Do you really need to carb-load? (#447)

Carbohydrate loading is often seen as a crucial element for improved performance - but how much is truly necessary? This article explores the research on glycogen and debates whether more is always better. By referencing both classic and modern studies, it examines the distinction between having enough stores and maximizing them. The conclusion is nuanced: although high glycogen levels can boost performance, extreme approaches rarely offer additional benefits and may even be counterproductive. In practice, it's more about finding a balance that suits you, your event, and your physical needs rather than maximizing everything.

Studies from the 1970s proposed more extreme glycogen-loading protocols that indeed resulted in very high muscle glycogen concentrations. However, the setup was so tough that it often negatively affected the athlete's performance. A classic protocol looked like this: a really hard and long session seven days before the competition to deplete the glycogen stores. After the session, no carbohydrates were consumed for three days, followed by a very high carbohydrate intake for four days leading up to the competition. No training at all was done during this entire week.

The result? Sure, the glycogen stores were maxed out — but the athletes didn't feel good. They were worn out after the tough session and lacked carbohydrate replenishment for three days, leading to poor recovery. The high-fat diet they consumed was unfamiliar to many and upset the stomach, and then switching to massive carbohydrate intake in the last days didn't make things better. Additionally, six days of total rest before the competition often left many with a heavy feeling in the body. A textbook example of when a protocol theoretically maximizes a measurement but in practice becomes counterproductive for the athlete — hence many skipped this setup.

We don't know everything. Early studies with protocols like the one above often compared low glycogen levels with high and found performance increases with carbohydrate loading in almost all cases. However, studies have rarely explored the difference between normal muscle glycogen and very high muscle glycogen. As far as I know, there's only one study today comparing high versus very high muscle glycogen—and it's a case study on an elite race walker (top‑50 in the world). This person typically consumed 12.7 g carbohydrates (CHO)/kg body weight/day in their regular diet (total 4507 kcal/day).

In the case study, he tested a two-day loading with either 13.7 g CHO/kg/day, 13.9 g CHO/kg/day, or 15.9 g CHO/kg/day. He increased glycogen stores by about 30 percent when he went from 12.7 g/day to 13.7–13.9 g/day and everything felt good. At 15.9 g/kg/day, stomach issues and bloating feelings arose. Conclusion: there’s always an upper limit — might as well find it. 😀 This person also gained weight between 1.5–1.8 kg due to the loading, something that can be negative in certain types of races (e.g., hilly and short races).

If we modify the availability of a substrate (for example, through dietary manipulation with carbohydrate loading), the regulation of metabolism during exercise is affected. Several metabolic pathways and control points are involved. Primarily, an increase in muscle glycogen concentration will increase the rate at which muscle glycogen is broken down to pyruvate during exercise (glycogenolysis). The enzyme responsible for breaking down glycogen (phosphorylase) is more active at higher glycogen concentrations. So more glycogen = increased glycogen utilization. The body loves its energy-efficient glycogen.

“Increasing muscle glycogen concentration will also enhance the speed with which muscle glycogen is broken down to pyruvate during exercise.”

Besides glycogenolysis, muscle glycogen also appears to be a strong regulator of another key enzyme in glycogen metabolism, namely pyruvate dehydrogenase (PDH). This enzyme converts pyruvate to acetyl‑CoA and is a rate-limiting step for carbohydrate oxidation. Starting exercise with higher muscle glycogen results in a greater exercise-induced increase in PDH activity, whereas lower glycogen levels reduce it. Hence: when carbohydrate availability is good, the body initiates more metabolic mechanisms to maximize carbohydrate burning.

As mentioned, there is so far only one case study comparing high and very high muscle glycogen. This study found that up to about 14 g of carbohydrates per kg of body weight per day still increased glycogen amounts without side effects for that person, but higher intake had negative effects.

To stir things up a bit, I also want to highlight a study by Melissa Arkinstall and colleagues. They found that glycogen usage was higher during training at 45 percent of VO2max when starting with high glycogen, compared to training at 70 percent of VO2max starting with low glycogen. In other words: with a lot of glycogen stored, participants burned more glycogen despite lower intensity than those who trained harder but with less glycogen at the start. So, the belief that higher intensity always results in higher carbohydrate utilization was not supported here — in this case, glycogen availability was a stronger driving force.

Practical example: imagine running a marathon in 3–4 hours and starting with either high or very high muscle glycogen. During the first hour, the concentration difference would be noticeable, but in both cases, there would be enough glycogen to maintain a fast pace. After two hours, glycogen levels will be low in both cases, and the difference minimal. Towards the end of the marathon, the levels are probably similar. So, is there really a clear advantage to a more extreme carbohydrate loading protocol?

One could argue for a slight advantage since the run is probably more economical (requiring less oxygen for a given workload) during the first hour. The counterargument is that oxygen uptake is rarely the limiting factor in that scenario. Until more studies compare high and very high glycogen, we may never get a definitive answer, but it seems that any benefits of extreme carbohydrate loading may be outweighed by the drawbacks of an overly intense regimen.

We already discussed this in our article on carbohydrate loading: you don’t need to carb-load like they did in the '70s for a whole week. In fact, in many cases, a 24-hour loading can be sufficient to achieve supercompensation.

In a 2002 study, participants cycled at 130 percent of VO2peak for 2.5 minutes, followed by a 30-second all-out sprint to quickly deplete glycogen stores and cause a "backfire." Does it seem unlikely that you can deplete local glycogen in just three minutes? At maximum intensity, it’s quite plausible—feel free to check out our basic glycogen article where a table shows consumption per second at different intensities.

At over 90 percent of max, you can locally in your working muscles burn just over 4 mmol (about 1 g) of glycogen per minute and kilo of muscle mass. Calculate for 10 kg of thigh muscles working near max, and that means about 120 mmol (30 g) of glycogen disappearing over three minutes.

The researchers measured glycogen levels before depletion and after 24-hour replenishment. Before depletion, it was 109 mmol/kg, and after 24 hours, it was 198 mmol/kg, nearly doubling in 24 hours with a carbohydrate intake of 10 g CHO/kg body weight. In other words, the same levels as in classic 2-3 day protocols, where participants often reach around 200 mmol glycogen/kg after loading.

This protocol isn't for everyone—an intense depletion session the day before a competition isn’t always wise—but it can be a useful trick if you realize 24 hours before the start that you’re not fully fueled. Evaluate it based on the type of competition and how you personally react.

The effectiveness of glycogen loading is a topic of debate.

– More glycogen = lower oxygen consumption because we can utilize more carbohydrates (saving about 7 percent oxygen).
– More glycogen = higher glycogen consumption = the benefit is used up in about 2 hours

If your race lasts longer than two hours, you might experience a marginal oxygen saving in the first two hours—a few heartbeats could be affected due to having a more oxygen-efficient fuel. But after two hours, you're likely on par with your fellow competitors. However, you might have fresher legs, which needs further exploration.

The longer the race, the more irrelevant the loading becomes. If you're cycling Vätternrundan in 12 hours or running 100 miles, the extra 100 g of glycogen you can store amounts to just a small percentage of your total carbohydrate needs. The intake during the race accounts for 99 percent of your energy plan and is undoubtedly the most critical.

But if it's a race of, say, 10–21 km, loading can be incredibly beneficial. In these distances, the intensity is high from the start and it's long enough for extra glycogen to really help. Here, however, you must weigh the benefits against a possible 1–2 kg weight gain at the starting line—it can slow you down as much as the loading helps if the course is hilly.

I don't have a perfect answer here. The conclusion is that you need to make your own assessment based on how you react to a 1-day, 2-day, or 3-day loading, what the course profile looks like, how much the loading increases your body weight, and how much you feel it affects you. As a coach: test in training, note how the gut, weight, and sensations are affected—and plan accordingly.