We often focus on the nutritional strategies before and during exercise to optimize exercise performance. However, the post-exercise period is as important as the pre- and during- exercise periods, especially when competitions or training sessions are close in time to each other and the recovery time between them is limited.
This time I want to focus on the recovery period. Concretely, I want talk about the relevance of glycogen replenishment after exercise and how the type of carbohydrate ingested affects the rate of glycogen replenishment and ultimately exercise performance.
It is a fact that endurance sports are becoming more and more popular over time. One of the hardest types of endurance competitions are multi-stage races, such as cycling Grand Tours, the Marathon des Sables, or innumerable Ultraman – 3-day triathlon – competitions, during which athletes compete for several days in a row.
Unlike in one-day competitions, the limited recovery time in multi-stage races provides an extra challenge to overcome: athletes only have around 15 hours – depending on the stage duration – to recover between stages.
But this is not unique of multi-stage races. This challenge also arises during intensive training periods where athletes train hard every day, or even twice a day, and during tournament-style competitions.
Ensuring that athletes recover quickly from each stage, training session, or match and go into the next one in the best possible conditions is crucial for optimizing their performance.
Since strong relationship between replenishment of endogenous carbohydrate stores with subsequent exercise performance has been well established by many studies, the main factor determining recovery time is glycogen repletion.
The main factor determining recovery time is glycogen repletion.Gonzalez et al. 2017
But we don’t like to stay on the surface of facts, phenomena, or theories. I am a big fan of breaking down every piece of knowledge and understand their basis and mechanisms by which it is thought to work. So, let’s dig into why glycogen repletion is so important during recovery, especially when optimal performance is required on more than one occasion with a limited interval between bouts!
Glycogen stores are crucial for optimal exercise performance
We are well aware of the importance of carbohydrates as the main fuel substrate during exercise of moderate-to-high intensity – this is the intensity at which races are won. You can take a look at an article about the determinants of substrate utilization to review this.
In our body, carbohydrates are stored as glycogen – glucose polymers – most abundantly within the skeletal muscle and the liver. However, the capacity to store carbohydrates is very limited compared to fat. So you can get an idea, more than 100,000 kcal are stored as fat while no more than 3,000 kcal are stored as carbohydrates in a typical 75-kg person with 15% body fat, which is not enough to support even one full day of racing.
Due to their limited storage capacity, endogenous carbohydrate stores are depleted within 90 minutes of moderate-to-high intensity exercise (≈75% VO2 max), which is not even half of the total duration of a single stage. In turn, glycogen depletion is strongly associated with the onset of fatigue and a reduction in exercise performance – with low glycogen stores, we will no longer be capable of keeping up with a race-pace intensity.
In an attempt to solve this issue that affects every endurance athlete, many studies have focused on designing nutritional strategies to replace endogenous carbohydrate stores as fuel. Carbohydrate ingestion during exercise has been shown to delay endogenous carbohydrate stores depletion and thus the onset of fatigue, which results in an improved exercise performance. I thoroughly explain this in a post about the effects of carbohydrate ingestion during exercise. However, despite delaying the depletion of glycogen stores, carbohydrate feeding during exercise does not completely prevent it and our endogenous carbohydrate stores end up being very low after several hours exercising at race-pace intensity.
Thus, if we want to optimize exercise performance for each stage, training session, or match, we must ensure that our glycogen stores are replenished as soon as possible and that they are full before the next exercise bout starts. In fact, it is well known that the rapid recovery of both muscle and liver glycogen stores after prolonged exercise are important determinants of the capacity to perform a subsequent bout of moderate-to-high intensity exercise.
Let’s see how nutrition after exercise targets glycogen repletion and how it is affected by the type of carbohydrates ingested.
Effect of fructose-glucose mixtures on glycogen repletion
In the hours following exercise, carbohydrate ingestion is a requirement for substantial replenishment of glycogen stores. The current sport nutrition guidelines for recovery from exercise recommend 1.0-1.2 grams of carbohydrate per kilogram of body mass per hour during the first four hours post-exercise for rapid refuelling. This rate of carbohydrate ingestion after exercise has been shown to maximize muscle glycogen repletion rates.
But our body is not that simple, and science does not settle for that.
In a previous blog, we saw the underlying mechanisms for the benefits of glucose-fructose co-ingestion during exercise. We described how this practice significantly increases carbohydrate absorption and delivery to peripheral tissues while reducing gastrointestinal distress, which ultimately improves exercise performance. You can read more about the underlying mechanisms that explain this in a previous post about absorption of carbohydrates.
Likewise, it could be argued that the use of multiple transportable carbohydrates during the recovery period further maximize glycogen repletion rates. But does it really increase the glycogen repletion rate compared to the ingestion of glucose alone? And, more importantly, does it positively impact on exercise performance?
Before we start building the answer to these questions, remember that there are two main carbohydrate stores in our body: the skeletal muscle and the liver. Despite having slightly different roles – while skeletal muscle glycogen directly provides a rapid and efficient fuel source and is also believed to be an important signalling molecule that regulates muscle function , liver glycogen plays a central role in blood glucose homeostasis, which is taken up by the skeletal muscle as an energy substrate for ATP resynthesis – they are both equally important. We must ensure that replenishment rates of both muscle and liver glycogen stores are maximized for optimal performance on subsequent bouts of exercise.
Let’s take a look at how the ingestion of fructose-glucose mixtures affect glycogen repletion in each tissue.
Muscle glycogen repletion
The main precursors for glycogen re-synthesis in skeletal muscle are glucose and lactate. Therefore, their availability to muscles is an important factor – if not the most – in maximizing muscle glycogen repletion and reducing recovery time. We want that carbohydrate availability to be high after exercise so that muscles resynthesize glycogen as fast as possible.
In turn, that carbohydrate availability is increased by ingesting carbohydrate-containing food or beverages. Besides, it is well known that the ingestion of multiple transportable carbohydrates – glucose and fructose mixtures – further increases the absorption of carbohydrates in the intestine, thus further enhancing carbohydrate availability to muscles while reducing gastrointestinal distress.
So, it could be hypothesized that the increased carbohydrate availability when using glucose-fructose mixtures could improve muscle glycogen repletion compared to the ingestion of glucose alone in isocaloric quantities.
However, when researchers looked at whether the use of multiple transportable carbohydrates increased muscle glycogen resynthesis over glucose alone, they found no differences. Co-ingestion of glucose with fructose did not improve muscle glycogen repletion over glucose. It seems that muscle glycogen repletion is not sensitive to the type of carbohydrate ingested. They are mostly dependent on the dose of carbohydrate ingested instead.
Despite current evidence not supporting that glucose-fructose co-ingestion accelerates muscle glycogen repletion, it is important to point out that this practice allows maximal rates of muscle glycogen repletion to be achieved with lower gastrointestinal distress.
Liver glycogen repletion
Unlike in muscle, fructose represents an important precursor for the synthesis of glucose – from which glycogen is then synthesized – in the liver. In fact, after being absorbed in the intestines, most fructose is taken up by the liver and converted to glucose and lactate, which then either enter the systemic circulation to be delivered to peripheral tissues or contribute to liver glycogen synthesis.
Bearing this in mind, there is a stronger hypothesis that co-ingestion of fructose with glucose accelerates liver glycogen synthesis compared to the ingestion of glucose alone.
Researchers who addressed this hypothesis observed that the ingestion of glucose-fructose mixtures significantly increases the rate of liver glycogen repletion compared to glucose alone. In fact, the rate of post-exercise liver glycogen resynthesis when co-ingesting fructose with glucose is typically twice that of glucose alone. It seems that the liver is more sensitive to the type of carbohydrates ingested than the skeletal muscle.
However, the purpose of these practices is to enhance exercise performance. So, the next question we should ask is whether the accelerated liver glycogen repletion observed when fructose is co-ingested with glucose translates into improved exercise performance. Let’s look at it!
Fructose-glucose mixtures enhance subsequent exercise performance
Not many studies have focused on the effects that the ingestion of fructose-glucose mixtures post-exercise have on exercise performance. However, some insights have come to light in the recent years.
In 2018, a group of researchers studied the effects of this practice in endurance runners and triathletes (Maunder et al. 2018). Subjects performed two bouts of running to exhaustion separated by 4 hours of recovery. During the recovery time, they were given drinks containing either glucose-based carbohydrates or fructose-glucose mixtures in isocaloric quantities. They found out that after fructose-glucose mixtures, athletes were able to exercise for 30% longer than with glucose-based carbohydrates.
This suggested an important impact of type of carbohydrate ingested on subsequent exercise capacity. However, this study didn’t bring evidence that it could benefit multi-stage competitions, since most of them have recovery periods longer than 4 hours between bouts of exercise and include an overnight fast during sleep. Liver glycogen stores are used overnight to supply the brain and other tissues with glucose. Therefore, the benefits of fructose-glucose co-ingestion post-exercise on liver glycogen repletion could be lost overnight.
In this line, in 2019, another group of researchers designed a study to address this issue (Gray et al. 2019). They asked 8 trained cyclists to perform an exercise bout to exhaustion and to consume either fructose-glucose mixtures or glucose-based carbohydrates alone for 4 hours following exercise. Subsequent exercise capacity was assessed after 15 hours of recovery, which included an overnight fast and a low carbohydrate breakfast. Interestingly, subsequent exercise capacity was improved by 20% after ingestion of fructose-glucose mixtures compared to glucose-based carbohydrates alone. This suggests that fructose-containing carbohydrates during the recovery period may improve the capacity of athletes to exercise the following day.
The rapid repletion of both muscle and liver glycogen stores after prolonged exercise is crucial for optimizing performance during multi-stage races, intensive training periods, and tournament-like championships.
While muscle glycogen repletion is mainly dependent on the amount of carbohydrate ingested, with little or no sensitivity to the type of carbohydrate, liver glycogen repletion is strongly influenced by the type of carbohydrate. The ingestion of fructose-containing carbohydrates significantly accelerates liver glycogen resynthesis, which ultimately improves exercise capacity in situations with a limited recovery time.
In line with this, athletes who want to optimize exercise performance during bouts of exercise close to each other in time should aim to take 1.0-1.2 grams of fructose-containing carbohydrates per kilogram body mass per hour for the first four hours of recovery.
- Gonzalez, Javier T., Cas J. Fuchs, James A. Betts, and Luc J.C. van Loon. 2017. “Glucose plus Fructose Ingestion for Post‐exercise Recovery—Greater than the Sum of Its Parts?” Nutrients 9 (4): 1–15.