The Most Influential Study in Sports Science Was Done on Regular Students, Not Elites. And It Got Muscle Glycogen Completely Wrong

Professor Tim Noakes has spent nearly five decades studying endurance performance, first as an athlete, then as one of the world’s leading sports scientists.

Recently, he and his colleagues published a groundbreaking paper that challenges one of the most sacred beliefs in endurance sports.

The claim? That muscle glycogen depletion—running out of stored carbohydrates in your muscles—is not what causes you to hit the wall.

Instead, it’s your blood glucose levels and brain function that determine when you slow down or stop.

The Paper That Changed Everything

For decades, sports science has operated on a simple principle: eat more carbs before exercise, store more glycogen in your muscles, perform better.

This idea stems from a famous 1967 study conducted on recreational physical education students in Stockholm. The research showed that athletes who carb-loaded could exercise longer than those who didn’t.

I was a medical student in 1969, 1967, this famous paper comes out from Stockholm where they used a muscle biopsy technique to measure the glycogen in the muscle. And they found that if you ate lots of carbohydrate before exercise, you could exercise for much longer.

But Noakes noticed something everyone else missed.

When he re-examined the data, he found that all athletes stopped exercising at similarly low blood glucose levels—not at the same muscle glycogen levels.

Two Glucose Pools: The Key Discovery

Noakes and his team identified two separate carbohydrate stores in the body, each with entirely different functions.

The small pool: blood glucose and liver glycogen (about 100-120 grams total). This pool exists to keep blood sugar stable and fuel the brain.

The large pool: muscle glycogen (can exceed 400 grams). This acts as a storage dump for excess dietary carbohydrate.

There are two pools of glucose in the body. The ones in the bloodstream and in the liver, and that has one function. To keep the blood glucose stable. It’s not to make you a better runner and to set a world record. It’s there to make sure your blood glucose stays stable.

Critically, these pools are regulated independently. Muscle glycogen doesn’t directly fuel performance—it influences which fuel you burn during exercise.

Why Muscle Glycogen Doesn’t Drive Performance

Noakes reviewed over 100 years of scientific literature and found 16 reasons why muscle glycogen is not the limiting factor in endurance performance.

One of the most compelling: every study showing carbohydrate improves performance also showed blood glucose falling in the control group.

I did an analysis, and it showed the only commonality in those studies was the blood glucose level at stopping when they stopped. There was little variation in it, but the muscle glycogen was much different between the guys who’d eaten lots of carbs and not.

In other words, carbs worked because they prevented hypoglycemia—low blood sugar—not because they refilled muscle stores.

When blood glucose drops below a certain threshold, the brain perceives danger and forces you to slow down to preserve fuel for critical brain function.

Muscle Glycogen as a Fuel Switch

Muscle glycogen doesn’t fuel performance directly, but it does determine what you burn.

High muscle glycogen elevates insulin, which suppresses fat oxidation and forces your body to burn more carbohydrate—even at rest.

If you eat a high carbohydrate diet and your muscle is full of carbohydrate, your insulin is elevated. And because your insulin is high, the muscles will burn the carbohydrate first. They will inhibit fat.

Conversely, starting exercise with low muscle glycogen keeps insulin low, allowing your body to burn fat at much higher rates—even at intensities up to 85% of maximum.

The Real Role of Carbohydrate During Exercise

So if muscle glycogen doesn’t matter, what does?

Blood glucose regulation.

During prolonged exercise (longer than about 2 hours), the liver struggles to produce enough glucose to meet demand from muscles and brain.

That’s when consuming carbohydrate becomes essential—not to fuel muscles, but to prevent hypoglycemia and keep the brain functioning optimally.

You only need 10 to 20 grams an hour to maximize your metabolism. That’s the fueling. But if you want to stimulate the brain, then you’ve got to take 120 grams an hour. And that’s the difference.

The 120 Grams Per Hour Controversy

Elite marathoners and triathletes now consume upwards of 120 grams of carbohydrate per hour during competition.

Noakes initially dismissed this as unnecessary—until evidence from world record performances forced him to reconsider.

Because I put myself out on Twitter, I went on a limb and the limb got cut off. You have all convinced me that taking 120 grams an hour works. So I changed within 2 minutes. I said, I apologize. I’ve been wrong.

The explanation? At such high doses, carbohydrate acts as a pharmacological stimulant to the brain, reducing mental fatigue and perception of effort.

It’s not physiology—it’s pharmacology.

Practical Implications for Everyday Athletes

For recreational runners and age-groupers, the message is clear but nuanced.

Pre-exercise carb loading is largely irrelevant. Whether you start with high or low muscle glycogen won’t significantly impact performance.

What you eat daily matters more for metabolic health. High-carb diets can promote insulin resistance, especially in those predisposed to it.

During exercise lasting over 2 hours, consume 10-20 grams of carbohydrate per hour to maintain blood glucose and prevent hypoglycemia.

Elite athletes chasing podiums may benefit from 120 grams per hour—but this is a brain stimulant, not a muscle fuel strategy.

If you are just wanting to run and complete the event, your 20 grams an hour is all you need. And a low carbohydrate diet is going to be more beneficial because you’re going to be at less risk of developing insulin resistance and type 2 diabetes.

The Insulin Resistance Risk

Noakes emphasizes a critical point often ignored in sports nutrition: chronic high-carbohydrate intake can lead to insulin resistance and type 2 diabetes, even in lean, active individuals.

In one study, three out of ten recreational athletes tested pre-diabetic on a high-carb diet. When switched to a low-carb, high-fat diet, all markers normalized.

More and more people in their 30s and 40s, recreational athletes, are coming to us with pre-diabetes. And they can’t understand it. I’m a runner. I’m lean. How can I possibly have this?

Noakes recommends checking fasting blood glucose, fasting insulin, and HbA1c with a physician who understands insulin resistance.

Training on Low Carbohydrate

Starting runs in a fasted state with low muscle glycogen doesn’t impair performance—it enhances fat oxidation.

Consuming a small amount of carbohydrate during exercise (10-20g/hour) maintains blood glucose without shutting down fat burning.

This strategy allows athletes to become highly fat-adapted while still preventing hypoglycemia.

If you stop eating carbohydrates, your rate of fat oxidation can triple in a few days. That’s very clearly shown.

The Central Governor and the Brain’s Role

Noakes is also known for his “Central Governor” theory, which posits that fatigue is regulated by the brain to prevent catastrophic failure.

Updated versions of this model incorporate emotion, intellect, and what researchers call the “impending action crisis”—the mental battle between your goal and the rising discomfort.

The brain ultimately determines your performance. But the paradox is that it’s the muscles telling the brain I’ve had enough. Your intellect and your emotion says, okay, that’s enough. And then you fall off the pace and then you quit.

This explains why taking carbohydrate can reverse fatigue so rapidly—it’s not refueling muscles, it’s reassuring the brain that glucose supply is secure.

The Norwegian Training Method

When asked for his final practical advice, Noakes pointed to the Norwegian method of polarized training.

This approach emphasizes high training volume at very low intensity, combined with strategic high-intensity sessions—avoiding the “gray zone” where most recreational athletes mistakenly spend their time.

Norwegian athletes like Johannes Høsflot Klæbo and Kristian Blummenfelt dominate across distances from sprints to ultra-endurance events using this model.

The key is how much below intensity training you’re doing. It’s got to be really low. And it was interesting—John Korir who won the Boston Marathon, someone showed his Garmin and he was running at 9 minutes a mile in training.

What This Means for You

The science is clear, even if it contradicts decades of conventional wisdom:

• Muscle glycogen is not the performance limiter you’ve been told it is
• Blood glucose regulation is what really matters during endurance exercise
• Carb loading before events has minimal impact on performance
• Small amounts of carbohydrate during exercise (10-20g/hour) prevent hypoglycemia effectively
• Ultra-high carb intake (120g/hour) works as a brain stimulant, not muscle fuel
• Low-carb diets improve fat oxidation and may reduce metabolic disease risk
• Most athletes would benefit from eating less carbohydrate daily and strategically using it during long efforts

Before making dramatic changes, Noakes recommends getting metabolic markers checked and working with a healthcare provider who understands insulin resistance.

The debate between Noakes and his colleague Professor Louise Burke is published in the American Journal of Clinical Nutrition, where both scientists present their evidence.

After 50 years of research, one thing is certain: the conventional carbohydrate-centric model of endurance performance is being fundamentally challenged—and the implications for athletes at every level are profound.