Why exercise sometimes leaves me exhausted
I’ve known the word glycogen since early in my life — but only in one context.
I learned about it through diabetes education in the 1980s when glycogen was explained as something the liver does: it stores sugar to help keep blood glucose stable between meals or overnight. That framing made sense for diabetes management, and I never questioned it.
What I didn’t connect, till the past few months, was how glycogen relates to exercise. I wanted to know why some workouts left me completely wiped out for the rest of the day.
Here’s the part I was missing about glycogen
Glycogen is stored in two places, not one:
- The liver, which helps regulate blood sugar and support the brain
- The muscles, which store their own fuel for movement
Most of our glycogen is actually stored in muscle, I now see, and muscles mostly rely on their own supply. They don’t easily “borrow” from the liver once muscle glycogen is gone.
So you can have:
- Normal blood sugar
- Decent diabetes numbers
- And still feel completely drained after exercise
That happens when muscle glycogen is depleted, even if liver glycogen is doing its job. It sounds so obvious now but nobody told me when I was younger. I was treated with a diabetes-first approach when asking for help for medical or fitness questions.
The science as I understand it
Harder activities like biking, sports, and long active days can burn through muscle glycogen quickly. Ok that’s something I didn’t know but verified at Mayo Clinic and other sources. When it isn’t restored, the body stays in a stressed, low-energy state. It doesn’t seem to borrow glycogen from the liver, or replenish from recently-eaten exercise snacks (gluten-free granola bars in my case), either. The result isn’t just physical tiredness, it’s also irritability, intense hunger, and feeling useless for hours after exercise.
For those of us who exercise frequently such as by riding a bike to work and back (like I did many times in my early years but not now), this explanation is long overdue. And it explains a lot of the mysterious ‘chronic fatigue’ I got labeled with.
My new understanding of post-exercise recovery
Exercise doesn’t end when the workout ends. As a diabetic, normally I do >30 minute exercise bouts after a solid meal, and if it’s closer to an hour or more, I’d eat fruit or granola bars during exercise, and then soon a meal time arrives and I should be covered. That’s not quite right according to muscle glycogen.
Instead, exercise ends when recovery is complete. Okay now I kinda get it. This glycogen thing also explains why I need many granola bars to raise blood glucose if it drops (I’m type 1 diabetic). Liver glycogen can already be depleted if my pre-exercise meal is too minimal, it seems. Then muscle glycogen is needed to sustain exercise and afterwards. Yikes, no wonder I’m super hungry no matter what my blood glucose is after major exercise. I’ve gotten older too, so 2 hours of playing anything wipes me out.
Better practices going forward
Before exercise
- A solid meal that includes protein, to keep blood glucose stable, in my case. I’ll experiment with this.
During exercise
- Snacks during exercise can help you keep going.
After exercise
- Those earlier snacks and meal don’t always prevent the crash afterward. Recovery needs to send a clear signal to the body that the stress is over.
For harder exercise, that usually means eating within an hour after exercise, preferably containing:
- Some protein
- Some carbohydrates
- A small amount of fat
This doesn’t require supplements or perfection. Regular foods have already proven best for me. I always try to have yogurt and cottage cheese on hand, plus eggs and milk, and a variety of cheese. Those are easy proteins to grab.
What to do on non-exercise days
Glycogen (the muscle type) isn’t just for managing physical activity. It’s also stored on light or rest days. This only happens if you eat consistently and don’t skip meals. I frequently skip breakfast…er, I mean I used to…but no longer since I read about this in the past few days. Skipping breakfast or undereating on “leisurely” days can quietly leave the glycogen bank half empty for the next exercise-rich day.
I’ve learned to plan for fatique and recovery differently
Exhaustion from exercise may not indicate poor fitness or poor training regimes. It may instead mean your body needs better recovery.
Although I was quite athletic with running and bike races in my early adulthood, recovery meant rest days for muscle rebuilding, not glycogen replenishment. The word glycogen only ever came up when I was talking with medical staff about diabetes.
So that was a lost opportunity. Now please do read up on glycogen yourself – I’m not a nutritionist and am only educated by information I could find online including ChatGPT (which conveniently provided the same information as Mayo Clinic and other go-to reputable sources (here’s just one article of many on exercise and glycogen).
Still using rules that don’t work for you?
I write for people who suspect the rules they were given don’t quite fit. If this article sparked your imagination, you can bookmark my home page or find more of my reflections on YouTube. My youtube button is up in the menu.
Reminder: find your own sources on this topic; I’m not a nutritionist, but an avid exerciser that learned something worth sharing to spark your curiosity.
More sources to explore on exercise recovery, fatigue, and glycogen:
- Search and look for articles hosted on National Institutes of Health (health topics section) or PubMed Central, such as “muscle glycogen metabolism” and “recovery after exercise”
- Try Mayo Clinic (library section) for a consumer-friendly feel. Search for “glycogen exercise recovery” and “muscle glycogen fatigue”
Notes:
- Cover image by Mabel Amber, from Pixabay