New Study Challenges Carbo-Loading for Endurance

For decades, athletes have followed a trusted formula for success: load up on carbohydrates. The pre-race pasta dinner is a ritual, built on the belief that packing muscles with stored carbs, known as glycogen, is the key to unlocking peak performance. This practice, called carbo-loading, became the gold standard after studies in the 1960s suggested that more glycogen meant more fuel in the tank.

But what if this long-held strategy is something we should reconsider? A comprehensive new review of over 160 studies challenges the foundation of carbo-loading, and the results were published in Endocrine Reviews. It suggests that the real key to endurance isn’t cramming your muscles with carbs, but rather maintaining stable blood sugar.

How Was The Study Conducted?

Researchers conducted a massive review, analyzing data from over 160 different studies published over more than 50 years. This comprehensive approach allowed them to connect the dots between decades of sports science research, from the earliest studies using muscle biopsies in the 1960s to modern experiments tracking fuel use in real time.

The analysis covered a wide range of topics, including the effects of different diets (high-carb vs. low-carb), the impact of ingesting carbohydrates during exercise and the body’s hormonal responses to various fueling strategies. The researchers re-examined studies that formed the basis of carbo-loading, looking at the data through a new lens. They synthesized findings from experiments that measured muscle glycogen, blood glucose levels, fat and carbohydrate oxidation rates and exercise duration to exhaustion.

What Did The Study Find?

The review’s conclusions turn conventional sports nutrition on its head. The central finding is that fatigue during prolonged exercise is not primarily caused by muscles running out of glycogen. Instead, the most critical factor is a drop in blood glucose levels, a condition known as exercise-induced hypoglycemia (EIH).

Here are the key takeaways:

• Blood sugar is key. The researchers found that when blood glucose falls, the brain initiates a protective response. It reduces the signals sent to the muscles, effectively putting the brakes on performance to prevent potentially dangerous low blood sugar in the brain. This suggests fatigue is a brain-regulated safety mechanism, not a local energy crisis in the muscles. The original 1967 study that popularized carbo-loading actually showed subjects experienced severe exercise-induced hypoglycemia.
• Small amounts of carbs are highly effective. The analysis showed that ingesting even small amounts of carbohydrates during exercise (as little as 10-15 grams per hour) was enough to reduce the risk of hypoglycemia and improve performance. Surprisingly, the review found no clear evidence that higher doses (like the sometimes recommended 90-120 grams per hour) provide a dose-dependent performance boost. The primary benefit seems to come from stabilizing blood sugar, not from providing massive amounts of external fuel.
• Carbo-loading doesn’t “spare” muscle glycogen. A popular theory is that eating carbs during exercise helps “spare” the glycogen stored in your muscles. The review found this to be largely untrue. In fact, high rates of carbohydrate ingestion often increase the rate at which muscle glycogen is burned by suppressing the body’s ability to use fat for fuel. The one thing that carb intake does spare is liver glycogen, which is the organ responsible for maintaining stable blood sugar.
• Fat is a more important fuel than believed. The paper highlights that even at high intensities, the body is capable of using fat for fuel much more effectively than previously thought, especially in athletes adapted to a lower-carbohydrate diet.

Limitations of the Study

As a comprehensive review, this work relies on interpreting previously published data. While it analyzes over 160 studies, the individual experiments had their own specific designs, participant groups and limitations. For instance, many of the foundational studies were conducted on small groups of male endurance athletes, so the findings may not apply equally to everyone.

Furthermore, the review presents a new interpretation of existing evidence rather than generating entirely new data. Because of these factors, we have to take the results with a grain of salt.

How Does This Apply To Real Life?

The practical implications of this review are important for athletes of all levels. It may be time to reconsider opting for lots of carbohydrates before a big workout. Instead, the goal may need to shift from maximizing carb intake to optimizing blood sugar stability.

This doesn’t mean abandoning carbs, as they are still a vital energy source. However, it does suggest that you may not need nearly as many as you think. Instead of huge pasta dinners and sugary gels every 20 minutes, a more moderate approach might be more effective. These results suggest that focusing on timing your nutrition to prevent blood sugar crashes may be even more important. A small, steady supply of carbs during a long workout or race may be enough to keep your brain happy and your muscles firing.

Our Expert Take

This comprehensive review published in Endocrine Reviews challenges decades of thoughts about carbo-loading and endurance performance. By examining more than 160 studies, the authors suggest that fatigue during prolonged exercise is driven not by muscle glycogen depletion, but by drops in blood sugar, which is a signal that prompts your brain to protect itself. Importantly, the research shows that just a modest intake of carbohydrates during activity is usually enough to prevent these blood sugar crashes, while larger amounts offer no extra benefit and may even impact your long-term health.

For athletes and fitness enthusiasts alike, the takeaway is simple: it may be best to focus less on overloading your body with carbs and more on keeping your blood sugar steady. This can be accomplished with smaller, well-timed doses of carbohydrates, allowing your body to use both carbs and fat efficiently for fuel. As science continues to evolve, a balanced and individualized approach, ideally guided by a registered dietitian, remains the smartest strategy for health and peak performance.