Base Knowledge: Glycogen

Back-up blood sugar stored in your liver and muscles needed for hard workouts.

Talking sports nutrition requires a certain degree of highfalutin jargon, so I’m starting a feature called “Base Knowledge” within which we define 25¢ words essential for any committed active person. Then, when I refer to them in later newsletters, I’ll just link back to the Base Knowledge article.

(This whole hyperlink thing is super useful. I think it could really catch on!)

Today, let’s talk glycogen.

When you exercise, you use two types of fuel: fat and carbohydrates. Together, you’ll hear them referred to as substrates, so when Dr. Fancy Pants says, “energy substrate utilization,” they’re talking about whether you’re burning carbs or fat.

Generally, the harder you push yourself, the more you use carbs. On the other hand, easier efforts tend to burn fat. Going for a walk or light jog burns more fat. Doing sprint drills or running uphill burns more carbs. It works on a continuum, not like a light switch, so for most exercise, you weave back and forth between fat and carb burning.

How your body accesses carbs and fat is wildly complex. If you’re looking for an in-depth explanation, check out Why Calories Don’t Count by Dr. Giles Yeo. (Not to be confused with Dr. Fancy Pants.) Long story short, fat can convert to carbs, carbs can convert to fat, and protein can convert to both. Anyone trying to simplify all this Nutri Fu© is in for a world of hurt—but I’m going to give it a go anyway.

Fat shuttles around in your blood as blood lipids called cholesterol and triglycerides. It’s also stored as adipose tissue, also known as body fat.

Carbs float around your blood as glucose, also known as blood sugar. When you put forth a hard effort, your body uses glucose for fuel. Unfortunately, we don’t have a ton of it. According to The American Journal of Physiology - Endocrinology and Metabolism, a 154-pound person has about 16 calories, or 4 grams, floating through your veins at any given time.

Luckily, we have a back-up supply of carbs called glycogen, which you’ll find it in your liver and your muscles. If you’re doing squats, your body will utilize the glycogen in your liver and your glute muscles, quad muscles, hamstring muscles, et al. as energy. Keep in mind that muscle glycogen is site-specific, so the glycogen in your biceps, for example, won’t come to the party on leg day.

When your glycogen stores are completely full, you can do about 90 minutes of intense activity before you run out. When you run out of glucose and glycogen, this is known as “bonking.” The French call it Le Fringale. You know it when it happens. It’s like hitting a wall—and not a soothing, Aldous Snow furry wall. I’m talking a Wile E. Coyote-flattening wall. Something that leaves a mark.

Most of the time, athletes tend not go full-bore out of the gate unless they’re a horse, a greyhound, or a cyclocross racer. Because fat and carb utilization works on a continuum, this means you might go more than 90 minutes before bonking, even if you don’t feed. On the other hand, if your glycogen stores aren’t full, that bonk could come much sooner. If you’re eating at a calorie deficit or you live a low-carb lifestyle, it’s likely you don’t have a ton of glycogen.

With all this in mind, if your sport of choice involves long, intense efforts, consider adding healthy carbs back into your diet. I’m not saying you need to apply for an Olive Garden Frequent Feeder Visa Card. I’m just saying that carby foods like whole grains, legumes, tubers, and fruit are super healthy for most people—especially athletes. They can play a big role in your diet and they can give you the energy you need to crush it.

Also, you might consider upping the carbs for a few days before a big effort. The fancy term for this is glycogen supercompensation. I’ll do an entire newsletter on this topic soon, but for now, just set aside all fear of carbs for three or four days before stepping up to the starting line.

Glycogen requires fluid to store in your muscles, so replenishing your glycogen can come with water weight gain. If this happens, remain calm. You did not gain three pounds of fat because you ate a bowl of pasta the night before your triathlon. That weight gain came partially from sodium and partially from increased glycogen stores. So, yay!

Glycogen is a hard thing to write about because it inspires me to go down so many groovy rabbit holes, but I think this should provide a solid baseline. We’ll geek out later.

But let me know if you have any questions!