For years, the field of sports nutrition has been simplified by a common analogy: the body functions like an engine, with glycogen acting as its fuel, leading to exhaustion when supplies run low. This straightforward perspective suggested that to enhance performance, one should consume a large amount of carbohydrates, continuously refilling the “tank” during exercise. However, recent research reveals that this understanding of nutrition and exercise is far more complex.
A review published in January 2026 analyzed over 160 studies on carbohydrate intake, metabolism, and usage, concluding that the simplistic “engine” analogy may not accurately capture the intricacies of our physiology. While carbohydrates are certainly not useless, their primary role in exercise may differ from common beliefs.
Understanding Muscle, Energy, and Our Assumptions
The long-standing model of sports nutrition primarily focuses on muscle, built upon three foundational assumptions:
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Muscles rely on glycogen.
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Fatigue occurs when glycogen is depleted.
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Thus, maximizing glycogen reserves and carbohydrate intake is essential.
This perspective gained traction in the 1960s when advancements such as muscle biopsies allowed researchers to measure glycogen levels before and after exercise. Observations indicated that athletes with higher glycogen levels could endure longer periods of intense exertion, leading to the widespread practice of “carb-loading” prior to competition.
However, a crucial element was often ignored: the fluctuations of blood glucose levels and the impact on the central nervous system as athletes approached their limits.
The Role of Blood Glucose and the Liver
The recent review emphasizes the importance of a less obvious component: the small amount of glucose circulating in the blood and the liver’s role in regulating its stability. At any moment, the blood contains only a few grams of glucose—a small reserve that is critical for brain function. When extended exertion causes blood glucose levels to drop and the liver can’t compensate, the body perceives a potential threat; continued depletion can lead to hypoglycemia and even brain damage.
The nervous system responds by activating a protective mechanism, reducing the number of motor neurons engaged, decreasing overall strength, and prompting a slowdown or halt, even if the muscles are still capable of contracting. Therefore, fatigue can be viewed not as a mere “empty fuel tank,” but rather as a built-in safety system to prevent injuries or accidents.
Experiencing “Hitting the Wall”
Anyone who has participated in a marathon or similar endurance event is familiar with the term “the wall.” Traditionally, this has been ascribed to a depletion of glycogen reserves.
Yet, more nuanced studies reveal a different picture. Research indicates that in groups that do not consume carbohydrates, blood glucose levels consistently drop, whereas in those who do, the decline slows or stops, allowing for sustained performance. Carbohydrates, therefore, seem to serve less to “feed the muscle” and more to maintain safe blood sugar levels, shielding the nervous system. For the most part, “the wall” appears to be the body’s way of activating its safety brakes.
There are strong arguments for this interpretation. When muscles truly exhaust their supply of ATP (adenosine triphosphate, the cellular energy currency), they exhibit rigidity akin to rigor mortis. However, athletes typically experience a gradual decline in performance rather than a sudden failure.
Determining Carbohydrate Needs
If the principal function of carbohydrates during exercise is to stabilize glucose levels and avert hypoglycemia, the pertinent question shifts from “how much do we need?” to “what is the minimum requirement in different situations?”
Notably, many previously established recommendations have come under scrutiny. High carbohydrate intakes of 60-90 grams per hour, once thought necessary for prolonged efforts, may still apply in specific circumstances, such as elite competitions. However, recent evidence suggests that much smaller amounts can have similar effects.
The recent review posits that 15-30 grams of carbohydrates per hour during extended activity can yield performance enhancements comparable to much larger quantities. The key takeaway is not merely the amount consumed, but rather the importance of preventing significant decrements in blood glucose levels. Once stabilized, increasing intake does not necessarily yield additional benefits.
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This shift transforms nutritional strategies. Instead of chasing ever-larger carbohydrate doses—which may lead to gastrointestinal discomfort, psychological reliance on energy gels, and increased costs—the focus can be refined to identifying the minimum level necessary to stabilize glucose, thus sustaining performance.
Moreover, exceeding certain carbohydrate thresholds can lead to unexpected consequences, including reduced fat oxidation and increased insulin levels, and in some cases, depletion of muscle glycogen instead of preservation. This contradicts the desired outcomes for athletes.
In essence, carbohydrates function as a buffer for glucose, not as an unlimited fuel source. Simply put, once the decline has been mitigated, further additions do not produce a corresponding advantage.
The Perspective of Low-Carb Athletes
Challenging the predominant carbohydrate-centric beliefs, the performance of athletes on low-carb diets is noteworthy. These athletes often exhibit remarkably high fat oxidation rates, even at intense levels (above 85% of VO2 max), with performance levels comparable to those following carbohydrate-rich regimens.
This doesn’t necessarily imply that fat is a superior fuel source. Rather, it signals that the body can adapt to utilize fat even during strenuous activities. The previously held belief that “high intensity requires carbohydrates” may not be as universally applicable as once thought.
This new understanding leans on the concept of metabolic flexibility, which describes the ability to switch between fuel sources based on demand and availability. A consistently high-carbohydrate diet, without strategic variation, can diminish the body’s signals to utilize fat, creating a dependence on quick energy sources like gels.
The solution is not a complete reduction in carbohydrate intake but rather a strategy of periodization—training methods that encourage greater fat utilization while strategically employing carbohydrates when necessary.
Reevaluating Our Relationship with Carbs
What does all of this imply for athletes in training or competition?
First, it clarifies the role of carbohydrates. They should not be idolized or dismissed; they are simply a resource. The focus should shift from “more is better” to key questions such as:
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What are my goals for today? Is it peak performance in the moment, or enhancing metabolic flexibility over time?
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How does my body respond? Monitoring for signs of hunger, fatigue, “hitting the wall,” digestive issues, and dependency is crucial.
Second, it highlights that performance ceilings are not solely influenced by muscle capabilities. The brain continuously monitors glucose and other fuel levels; when it detects an imbalance, it lowers power output. When properly applied, carbohydrates can postpone the engagement of this regulatory mechanism, primarily by keeping glucose levels stable rather than simply replenishing glycogen stores.
Lastly, it’s important to acknowledge that general guidelines are just starting points. Individuals with diabetes, a tendency toward hypoglycemia, or those on specific medications should seek tailored guidance, preferably under professional supervision.
The future of sports nutrition does not lie in an increased reliance on sugar, but rather in training the metabolism to utilize the most effective fuel source at any given time. Carbohydrates will continue to have a significant role, primarily as a minimal effective dose for the brain, not as an unquestionable source of muscle energy.
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