Are you actually carb loading…or just eating a bowl of pasta for dinner?

By 

Giles Elmore (BSc Hons) MRSPH

The evening before a marathon, Ironman or big sportive, two competitors sit down at the dinner table and order the same meal. Both get an enormous bowl of pasta and finish every mouthful. On the face of it they're doing exactly the same thing. Physiologically, however, they could be achieving very different results.

In this scenario, one athlete is simply replenishing the carbohydrates they've used throughout the day. They could eat one bowl of pasta or five, but without a specific build up, they are simply restoring glycogen to normal levels.

The second athlete has spent the previous few days creating the conditions that allow carbohydrate loading to occur. Their muscles have become progressively more receptive to storing glycogen. Enzyme activity has increased, glucose transport into the muscle has been enhanced, training volume has been reduced, and their muscles are now capable of storing considerably more glycogen than they normally would. That final bowl of pasta isn't creating the adaptation; it is simply providing the final raw material for a physiological process that has been developing over several days.

This highlights perhaps the biggest misconception surrounding carbohydrate loading. Most people believe it means simple eating a large bowl of pasta, bread or pizza the night before an event. In reality, carbohydrate loading has very little to do with a single meal. It is a carefully orchestrated physiological process that temporarily increases the muscle's capacity to store glycogen. Understanding how this happens requires us to look inside the muscle itself.

Carbohydrate loading begins with exercise, not food. Whenever we eat carbohydrates, they are broken down into glucose. Some of this glucose is used immediately to provide energy, while any surplus is joined together into long, highly branched chains called glycogen and stored in our muscles and liver, ready to be broken back down into glucose whenever rapid energy is required.

Every hard ride, long run or demanding training session reduces those glycogen stores. However, at the same time that glycogen levels fall, the muscles also become increasingly receptive to replacing it. Glucose transport into the muscle increases through greater GLUT-4 activity, insulin sensitivity improves, and perhaps most importantly, glycogen synthase, the enzyme responsible for converting glucose into glycogen, becomes highly active.

This period is often referred to as the post-exercise "open window". While glycogen synthesis is at its greatest during the first few hours after exercise, the phrase can be slightly misleading. Rather than representing a brief opportunity that suddenly disappears, it is better thought of as a period during which the muscle becomes exceptionally efficient at storing carbohydrate. Training, therefore, doesn't simply deplete glycogen, it temporarily changes the muscle's ability to replace it.

At the same time, the muscle's storage machinery also changes. Glycogen synthase remains highly active, allowing glucose to continue being converted into glycogen even as stores approach their normal resting level. When this enhanced storage capacity is combined with reduced training volume and a sustained high carbohydrate intake, glycogen synthesis begins to exceed glycogen breakdown. This is the physiological basis of glycogen supercompensation, first described in a study by Bergström and Hultman back in 1967.

So, if it improves performance so effectively, why don't endurance athletes simply remain carbohydrate loaded all year round? The answer lies in the balance between storage and use. During a normal training week, glycogen stores are constantly being depleted by training and replenished through diet. The muscles rarely have an opportunity to accumulate glycogen beyond their normal resting level because the next ride or run simply begins using it again.

However, a taper before a big event can change this completely. Training volume fall, glycogen utilisation drops, yet carbohydrate intake remains high. The muscles continue behaving as though they need to replenish glycogen, but very little is being used. Storage therefore begins to exceed breakdown, allowing glycogen concentrations to rise beyond their normal physiological level.

Glycogen isn't stored as one large reservoir within the muscle. Instead, it is distributed throughout every muscle fibre in thousands of microscopic storage sites. Recent research suggests that during carbohydrate loading the muscle temporarily increases the number of these storage sites, effectively creating more places where glycogen can be deposited. Rather than simply filling existing stores, the muscle increases its overall storage capacity, allowing substantially more glycogen to be stored before competition.

Test show that when done successfully, the effect is quite significant. Successful carbohydrate-loading protocols have increased measured muscle glycogen concentrations to approximately 1.7 to 1.9 times normal baseline values in some studies. Few nutritional strategies produce such dramatic physiological adaptations.

The obvious question then becomes whether these larger glycogen stores actually improve performance. The answer is an emphatic yes. Since Bergström and Hultman's pioneering work, decades of research have consistently demonstrated that athletes beginning prolonged endurance events with super compensated glycogen stores perform better than those starting with normal glycogen levels. Importantly, carbohydrate loading doesn't increase VO₂max, improve lactate threshold or make an athlete inherently stronger. The engine itself remains the same. Instead, it delays the point at which the muscles begin running short of their preferred fuel.

A comprehensive review by Hawley et al. (1997), together with more recent work by Burke et al. (2011), concluded that carbohydrate loading offers little measurable benefit for events lasting less than approximately 90 minutes. However, for prolonged endurance events where glycogen availability becomes limiting, the evidence is remarkably consistent. Athletes who carbohydrate load typically improve performance by around 2–3%, while delaying the onset of fatigue by approximately 20%.

At first glance, a two or three percent improvement may not sound particularly impressive, but for a three-hour marathon, a 2–3% improvement represents approximately 3½ to 5½ minutes. During a six-hour sportive or Ironman bike leg, it equates to somewhere between 7 and 11 minutes, achieved not through additional fitness or harder training, but simply by arriving on the start line with maximised glycogen stores and pushing the gradual decline in power that normally accompanies prolonged exercise to begin later.

So, does the famous pasta dinner the night before a marathon or sportive help? Well, yes! but probably not in the way most people believe. If you've trained normally throughout the week and simply eat one enormous bowl of pasta the evening before your race, you'll almost certainly replenish liver glycogen and continue restoring muscle glycogen overnight. That's beneficial, but it isn't carbohydrate loading. True carbohydrate loading requires creating the physiological conditions that allow muscles to temporarily store more glycogen than they normally can. The pasta is simply the final piece of a process that has already begun.

One final point that often surprises athletes, is most will gain between one and two kilograms during a successful carbohydrate-loading protocol. The immediate assumption being that during their reduced activity that week, they have gained unwanted fat. However, what is happening, is every gram of glycogen stored is accompanied by approximately three grams of water, meaning the additional body weight is part of the performance adaptation itself. Those extra kilograms aren't body fat; they're the fuel and the water required to store it.

Ultimately, carbohydrate loading is far more sophisticated than simply eating more carbohydrates. It is a temporary physiological state in which exercise, tapering, enzyme activation, enhanced glucose transport and carefully planned nutrition combine so that on race day we can maximise one of the body's most valuable performance resources.

If you feel uncertain about whether your daily nutrition is truly supporting your health, performance, and long-term wellbeing, G2 Nutrition offers a highly personalised diet analysis with a bespoke 6-week optimisation programme, available for £250.

This is a tailored, results-driven service designed to elevate your approach to nutrition, refining not only what you eat, but how your body performs, recovers, and thrives day to day.

To enquire, please contact me at giles@g2nutrition.com

Clients of G2 Nutrition consistently experience:

•     Refined body composition and physique

•     Sustained, elevated energy levels

•     Measurable improvements in key health markers

•     Enhanced immune resilience and reduced susceptibility to illness and injury

•     Noticeable gains in sporting performance

•     Improved cholesterol and cardiovascular health

And a deeper, lasting understanding of intelligent nutrition for themselves and their family

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