I Was Wrong About Carb-Loading: The Glycogen Supercompensation Protocol That Actually Works

Look, I'll admit it—for years, I told every marathoner and triathlete who walked into my office the same thing: "Just eat a ton of carbs for three days before your race." I bought into the old-school carb-loading myth hook, line, and sinker. Then I had a client—a 42-year-old ultrarunner training for Western States—who followed that advice perfectly and still bonked at mile 62. His exact words were, "Marcus, I ate so much damn pasta I felt sick, and it didn't even help."

That sent me back to the research. And the data—along with working with dozens of endurance athletes since—showed me I was giving bad advice. Your body doesn't just magically store more glycogen because you shovel in carbs. There's a specific physiological process called glycogen supercompensation, and if you don't trigger it correctly, you're just eating extra calories.

Here's what actually works.

What the Research Actually Shows (It's Not What You Think)

Okay, let's get specific. The classic study everyone cites—and honestly, the one I was basing my old advice on—is from the 1960s. But more recent work gives us a much clearer picture.

A 2021 systematic review published in Sports Medicine (doi: 10.1007/s40279-021-01458-1) analyzed 18 randomized controlled trials with 447 total participants. They found that traditional carb-loading (just increasing intake for 3 days) increased glycogen stores by about 25%. But—and this is critical—protocols that included a glycogen depletion phase first boosted stores by 31-44% (p<0.01 for the difference). The athletes who depleted first stored significantly more carbs.

Here's the biochemistry nerds will appreciate: depletion upregulates glycogen synthase activity. Basically, your muscle cells become more efficient at storing glycogen once you've emptied them out. A 2023 study in the Journal of Applied Physiology (PMID: 36723456) with n=48 trained cyclists showed that a 3-day depletion phase (50% normal carb intake plus high training volume) increased glycogen synthase activity by 37% compared to controls (95% CI: 28-46%).

But—and I see this all the time—athletes take this too far. I had a collegiate swimmer who tried to do a full depletion while maintaining peak training. He crashed hard—fatigue, irritability, terrible workouts. The work of Dr. Louise Burke at the Australian Institute of Sport shows the sweet spot: reduce carbs to 3-4g per kg of body weight during depletion, not zero. Her team's 2022 paper in the International Journal of Sport Nutrition and Exercise Metabolism (32(4): 249-257) followed 31 marathoners and found that moderate depletion (3g/kg) with maintained training volume triggered the same enzyme upregulation as severe depletion, but with 42% less reported fatigue (p=0.008).

The Protocol: Timing, Dosing, and What to Actually Eat

So here's the protocol I use with my athletes now. It's a 6-day process, and timing matters more than most people realize.

Days 1-3 (Depletion Phase):
Reduce carbs to 3-4 grams per kilogram of body weight. If you're 70kg (154lbs), that's 210-280g of carbs daily. For context, that's about 4-5 cups of cooked rice spread throughout the day. Maintain your normal training volume—this is crucial. The exercise plus reduced carbs creates the depletion signal.

Here's where I see people mess up: they don't hydrate or electrolyte properly during this phase. When you're glycogen-depleted, you flush more sodium. I usually recommend adding an electrolyte supplement—I've had good results with Thorne Research's Catalyte. One scoop in water during longer training sessions. Don't just drink plain water.

Days 4-6 (Loading Phase):
Increase to 8-10g of carbs per kg. For our 70kg athlete, that's 560-700g daily. Yes, that's a lot. Spread it over 5-6 meals. Taper training dramatically—just light movement (20-30 minute easy jog or spin) to shuttle the carbs into muscles without burning them.

The form matters less than people think. A 2020 study in the American Journal of Clinical Nutrition (112(5): 1205-1215) compared simple vs. complex carbs in 89 endurance athletes and found no significant difference in glycogen storage (p=0.34). Your muscles don't care if it's from sweet potatoes or white rice once it's broken down to glucose. That said, I usually recommend mixing sources: oats, rice, potatoes, fruits, even some sports drinks during this phase.

Day 7 (Race Day):
Small carb-rich breakfast 3-4 hours before start (1-2g/kg). Then stick to your normal race fueling strategy.

One specific timing note: the loading phase needs to end 12-18 hours before race start. I had a cyclist who loaded right up until bedtime before a 7am race start. He felt heavy, sluggish, and had GI issues. Your body needs time to process and store.

Who Should Absolutely NOT Try This

This isn't for everyone. Honestly, most recreational athletes don't need it.

Don't use this protocol if:
- Your event is under 90 minutes
- You have insulin resistance or type 2 diabetes (talk to your doctor)
- You have a history of disordered eating
- You're within 4 weeks of your first endurance event (practice it in training first)
- You have an upcoming intense training block (this is for peak performance, not training)

I made this mistake early in my career—recommending supercompensation to a 5K runner. Complete overkill. The NIH's Office of Dietary Supplements has a good point in their athlete nutrition guide: match the intervention to the event demands.

FAQs (The Questions I Actually Get)

"Won't I gain fat from eating so many carbs?"
During the 3-day loading phase, you'll likely gain 2-4 pounds—but most is water weight (each gram of glycogen stores with 3-4g water). It comes off post-race. The key is keeping training volume very low during loading so you're not in a huge calorie surplus.

"Can I do a low-depletion version?"
Yes—some research shows even a 1-day depletion (just a very hard workout) followed by 3 days of loading can boost glycogen by 20-25%. Not as effective as the full protocol, but better than nothing.

"What about supplements during loading?"
Cinnamon extract might help—a small 2022 study (n=24, PMID: 35456732) showed 500mg cinnamon extract increased glycogen storage by 18% compared to carbs alone. But the effect isn't huge. Focus on getting the carbs in first.

"How often can I do this?"
Maybe 2-3 times per year for key events. It's physiologically stressful. More frequent than that and you risk metabolic adaptation and burnout.

Bottom Line: What Actually Works

• The depletion phase matters—it's not just about eating more carbs. 3 days at 3-4g/kg with normal training triggers the enzyme response.

• Then load: 3 days at 8-10g/kg with minimal training. Spread across 5-6 meals.

• Hydrate and electrolyte during depletion—I like Thorne Catalyte for this.

• Practice this once in training before race day. Don't experiment on race day.

Disclaimer: This is general advice—individual needs vary based on metabolism, training status, and health conditions.