Is Carb-Loading Useless? What the Science Really Says

In the final day or two before a marathon, we’re all given the same advice. Eat more carbohydrates, reduce training, top off glycogen, and arrive at the start line with the tank full.

It is one of the most familiar rituals in endurance sports, and the underlying physiology is solid. Muscle glycogen (think of it as stored carbohydrates) is a critical fuel source during prolonged moderate- to high-intensity exercise. When glycogen availability falls, the ability to sustain pace or power can suffer.

But how much of a performance boost does “carb-loading” really provide?

A new commentary in the International Journal of Sport Nutrition and Exercise Metabolism asked whether traditional carbohydrate loading provides an additional performance benefit once athletes are already fueling before and during the race in ways that reflect current practice.

This is an important distinction because much of the original carbohydrate-loading literature was built in an era when in-race fueling was far less developed than it is today.

Many studies tested athletes after limited pre-exercise carbohydrate intake, with no carbohydrate provided during the exercise test. In that setting, it is not surprising that starting with more glycogen improves endurance capacity. But that is not the same scenario as a marathoner eating breakfast, taking gels every 25–35 minutes, and consuming a carbohydrate drink on course.

The paper does not argue that glycogen is unimportant, but it does challenge the certainty surrounding one specific idea—that pushing muscle glycogen above normal through aggressive 24–48-hour carbohydrate loading reliably improves performance beyond what is achieved by adequate pre-race fueling and optimized carbohydrate intake during the event.

The authors reviewed the carbohydrate-loading literature with two methodological questions in mind.

First, were the studies double-blind and placebo-controlled? In other words, did participants and researchers actually not know whether the athlete was receiving the carbohydrate-loading intervention or the comparison condition? (This is important for reasons we’ll discuss later).

Second, did the studies provide carbohydrates during exercise in a way that resembles real-world endurance competition?

Both questions are important because performance is physiology filtered through perception, expectation, motivation, pacing, and belief. If an athlete knows they are carbohydrate-loaded, that knowledge may influence how hard they are willing to push, how they interpret fatigue, or how confident they feel late in a test. Likewise, if an athlete is asked to perform prolonged exercise without carbohydrate intake, the test may exaggerate the benefit of pre-exercise glycogen stores compared with a real race where carbohydrate is consumed throughout.

To evaluate this, the authors identified carbohydrate-loading studies that compared a moderate or “normal” carbohydrate intake (roughly 4–6 grams per kilogram per day) with a higher carbohydrate intake (above 7 grams per kilogram per day for at least 24 hours). The studies also had to include an endurance performance or work-capacity test lasting at least 90 minutes.

From an initial search of more than 10,000 studies, they narrowed the field to (just) 14 eligible studies.

That alone tells us something—carbohydrate loading is one of the most deeply embedded practices in endurance sport, yet the performance evidence base that directly tests it under relevant conditions is not especially large.

• Of the 14 studies, only two used double-blind, placebo-controlled designs.
• Only four provided carbohydrates during the performance test.
• The two studies that did both—double-blinding plus in-exercise carbohydrate provision—did not show a clear performance benefit from carbohydrate loading.

Those two studies were small, with only 7 and 9 well-trained athletes, and only produced relatively modest increases in muscle glycogen: 18% in one study and 25% in the other. Classic carbohydrate-loading protocols can sometimes increase glycogen by 30–90%, so it is possible that these trials did not even create a large enough separation between “normal” and “loaded” conditions to fully test the question.

The largest benefits in the literature tended to come from studies that were not double-blind and did not provide carbohydrates during exercise. These studies often used time-to-exhaustion tests, where athletes continue at a fixed pace (or work output) until they can no longer maintain it. Time-to-exhaustion tests can be useful for investigating fatigue, but they do not fully mimic racing, where athletes pace themselves, respond to discomfort, and make constant decisions about effort.

This is where the interpretation gets a bit tricky. If carbohydrate loading improves your time to exhaustion in a fasted or underfueled test, that may demonstrate that glycogen availability matters; but it does not necessarily prove that aggressive carbohydrate loading adds a meaningful benefit for you arriving at the start line fed and consuming carbs during the race.

Even among the studies that did provide carbohydrate during exercise, the doses were typically around 60–70 grams per hour. That was consistent with older guidelines and still represents solid fueling, but it is below the 90–120 grams per hour now being discussed and used by some highly trained endurance athletes. So these studies still don’t fully reflect the highest end of modern fueling practice.

The Placebo Problem: When Belief Becomes Part of the Performance

One of the more interesting parts of this paper is its discussion of belief.

In endurance nutrition, we often talk about interventions as though they act only through pure physiology. Carbohydrates increase glycogen. Caffeine affects the central nervous system. Sodium bicarbonate buffers hydrogen ions. Those mechanisms are real, but performance outcomes are also shaped by expectancy. What we think a supplement/intervention should do based on what we’ve read or heard.

Let me be clear here. Placebo effects in sport are not imaginary. They are measurable. If an athlete believes they have consumed something performance-enhancing, performance can improve even when the intervention itself is inert (that’s a nice word for “useless”). This is especially relevant in nutrition research because many interventions are difficult to blind. Athletes can often tell whether they are receiving caffeine, carbs, sodium bicarbonate, or a low-carb diet. Researchers may know too, which can influence encouragement, testing conditions, or subtle cues during performance trials.

In this commentary, the authors highlight that placebo effects for nutritional interventions have been estimated around a small-to-moderate effect size, statistically speaking, which aligns with many of the effect sizes observed in these carb-loading studies, meaning we cannot confidently separate the physiological effect of carbohydrate loading from the performance effect of believing you are carbohydrate-loaded.

This might be a somewhat uncomfortable (though useful) idea for runners. The pre-race carb-loading ritual may work partly because it changes physiology and partly because it changes confidence. Those two are not mutually exclusive. Believing you’re well-fueled may allow you to pace more assertively, tolerate discomfort better, or avoid spiraling when the race starts to hurt. In a marathon, that psychological state isn’t trivial.

The mistake here would be to dismiss belief as “not real.” Belief is part of performance. But if we want to know whether above-normal glycogen stores independently improve performance, we need studies that can isolate that physiology from expectations, and that hasn’t been done yet.

How Modern In-Race Fueling Changes The Equation

A decade or two ago, many runners were still racing on low-to-moderate carbohydrate intakes, often taking gels late or inconsistently. Now, more athletes are practicing high-carb fueling in training, using different types of carbohydrates, and targeting 60, 75, 90, or even 100+ grams per hour.

That changes the context of the carb-loading question.

If you are not taking in carbohydrates during the race, pre-race glycogen matters enormously because it is doing nearly all the work.

But if you are supplying carbohydrates throughout the event, the relationship becomes more nuanced. Muscle glycogen still matters, especially at marathon intensity, but exogenous carbohydrates can help preserve blood glucose, support carbohydrate oxidation, and potentially reduce reliance on endogenous (internal) stores.

The best strategy is therefore probably not to load as much as possible and hope the tank lasts. It’s to start with high availability, then keep carbohydrate availability high for as long as possible.

This also helps explain why gastrointestinal tolerance matters so much. A runner who forces down 12 g/kg/day of carbohydrate before race day but arrives bloated, uncomfortable, and unable to execute their fueling plan may be worse off than a runner who eats a slightly more moderate carbohydrate load but feels good and fuels consistently during the race.

What this means for runners

This paper is NOT a reason to stop carb loading. It’s a reason to modernize how you think about it.

The goal before a marathon or long race should be to arrive rested and well-fueled—not simply to hit the highest possible carbohydrate number in the days before the race. In practice, that means:

• increasing carbohydrate intake during the final 24–48 hours.
• reducing training enough to avoid glycogen depletion.
• choosing familiar, low-fiber, low-fat foods if your gut is sensitive.
• practicing your race breakfast.
• making in-race fueling a central part of the plan.

For many runners, a moderate-to-high-carbohydrate approach that is comfortable and repeatable will beat an extreme protocol that causes tummy troubles.

I found this commentary useful because it pushes sports nutrition to hold itself to a higher standard. If we want to know whether carbohydrate loading independently improves performance, we need studies that are blinded, placebo-controlled, and designed around how athletes actually race—fed before exercise, fueled during exercise, and tested in performance formats that resemble competition.

Until then, the practical advice is balanced. Do not race depleted. Do not ignore carbohydrates. But don’t assume that the biggest pre-race carb load is automatically the best one.