Landmark Review Challenges Sports Nutrition’s Carb Obsession and Points to Health Risks

By Andrew Koutnik, Ph.D.

For decades, athletes have heard the same refrain: load up on carbs or you’ll “hit the wall.” Consuming pasta dinners and copious sports drinks or gels has become standard advice—from textbooks, dietitians, and sports nutrition companies alike. A major publication in Endocrine Reviews calls that guidance into question.

My colleagues and I—researchers from the University of Cape Town, Grove City College, The Ohio State University, the University of South Florida, and Florida State University—analyzed more than 160 exercise performance studies and nearly 600 papers in sports nutrition, spanning over a century of research. Our meta-analysis is the largest, most comprehensive review ever conducted on carbohydrate intake, exercise metabolism, and performance.

Our conclusion is clear-cut: “Carb-loading” doesn’t boost performance the way it’s widely believed to—and high-carb fueling strategies may put athletes’ long-term health at risk.

A summary of our key findings:

• The strongest predictor of workout fatigue is blood sugar—the source of brain energy—not muscles running out of stored carbs or the amount of carbs burned.
• Many athletes are consuming 6 to 9 times more carbs than necessary to maintain performance.
• High-carb fueling may carry long-term health tradeoffs, including higher insulin levels, lower fat oxidation, and other biomarkers consistent with prediabetes.
• Athletes can perform just as well on a lower carb intake, as long as blood sugar and brain energy are stable.
• Carbs during exercise help most when they prevent blood sugar from dropping to maintain the brain’s energy source. Consuming more carbs after blood sugar is stable doesn’t provide any additional performance benefits.

Our research is especially relevant in light of recent updates to the Dietary Guidelines for Americans, which call for reduced consumption of refined carbohydrates and note that people with certain chronic diseases may experience improved health outcomes when following a lower carbohydrate diet.

Below, we walk through what the data shows and the important implications for you from its findings.

The Myth: Athletes Fatigue When Muscles “Run Out” of Fuel

The reasoning behind carb-loading goes like this:

• Muscles store carbs as glycogen (about 300-500 grams).
• During long or hard exercise, glycogen runs low.
• Once glycogen is depleted, muscles can’t produce enough energy, and you “hit the wall.”

With this logic, more carbs must equal better performance: the more you eat, the more glycogen you store, the longer you can go. Accordingly, the Academy of Nutrition and Dietetics, Dietitians of Canada, American College of Sports Medicine, and International Society of Sports Nutrition all recommend up to 8-12 grams of carbs per kilogram of bodyweight. That could push an average bodyweight male and female to eat as many as 1,000 and 800 grams of carbs per day, respectively.

In our analysis, we examined the data closely—especially what was happening metabolically at the moment when athletes fatigued. A very different picture emerged.

The Reality: Athletes Fatigue When Blood Sugar (Brain Energy) Drops

Across hundreds of studies, we found that the best predictor of an athlete’s fatigue wasn’t muscle glycogen or the number of carbs they burned—it was blood sugar.

Normal blood sugar levels amount to about one teaspoon of glucose circulating in the bloodstream. That’s all. So when that small pool drops too low during exercise, the brain senses danger and hits the brakes on physical effort, producing the feeling that most athletes describe as “hitting a wall.”

In our review, we found:

• The data are unmistakable: In 88% of studies where carbs enhanced performance, they did so by holding blood sugar steady. The decisive factor wasn’t fueling muscles—it was protecting the brain from falling glucose.
• Muscle glycogen (stored carbs in muscles) and carbohydrate oxidation levels (the amount of carbs burned) did not reliably predict performance outcomes.

In other words, carbs help mainly when they prevent blood sugar and brain energy from crashing, not because muscles run out of fuel. This has profound implications for how much and what type of fuel athletes should consume.

Many Athletes Eat Six to Nine Times More Carbs Than Needed

Current guidelines recommend 60-90 grams of carbs per hour for endurance exercise—roughly a bowl of pasta, two baked potatoes, or 6-9 pieces of toast for every hour an athlete is moving. In practice, athletes consume 30-60 grams of carbs per hour on average.

Our analysis suggests this is far more than necessary to maintain blood glucose and brain energy.

We found that as little as 10 grams of carbs per hour is typically enough to keep blood sugar stable and maintain performance—even for strenuous, prolonged exercise lasting two hours or more. Once blood sugar is stabilized, more carbs do not reliably translate to better performance.

Beyond a modest dose, extra carbs mostly further:

• raise insulin.
• suppress fat burning.
• force reliance on sugar burning.
• increase glycogen breakdown.
• increase the risk of gastrointestinal distress.

The Health Risks of High-Carb Fueling

One of the more concerning findings in our review is what happens when athletes follow aggressive high-carb fueling strategies. In otherwise healthy athletes, we found that high carb ingestion created a metabolic profile similar to prediabetes and diabetes, with:

• elevated blood insulin levels.
• low rates of fat oxidation, or low fat burning.
• greater reliance on sugar for energy, including muscle glycogen.

When the body is too dependent on sugar for energy, it loses metabolic flexibility—making it harder to switch gears from burning carbs to burning fat efficiently. That can be a liability for athletic performance and long-term health alike.

These findings build on our previous research showing that athletes consuming high-carb diets over 4 weeks can develop prediabetes blood sugar levels—despite exercising frequently, having low body fat, and above-average cardiorespiratory endurance. Yet, when athletes lowered their carb intake, these markers improved or normalized rapidly.

This suggests that the very fueling strategies marketed for “optimal performance” may put athletes’ metabolic health at risk in the long term.

Athletes Don’t Need High Carb Intake, Just Blood Sugar and Brain Energy Stability

Our review also examined studies comparing athletes on higher- and lower-carb diets.

We found that endurance athletes with lower-carb intake, lower muscle glycogen, and lower dependence on glucose for energy can perform just as well as high-carb athletes, as long as blood sugar and brain energy sources remain stable.

In low-carb athletes, we see that they have:

• lower blood insulin levels.
• enhanced fat oxidation, or higher rates of fat burning.
• no loss of performance when blood glucose and brain energy are maintained.
• better metabolic and fuel flexibility—efficiently switching between burning carbs and fat, and making and using alternative brain fuels (glucose, ketones, and lactate).

This directly challenges the idea that carbs are an obligatory fuel for performance. The body can maintain brain energy by using smaller amounts of glucose as needed—much of which it can produce itself—and run very effectively using fat as fuel.

Use Carbs as a Tool, Not a Requirement

The carb-centric, muscle-centric model of performance has shaped sports nutrition for half a century. It influences what dietitians and coaches recommend; the products sports nutrition companies formulate and sell; and how athletes at every level eat and drink before and during competition.

Our review suggests that this model is not aligned with the data, misguided, and now outdated. The evidence points to a simple, but profound shift in how we should think about fueling:

• The primary role of carbs during exercise is to protect the brain by preventing blood sugar from crashing.
• Once that is achieved, more is not better. Performance does not appear to scale with higher and higher carb intake.
• Regular high-carb fueling can push athletes toward an unhealthy metabolic profile, potentially raising the risk of prediabetes.
• This also raises the question of whether carbohydrates are required at all since the brain can run on multiple fuel sources (glucose, ketones, lactate).

Carbs are a tool, not a requirement. For many athletes, the needed “dose” is far smaller than what they’ve been told. As little as 10 grams per hour will suffice for strenuous exercise longer than 60 minutes, and exercise for less than 60 minutes doesn’t require any carbs at all. By focusing on blood sugar and brain energy stability rather than maximum carb intake, athletes can sustain their performance in training and competition while preserving their metabolic health.

It’s time for sports nutrition to move beyond carb obsession and towards a more nuanced, health-conscious view: Fuel just enough to keep the brain safe—and let the body do what it’s already built to do.

Andrew Koutnik, Ph.D., is a principal investigator and research faculty at Florida State University’s Institute of Sports Sciences and Medicine, a visiting research scientist at the Florida Institute for Human and Machine Cognition, and a member of the Coalition for Metabolic Health’s advisory committee.