Why Are Carbohydrates Important for Athletes?

Carbohydrates are the body’s preferred fuel for moderate-to-high intensity exercise, and the harder an athlete works, the more the body depends on them. Stored in muscles and the liver as glycogen, carbohydrates supply energy faster than fat can, and when those stores run low, performance drops sharply. But the importance of carbs for athletes goes well beyond just having enough fuel in the tank. They affect how the brain perceives effort, how the immune system responds to hard training, and how quickly the body recovers between sessions.

Why the Body Favors Carbohydrates at Higher Intensities

Both fat and carbohydrate supply energy during exercise, but they do not share the workload equally at all effort levels. At low intensities, fat oxidation handles a large share of energy production. As intensity climbs, the body shifts toward burning more carbohydrate and less fat. Researchers have described a “crossover point,” the exercise intensity at which carbohydrate-derived fuels become the dominant energy source, with further increases in effort driving an even greater reliance on carbohydrate and a corresponding drop in fat oxidation.¹ For most people, this crossover happens somewhere around moderate intensity. The practical upshot is straightforward: if you are training or competing at anything above an easy pace, carbohydrate is your primary fuel.

Endurance training does push that crossover point a bit higher, meaning a trained athlete can burn more fat at a given intensity than an untrained person. But even elite endurance athletes eventually hit a ceiling on fat oxidation and become heavily dependent on carbohydrate during race-pace efforts. Fat simply cannot be broken down fast enough to sustain the energy demands of high-intensity work.

Glycogen and the Onset of Fatigue

Muscles store carbohydrate as glycogen, and that glycogen is the most immediately accessible fuel for hard efforts. The relationship between depleted glycogen and fatigue has been studied for decades, and the connection is clear: when muscle glycogen drops too low, the muscle cannot produce energy fast enough to maintain the exercise intensity.² This is not just a matter of running out of gas in a general sense. Emerging research points to a more specific mechanism involving calcium handling inside muscle cells.

For a muscle fiber to contract, calcium needs to be released from internal storage compartments called the sarcoplasmic reticulum. When glycogen in the muscle gets low, that calcium release is impaired, and the force of contraction drops. This happens even when the muscle’s overall energy supply has not completely bottomed out, which suggests glycogen plays a direct role in the contraction process beyond simply providing fuel.^{3 4} In practical terms, this means an athlete can feel the effects of low glycogen not just as general tiredness but as a distinct loss of power, slower sprinting ability, and reduced coordination.

The Liver’s Role in Keeping Blood Sugar Stable

Muscle glycogen feeds the muscles directly, but it stays locked inside the muscle fibers that store it. The liver, on the other hand, can release glucose into the bloodstream, which feeds the brain and other tissues during exercise. When liver glycogen runs low, blood sugar can drop, leading to lightheadedness, difficulty concentrating, and that shaky, hollow feeling sometimes called “bonking.” Research in animal models has shown that higher liver glycogen levels help maintain blood glucose during exercise and improve endurance capacity, likely by keeping a steady glucose supply to the brain and working muscles.⁵

This two-tank system means athletes need to think about both muscle and liver glycogen. Pre-exercise meals and carbohydrate loading top off both stores, but because the liver’s reserves are smaller and get tapped to maintain blood sugar around the clock, they are more vulnerable to depletion, especially if an athlete trains first thing in the morning after an overnight fast.

Carbohydrate Loading Before Competition

The idea of carb loading has been around since the 1960s, and the science backs it up. By eating a high-carbohydrate diet in the days before a major event, athletes can push their muscle glycogen above normal resting levels, a state called supercompensation. One study in trained subjects found that a carbohydrate-loading protocol nearly doubled muscle glycogen compared with baseline, and those elevated levels held steady over the following three days.⁶

That last detail matters practically. It means an athlete who loads on Wednesday and Thursday can still benefit on a Saturday race, as long as they keep eating normally in between. The early protocols involved a brutal glycogen-depletion phase before loading, but modern approaches simply increase carbohydrate intake for one to three days before competition while tapering training. For events lasting roughly 90 minutes or more, the extra glycogen can meaningfully delay the point at which fatigue sets in.

Fueling During Exercise

Taking in carbohydrate during long efforts helps in two ways. First, it provides an external glucose supply that the muscles and brain can use, reducing the drain on stored glycogen. A recent meta-analysis estimated that consuming carbohydrate during about 100 minutes of exercise spares roughly 24 millimoles per kilogram of dry muscle weight of glycogen compared with a placebo, a small but consistent effect that could become meaningful in a long race or back-to-back training sessions.⁷

Second, the rate at which carbohydrate can be absorbed through the gut sets a ceiling on how much external fuel the body can use. Glucose alone tops out because the intestinal transporter responsible for its absorption has limited capacity. Researchers found that mixing glucose (or maltodextrin) with fructose gets around this bottleneck, since fructose is absorbed through a different transporter. Studies have shown that combining the two can increase the rate of carbohydrate being burned from ingested sources by up to about 65% compared with glucose alone.^{8 9} This is why most modern sports drinks and gels designed for endurance events use a glucose-fructose blend rather than a single sugar type. The ratio and specific formulation can also affect gut comfort.¹⁰

Training the Gut to Handle More Carbohydrate

One reason many athletes struggle with during-exercise nutrition is simple: their gut is not used to it. Eating or drinking carbohydrate at high rates while running, cycling, or swimming can cause nausea, bloating, and cramping. But the gut is surprisingly trainable. Habitually consuming carbohydrate during training increases the density and activity of intestinal glucose transporters, which allows more carbohydrate to be absorbed and reduces the undigested sugar that sits in the gut causing trouble.¹¹

A systematic review found that repeating a carbohydrate-feeding protocol during exercise for two weeks reduced gut discomfort by an average of about 47% and cut carbohydrate malabsorption by roughly 45 to 54%.¹² For athletes who want to push their carbohydrate intake toward the higher end of recommendations during racing, practicing in training is not optional. The gut needs to adapt just like the muscles and cardiovascular system do.

Recovery and Refueling After Exercise

Once a hard session is over, the priority shifts to replenishing glycogen stores, especially when another session is coming within 24 hours. Consuming carbohydrate during recovery substantially speeds up glycogen resynthesis compared with water alone. Research shows that taking in about 1.2 grams of carbohydrate per kilogram of body weight per hour, starting immediately after exercise and continuing at regular intervals, maximizes the rate of replenishment.^{13 14}

A common question is whether adding protein speeds things up. Two meta-analyses have found that when carbohydrate intake is already adequate, adding protein does not significantly increase the rate of glycogen resynthesis.¹⁵ The apparent benefit of protein in earlier studies came largely from the extra calories it provided, not from the protein itself. However, if an athlete cannot stomach enough carbohydrate in the recovery window, adding some protein can make up for the shortfall in total energy and still support reasonable glycogen repletion. Protein also has its own recovery benefits for muscle repair, so combining the two makes practical sense even if it does not speed glycogen storage per se.

Carbohydrate and the Brain

Some of the performance benefit of carbohydrate during exercise does not come from fueling the muscles at all. Studies using mouth-rinse protocols, where athletes swish a carbohydrate solution and spit it out without swallowing, have found measurable improvements in performance. Brain imaging reveals that carbohydrate receptors in the mouth activate regions associated with reward and motor control, even though no calories are actually absorbed.^{16 17}

This effect is most pronounced during shorter, high-intensity bouts lasting around an hour, where muscle glycogen is not yet severely depleted but the brain still limits effort output. It suggests the nervous system uses the presence of carbohydrate in the mouth as a signal that more energy is available, allowing it to “release the brakes” on effort. For athletes in events too short for significant digestion but too long for a pure sprint, a carbohydrate mouth rinse can provide a small but real edge.

Protecting the Immune System During Heavy Training

Hard endurance exercise triggers a spike in stress hormones like cortisol and epinephrine, which suppress immune function in the hours after training. This post-exercise immune dip is one reason athletes going through heavy training blocks are more vulnerable to upper respiratory infections. Consuming carbohydrate before and during exercise blunts that hormonal surge. A systematic review of studies in experienced endurance athletes found that carbohydrate ingestion was an effective strategy for attenuating rises in cortisol and epinephrine during exercise.¹⁸

The immune protection goes beyond just lowering cortisol. Research has shown that carbohydrate ingestion during exercise reduces the death of T-lymphocytes, a type of immune cell critical for fighting infections. In one study, athletes who consumed carbohydrate maintained normal T-cell function after exercise, while those taking a placebo showed significant impairment, and the protective effect appeared to be independent of cortisol levels.¹⁹ General guidelines suggest that consuming roughly 30 to 60 grams of carbohydrate per hour during sustained intensive exercise helps limit the degree of exercise-induced immune depression.²⁰

Team Sports and Sprint-Based Demands

Carbohydrate is not just an endurance fuel. In team sports like soccer, basketball, and rugby, athletes perform repeated high-intensity sprints interspersed with lower-intensity running. Those brief sprints rely heavily on muscle glycogen broken down anaerobically. Over the course of a full match or training session, repeated sprinting progressively drains glycogen stores, and as they fall, sprint power drops and overall work rate declines.²¹ This is why the second halves of soccer matches often feature slower play and fewer high-speed runs. Increasing glycogen stores before competition and consuming carbohydrate during halftime can delay these performance declines.

The Ketogenic Diet Debate

Given how central carbohydrate is to high-intensity performance, what happens when athletes try to do without it? Ketogenic and low-carbohydrate, high-fat diets have attracted attention as an alternative fueling strategy, especially for ultra-endurance events. The physiology is real: after several weeks on a strict ketogenic diet, the body dramatically ramps up fat oxidation, roughly doubling the rate at which fat can be burned during exercise and shifting the peak fat-burning intensity higher.²²

The problem shows up at higher intensities. Burning fat requires more oxygen per unit of energy than burning carbohydrate. Studies on elite race walkers have demonstrated that keto-adapted athletes had markedly increased fat oxidation but also a higher oxygen cost of movement at any given speed, meaning reduced exercise economy.²³ When these athletes were tested in real-world race conditions, the high-fat group performed worse than those on high-carbohydrate diets, despite improvements in peak aerobic capacity. A follow-up study confirmed these findings in another cohort, including female athletes, reinforcing that the performance impairment is consistent and reproducible.²⁴

Keto-adaptation may also impair the muscle’s ability to use whatever glycogen it does have for oxidative energy production, compromising performance above about 80% of maximum aerobic capacity. For athletes whose events involve surges, sprints, or sustained hard efforts, this is a significant limitation. The evidence so far suggests that while a ketogenic diet can sustain low-to-moderate intensity exercise, it comes at a real cost to the ability to go hard when it counts.

Strategic Low-Carbohydrate Training

There is a more nuanced approach than going fully ketogenic. Some coaches and sport scientists use periodized carbohydrate availability, sometimes called “train low, compete high.” The idea is to deliberately restrict carbohydrate around certain training sessions to amplify the cellular adaptations those sessions produce, then restore full carbohydrate availability for key workouts and competitions. Research has shown that exercising with low glycogen availability increases the signaling pathways involved in mitochondrial biogenesis, the process by which muscles build more and better mitochondria for aerobic energy production.²⁵

The trade-off is that training quality suffers during the low-carbohydrate sessions. Athletes cannot hit the same intensities or volumes when glycogen is restricted, so the approach works best when applied selectively to easier, aerobic sessions rather than to the high-intensity workouts that drive speed and power gains. This kind of periodization requires careful planning and is not the same as chronically eating a low-carbohydrate diet, which, as the ketogenic diet research shows, tends to hurt performance more than it helps.

How Carbohydrate Aids Hydration

Carbohydrate does more than provide energy. In the gut, the active transport of glucose and sodium creates osmotic gradients that pull water across the intestinal wall, speeding fluid absorption. This is the same principle behind oral rehydration solutions used to treat dehydration globally. Dilute, slightly hypotonic glucose-sodium solutions are among the most effective ways to promote net water uptake during exercise.²⁶ Sports drinks exploit this coupling deliberately: the carbohydrate is not just fuel but a vehicle for faster rehydration. Athletes who drink plain water during long events absorb it more slowly than those drinking a properly formulated carbohydrate-electrolyte solution.

Sex Differences in Carbohydrate Metabolism

The hormonal environment influences how the body uses carbohydrate during exercise, and this has meaningful implications for female athletes. Estrogen promotes glucose availability and uptake in slow-twitch muscle fibers, which are the workhorses of endurance exercise. During the luteal phase of the menstrual cycle, when estrogen is higher, women tend to store more muscle glycogen and rely less on it during exercise, shifting somewhat toward fat oxidation instead. Progesterone can counteract some of estrogen’s effects.²⁷

What this means in practice is that carbohydrate needs and metabolism are not static across a menstrual cycle. In the early follicular phase, when estrogen is low, women may depend more heavily on glycogen and could benefit from higher carbohydrate availability around key sessions. In the luteal phase, the enhanced glycogen storage capacity and greater fat oxidation may slightly buffer against glycogen depletion. These differences are real but modest, and most general fueling guidelines still apply. The more important point is that female athletes should not assume their fuel needs are identical day to day across a cycle.

Individual Variation in Carbohydrate Digestion

Not everyone processes dietary carbohydrate with equal efficiency. One source of variation is the AMY1 gene, which encodes salivary amylase, the enzyme that begins starch digestion in the mouth. Humans carry different numbers of copies of this gene, and research has found a significant positive correlation between AMY1 copy number and salivary amylase activity both before and after exercise.²⁸ People with more copies produce more amylase and begin breaking down starch faster. While the downstream performance effects of this variation are still being studied, it is one piece of a larger puzzle explaining why two athletes on the same fueling plan can have different gut responses and different rates of energy availability. This kind of genetic variation reinforces why experimenting with carbohydrate sources, amounts, and timing during training is more useful than rigidly following a one-size-fits-all protocol.