When we combine our sedentary lifestyles with our ultra-processed diets, the inevitable result is that our metabolisms begin to malfunction. This “metabolic syndrome” takes many forms, but type 2 diabetes, polycystic ovary syndrome, and hepatic steatosis (fatty liver) are some of the main presentations. These disorders all share a commonality: elevated insulin levels with impaired insulin signaling. In this article, we’ll discuss how age related muscle loss contributes to this root problem. I’ll discuss how strength training reduces the risk for diabetes in particular, and metabolic dysfunction in general, and how this form of exercise offers benefits no other approach can give for normalizing insulin signaling in the body.
First, here’s a recap of what we discussed in Part 1 of this article series1.
Our body uses insulin to control how much energy is in our blood. Insulin tells muscles and other tissues to absorb extra blood sugar after a meal.
However, when muscles are already "full" of glycogen (stored glucose), there is less room to store the incoming sugars, and the body has to work harder to get our blood sugar level back to baseline. As a result, our body produces extra insulin, which has negative effects on many tissues, including the fat cells, kidneys, liver, and ovaries.
By finding ways to burn our muscle glycogen, such as sprint interval training, we can make room for the incoming blood sugars for our meals. When our muscles are "empty," they need less insulin to absorb sugars, which lowers our need for insulin. This prevents metabolic dysfunction and reduces some of the resulting problems for the female reproductive system.
A key idea to remember; when we are strong and active, the starches and sugars we eat are used to replenish the glycogen (emergency fuel) in our strongest muscle fibers. The muscles are sponges we should regularly wring out.
Shrinking Sponges means less insulin sensitivity.
We’ve established that we want to “wring our muscle sponges out” regularly with exercise, depleting our muscle glycogen and thus improving our sensitivity to insulin. But we also want to make sure our muscles are big and healthy, and not shrinking with age. A shrinking sponge can’t absorb much blood sugar! And it turns out that muscle mass and insulin sensitivity are closely related. As people lose muscle mass with age, it is typical for fasting insulin levels to rise, which is the earliest warning sign of metabolic dysfunction and pre-diabetes2.
After strenuous exercise, our muscles start sucking up glucose (or “blood sugar”) to replace the stored energy (“glycogen”) they just burned. And a large, well-trained muscle can store a lot of muscle glycogen!
What’s more, properly performed strength training (controlled tempo, moderately heavy, pushing close to failure) builds muscle preferentially in fast twitch, glycogen-burning muscle fibers3. These big, strong fibers are used during very high efforts, and store most of their energy right inside the cell as glycogen. This means that when we increase our muscle mass with high effort strength training, we make our sponges a lot bigger!
Age-related muscle loss leads to reduced glycogen storage capacity.
I want to paint a simple picture to help you understand the effect that muscle loss has on your body’s ability to metabolize dietary starches like bread, rice, pasta, bananas, or potatoes.
Jennifer is a healthy young woman with 50lbs of muscle mass. She played sports in high school, and is an avid hiker in her early 20’s. However, time goes by, and she gets busy with her career. She has two kids, and due to the time commitments of work and her family, her life becomes increasingly sedentary. From age 25 to age 45, Jennifer loses at least 5lbs of muscle (a conservative number – it can be higher!4) despite her trying to walk daily and take the occasional spin class.
At some point in her 30’s, she noticed her metabolism “slowed down.” The same diet that used to leave her feeling energetic and satiated now leaves her bloated and lethargic. Now that she is nearing 50, her weight is creeping up steadily while her strength and agility are declining.
Why did her metabolism actually “slow down?”
It’s easy to blame all of this on hormonal changes. And hormones certainly do play a role in slowing our metabolism. But much of the decline we experience with age is simply due to age-related muscle loss. As I have discussed at length, our comfortable lives rarely involve the activation of our strongest muscle fibers, even if we try and stay physically active5! These fibers atrophy due to disuse, leaving us weaker, less agile, and less metabolically healthy than we were in our youth. In the worst case, our muscle tissue can start to be replaced with fatty deposits6 – a major indicator of poor metabolic health and functional decline.
Our biggest, strongest muscle fibers aren’t very good at burning stored body fat for energy. Instead, they store energy in the form of glycogen, which they make from glucose (the carbohydrate fuel we get from starches like bread, pasta, rice and potatoes). When our muscle mass shrinks due to disuse, the body’s ability to store glycogen also declines, and with that, our ability to process starches in our diet also gets worse. The same meal that might have been quickly absorbed just 10 years ago now causes a large surge of both blood sugars and insulin, and this spike in insulin can have the effect of making us feel sluggish and fatigued, since one of its actions is to prevent body fat from being released for energy.
Sedentary people store 40-60% less muscle glycogen!
We said that Jennifer lost 5lbs, or 10%, of her muscle. Unfortunately, she didn’t just lose 10% of her glycogen storage capacity. As she became sedentary, she went from having 50lbs of healthy, well-conditioned muscle mass, to 45lbs of less healthy, atrophied muscle mass. Well trained muscle stores almost 40-60% more glycogen than deconditioned muscle7. Thus, a well trained person with 50lbs of muscle might be able to store 350-400 grams of glycogen in their muscle mass, whereas a sedentary person with 45lbs of muscle might be able to store only 200-250 grams.
This means that Jennifer may have lost 100 to 200 grams of glycogen storage. This is a massive drop, and we have to think a bit to understand why. If you can store 400 grams of glycogen, and you do a hard workout that burns 100 of those grams, this means that your body will need 125-150 grams of extra dietary carbohydrate in order to replenish your muscles (since some will be used immediately by the brain). When you eat those carbohydrates, you will have a modest insulin release – only enough to signal your tissues to store the dietary carbohydrates.
On the other hand, if your muscles have a diminished storage capacity for glycogen, then that same extra 125-150 grams of carbohydrates from a dinner out with friends essentially has nowhere to go when you eat it. This is doubly true if whatever capacity you have is already full because you haven’t trained intensely in years. Beyond the little your brain needs, the extra glucose will lead to a lot of extra insulin in the blood stream. And this is where we get to the “slowing metabolism” and increased risk for metabolic dysfunction.
This is the big takeaway. Age related muscle loss is predisposing us to metabolic dysfunction and type 2 diabetes by removing our bodies’ primary means of storing carbs and sugars from our diets.
A concrete example: Birthday Cake
When we strength train consistently and preserve our lean muscle mass, we also preserve our ability to metabolize the carbs in our diet in a healthy way. Jennifer with 45lbs of untrained muscle can’t really tolerate a slice of birthday cake. Her blood sugars and insulin levels rise significantly after she indulges, and since the muscle mass has no extra room for the sugars, it takes many hours for her insulin levels to fall back to normal.
While this is happening, she feels lethargic. This is because insulin prevents body fat from being released for energy. And the excess insulin can cause an overcorrection to blood sugars, dropping them too low and causing fatigue and strong cravings for snacks. (This will sound familiar to many people an hour or two after lunch!)
But Jennifer with 50lbs of well-trained muscle mass absorbs the birthday cake with ease! One indulgence doesn’t crash her energy levels or derail her diet with cravings because she has maintained a flexible, healthy metabolism thanks to her robust muscle mass. In this virtuous cycle, she is less likely to fall completely “off the wagon” after just one indulgent dessert. She simply doesn’t have as big of a blood sugar swing thanks to her hungry muscle “sponges” quickly absorbing the sugars from the cake.
The Big Takeaway: Strength Training Reduces Diabetes Risk
Type 2 Diabetes develops when our pancreas can no longer produce enough insulin to overcome the insulin resistance in our tissues. We can prevent this problem before it starts by eating a whole foods diet high in lean protein and fibrous fruits and vegetables, and by keeping our muscle tissues robust and healthy. Strength training preserves our muscle mass, maintaining our body’s ability to quickly absorb and store the carbohydrates in our diets. And by keeping our muscle sponges large and absorbent, strength training lowers the amount of insulin our pancreases must produce, which reduces our diabetes risk.
- https://www.strength-space.com/sprint-intervals-for-pcos/ ↩︎
- https://pmc.ncbi.nlm.nih.gov/articles/PMC3275515/ ↩︎
- https://www.strength-space.com/strength-training-prevents-muscle-aging/ ↩︎
- https://pmc.ncbi.nlm.nih.gov/articles/PMC3429036/ ↩︎
- https://www.strength-space.com/is-staying-active-enough/ ↩︎
- https://www.strength-space.com/what-is-fatty-muscle/ ↩︎
- Skeletal Muscle Glycogen Content at Rest and During Endurance Exercise in Humans: A Meta-Analysis | Sports Medicine ↩︎