Ask an endurance athlete what “carb loading” means and you may get a familiar answer: a huge bowl of pasta the night before a race.

Ask someone who studied sports nutrition a little longer ago, and the answer may sound more complicated: first exhaust your glycogen stores, eat very little carbohydrate for several days, then dramatically increase carbohydrate intake before competition.

That second answer comes from the classic carbohydrate-loading protocol.

So, if we turned it into a quiz:

In the classic two-phase carbohydrate-loading strategy, which sequence was used to maximize muscle glycogen stores?

A. Hydration → Fluid restriction
B. Glycogen depletion → Carbohydrate repletion
C. Protein loading → Fat adaptation
D. Electrolyte depletion → Sodium loading

The answer is B: glycogen depletion followed by carbohydrate repletion.

It is historically correct. But if an athlete followed that exact protocol today, a sports nutrition practitioner might reasonably ask: do we actually need the depletion phase anymore?

The answer, for most athletes, is no.

And that is where the story of carbohydrate loading becomes much more useful than simply memorizing the right answer to a multiple-choice question.

Where did the “deplete, then reload” idea come from?

Some of the foundational carbohydrate-loading research dates back to the 1960s.

In a landmark study, Bergström and colleagues used muscle biopsies to examine how exercise and different diets changed muscle glycogen concentrations. After glycogen-depleting exercise, a carbohydrate-rich diet produced muscle glycogen concentrations above normal levels. Higher starting glycogen stores were also associated with a greater capacity for prolonged exercise.

This helped establish the concept we now call glycogen supercompensation.

The logic was straightforward:

Train hard and reduce glycogen → temporarily restrict carbohydrate → then sharply increase carbohydrate while reducing training → store more glycogen than usual.

For endurance sport, that mattered. Muscle glycogen is an important carbohydrate reserve, particularly when exercise is prolonged and performed at moderate-to-high intensities. When glycogen availability becomes limiting, maintaining the desired pace becomes increasingly difficult.

So the original protocol was not a gimmick. It was built on an important physiological discovery.

But sports nutrition rarely stands still.

The depletion phase turned out to be less important than we thought

The classic method was demanding.

Athletes could spend several days training hard while eating relatively little carbohydrate before entering the high-carbohydrate phase. In practice, that could mean fatigue, poor-quality training, irritability and an unnecessarily difficult race week.

Later research began testing whether athletes could achieve high muscle glycogen concentrations without such an aggressive depletion phase.

A 1981 study by Sherman and colleagues compared different exercise and dietary approaches and helped demonstrate that glycogen supercompensation could be achieved with a more moderate strategy rather than requiring severe carbohydrate restriction beforehand.

Over time, the practical model changed.

Instead of deliberately running the fuel tank close to empty, modern carbohydrate loading generally focuses on two things:

increase carbohydrate availability and reduce the amount of glycogen being used.

In other words:

eat more carbohydrate + taper training.

Contemporary sports nutrition guidelines commonly recommend approximately 10–12 g of carbohydrate per kg of body mass per day for 36–48 hours when full carbohydrate loading is appropriate, particularly before endurance competition lasting around 90 minutes or longer.

The athlete can arrive at the start line with highly stocked glycogen stores without deliberately making the preceding days miserable.

And the evidence is still developing

Carbohydrate loading may be an old topic, but that does not mean the science is finished.

A 2026 study examined endurance-trained individuals consuming 6, 8 or 10 g/kg/day of carbohydrate for 48 hours, while following training designed to resemble real-world pre-competition preparation.

Muscle glycogen concentration was highest after the 10 g/kg/day condition, and carbohydrate intake showed a positive dose-response relationship with muscle glycogen. Interestingly, the very-high-carbohydrate condition did not produce a detectable increase in most gastrointestinal symptoms, although participants did report greater fullness.

That finding is useful for practitioners because race preparation does not happen in a laboratory vacuum. Athletes may still complete short training sessions during a taper. They also have to actually eat the prescribed food, tolerate it and feel comfortable enough to compete.

A nutrition strategy is only useful if the athlete can execute it.

Carb loading is not simply “eat as much carbohydrate as possible”

This is where practical sports nutrition becomes more important than memorizing a number.

A recommendation such as 10–12 g/kg/day is a performance target for a specific situation. It is not a universal daily carbohydrate prescription.

Carbohydrate loading is most relevant when the event is long enough for glycogen availability to become an important performance limitation—for example, marathon running, long-distance cycling, triathlon and other prolonged endurance events. Current recommendations particularly target events lasting around 90 minutes or longer.

For a short gym session or a brief competition, deliberately consuming huge quantities of carbohydrate for two days is unlikely to provide the same benefit.

The first question should therefore never be:

“How many carbs should I load?”

It should be:

“Does this athlete and this event actually require carbohydrate loading?”

The practical challenge: high carbohydrate intake can be a lot of food

Another common mistake is assuming carbohydrate loading means simply increasing dinner portions.

In reality, achieving very high carbohydrate targets through normal food can become uncomfortable surprisingly quickly.

Huge servings of high-fibre foods, very fatty meals and unfamiliar foods immediately before competition may leave an athlete feeling heavy or experiencing gastrointestinal problems—the exact opposite of the goal.

For this reason, an effective loading plan often distributes carbohydrate across meals, snacks and drinks throughout the day, rather than relying on one enormous pasta dinner.

Depending on the athlete, familiar lower-fibre carbohydrate foods may also become more useful close to competition because they allow higher carbohydrate intake without an unnecessarily large food volume.

Rice, bread, cereals, noodles, potatoes, fruit, juice, sports drinks and other familiar carbohydrate sources can all have a place. There is no requirement for carbohydrate loading to come from one “perfect” food.

The priority is hitting the carbohydrate target without compromising gastrointestinal comfort, hydration or the athlete's willingness to eat.

More glycogen can also mean more body mass

There is another detail athletes sometimes misunderstand.

Increasing muscle glycogen storage also increases associated water storage. Research commonly reports approximately 3–4 g of water associated with each gram of stored muscle glycogen, and glycogen supercompensation can therefore increase body mass temporarily.

For a marathon runner, that temporary increase may be an acceptable trade-off for improved carbohydrate availability.

For an athlete competing in a weight-category sport, however, the calculation may be completely different.

This is a good example of why performance nutrition cannot simply copy recommendations from one sport into another.

The physiological strategy may be sound, while the practical decision still depends on the sport.

What about “training low”?

Another source of confusion comes from modern discussions around deliberately training with reduced carbohydrate availability.

There is research interest in manipulating carbohydrate availability during selected training sessions to influence cellular signalling and training adaptation. But that is a training-periodization strategy, not the same thing as deliberately entering an important competition with poor carbohydrate availability.

When performance is the priority, carbohydrate availability should generally match the work required.

That distinction matters.

An athlete might deliberately complete a selected low-glycogen training session at one point in a training programme, yet deliberately maximize glycogen before a major race.

These strategies are not contradictory. They are answering different questions.

Training nutrition is about adaptation. Competition nutrition is about performance.

The skill lies in knowing when each goal matters most.

So what should athletes actually do?

For an athlete preparing for a prolonged endurance event, the modern approach is much simpler than the original depletion-and-loading protocol.

First, determine whether carbohydrate loading is actually relevant to the event.

If it is, the athlete can taper training while substantially increasing carbohydrate intake during the final 36–48 hours, with current guidelines commonly using 10–12 g/kg/day as a target for maximal glycogen loading.

But the number is only the starting point.

The plan should also consider:

• the athlete's usual carbohydrate intake;
• gastrointestinal tolerance;
• body size and total food volume;
• competition timing;
• familiar foods;
• hydration;
• training completed during the loading period; and
• whether temporary increases in body mass matter for the sport.

Most importantly, race week is not the time to test carbohydrate loading for the first time.

A well-designed performance nutrition strategy should be practised during training or lower-priority competition so the athlete already knows what foods, volumes and timing work.

GPNi Practical Takeaways

The old exam answer remains useful:

Classic carbohydrate loading = depletion + repletion.

But professional practice needs the next layer of understanding.

The deliberate glycogen-depletion phase is largely a historical feature of carbohydrate loading and is not normally required to achieve glycogen supercompensation in trained athletes today. Modern practice relies more on high carbohydrate availability combined with reduced training load.

Carbohydrate loading is also not simply “eating lots of carbs before competition.” It is a targeted strategy for events in which high glycogen availability is likely to matter.

And perhaps most importantly, the best plan is not necessarily the one that looks perfect on paper.

It is the one the athlete can digest, tolerate, practise and execute when performance matters.

That is the difference between knowing sports nutrition and applying performance nutrition.

References

1. Bergström J, et al. Acta Physiol Scand. 1967;71:140–150. doi:10.1111/j.1748-1716.1967.tb03720.x.
2. Sherman WM, et al. Int J Sports Med. 1981;2:114–118. doi:10.1055/s-2008-1034594.
3. Burke LM, et al. J Sports Sci. 2011;29(Suppl 1):S17–S27. doi:10.1080/02640414.2011.585473.
4. Thomas DT, Erdman KA, Burke LM. Med Sci Sports Exerc. 2016;48:543–568. doi:10.1249/MSS.0000000000000852.
5. Jones RO, et al. Scand J Med Sci Sports. 2026;36:e70312. doi:10.1111/sms.70312.