus research is so fascinating.
A small muscle deep in the calf gives us a window into something much larger.
Blood sugar isn't simply a story about sugar.
It isn't simply a story about carbohydrates.
And it isn't even simply a story about insulin.
It's also a story about what the body is being asked to do with the energy we give it.
Sometimes the signal beneath the surface isn't telling us that we need to take something else away.
It may be telling us that something important needs to be used.
Next Week: The Problem With Sitting May Be What Isn't Happening
In the final part of this series, we'll look more closely at inactivity.
Because perhaps the problem with sitting isn't simply that we're not exercising.
Perhaps long periods of stillness remove metabolic signals the human body once received throughout the day without our ever having to think about them.
And that raises a very different question:
What happens when movement stops being part of everyday life and becomes something we schedule for an hour — and then sit for the other fifteen?
That's where we'll go next.
Research Behind This Conversation
Hamilton MT, Hamilton DG, Zderic TW. A potent physiological method to magnify and sustain soleus oxidative metabolism improves glucose and lipid regulation. iScience. 2022;25(9):104869.
Richter EA, Hargreaves M. Exercise, GLUT4, and skeletal muscle glucose uptake. Physiological Reviews. 2013;93(3):993–1017.
Sylow L, Kleinert M, Richter EA, Jensen TE. Exercise-stimulated glucose uptake — regulation and implications for glycaemic control. Nature Reviews Endocrinology. 2017;13:133–148.
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