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In this Episode you'll learn about:
- When and how is glycogen (the stored form of carbohydrate in the body) used at rest and during exercise (glycogen metabolism 101)
- What is the relation between glycogen and performance?
- What is the relation between glycogen and training adaptations?
- How much glycogen do we store, and how quickly do these stores get depleted during training and racing?
- When and why is fast and complete glycogen resynthesis important for athletes?
- How can we influence glycogen resynthesis rates through diet, types and amount of carbohydrate replenishment, protein co-ingestion, nutrient timing etc.?
- For what level of athletes is fast glycogen resynthesis important?
- The positive and negative effects and consequences of deliberately or unknowingly delaying glycogen resynthesis
- I am an exercise physiologist, at first as a University Professor and later I came to join the sports drink manufacturer Gatorade.
I worked at Gatorade between 1985-2008 as their sports laboratory director, a part of the company that I came to develop from the ground.
During the last 12 years I have been working as an independent consultant for both small and big companies, which I have enjoyed in so many ways.
Basics of glycogen metabolism
- The most obvious take away is that the glycogen stores in our muscles and liver is extremely important for our ability to do high intensity work, which is true for both strength training but maybe most so for endurance training or racing.
Therefore, to be able to do high intensity work, one must make sure to be well fueled before the high intensity sessions.
For athletes who train plenty of volume, they must be really diligent to make sure that they are getting well fueled at a more or less constant basis.
- Our muscles and liver are most prone to restore the glycogens lost during hard work immediately after a race or a high intensity workout.
This process takes between 24-60h.
- We can also increase our glycogen stores in our bodies quite significantly by taking in plenty of carbohydrates at the same time as training volume is reduced.
- Glycogen is broken down whenever muscles are contracted, for most people glycogen is the primary energy substrate when the intensity gets over 65 % of VO2max, which basically is most exercise.
- During exercise, several hormones signal that the demand for energy is high, which leads to a consequent release in glycogen in the blood stream available for the muscle cells.
- Glycogen is extremely important in order to be able to do high intensity work because it is much faster and requires less oxygen to break down compared to fatty acids.
- However, one aspect that is important to point out is that as we get aerobically fitter, our ability to break down and utilize fatty acids as a fuel gets better and we become less reliable on glycogen metabolic pathways.
- Recent research also suggest that the level of glycogen stores not only is important as a storage for energy but also plays several important roles in signaling the total energy state of the body, which in turn could affect training adaptions.
For instance, if one wishes to target fat metabolism, one should aim to exercise during low glycogen stores as this stimulates the enzymes involved in the fat oxidation process to be more active.
- The glycogen content in the muscles of an untrained person is typically 80 mmol/kg muscle tissue and for an endurance trained muscle that same number is 120 (in world class athletes that number is maybe between 120-200 units, however, that assumes that the athlete is able to fully compensate for their very large carbohydrate consumption rates).
In a super compensated state, i.e. after 2-3 days of light training and a high carbohydrate intake, one can reach store numbers of 200 units.
- The glycogen state of the body and muscles are often overlooked as an important parameter for performance as well as ability to recover properly and reducing the risk for illness and injury.
I would say it’s not uncommon that many athletes engaged in high volume training do not reach their full potential on many of their quality sets due to lack of the right energy substrate for these high intensity workouts.
Hence, my recommendation is that athletes should be really diligent in terms of making sure that they take in sufficient of carbohydrates, and in this process, consulting a sports dietitian could be really helpful.
- As we engage in any kind of activity, the muscle glycogen stores start to deplete but the right of that depletion is dependent on the intensity.
During work close to VO2max intensity, the stores decrease rapidly since the muscles then are almost 100 % dependent on glycogen.
During medium intensity exercise, the glycogen stores typically become a limiting factor for continues exercise after around 2h.
- When glycogen stores shrink, once can roughly talk about two ”thresholds”, the first one being when the stores are down to around 50 % of normal levels.
At this point performance declines, but plenty of research suggest that driving down the muscle glycogen to such levels stimulate plenty of potential great training adaptations in the muscles enhancing performance.
This state could be reached either by accident (when an athlete fails to replenish sufficiently from workout to workout) or by purpose.
A common strategy, which in research settings have proven very effective is the sleep low-train low strategy, which means that you empty your glycogen stores by a high intensity workout in the evening or afternoon, after which you take in really little carbohydrates before you go to sleep and then engage in an activity again in the morning when you are really low on glycogen.
When glycogen stores have dropped to around 75 %, this is where most people experience that they ”hit the wall” and exercise intensity must drop.
- One can summarize it as it is probably a good thing that you’re glycogen stores are fairly depleted during certain specific time frames as this leads to further training adaptations, but one mist be really diligent to make sure that you’re fully replenished for quality sessions in order to get the most out of them.
One should also maybe add that these sleep low-train low strategies should only be implemented after an athlete have made sure that they do the basics right both when it comes to training and nutrition.
- The research suggest that it takes between 24-48h to fully replenish the glycogen stores after quite a high degree of glycogen depletion.
The rate of the replenishment is so-called bi phasic, meaning that the replenishment goes really quick in the beginning and then the rate declines as the stores get more and more filled up.
This is why it is particularly important to refuel immediately after a session as you then have the ability
to quickly get your glycogen stores to a decent level, in addition to that the muscles are really susceptible to taking up glycogen to refill their stores.
The recommendation is to try and take in 1-1.2 grams of carbohydrates per hour and kg body weight, ideally taking in 0.5-0.6 per 30mins.
After that, one should aim to have a proper meal within the next hour after the workout.
In terms of what type of carbohydrates one should aim to take in, that doesn’t seem to matter too much (main thing is that you take in ample amounts of carbohydrates of any kind).
- Also, research suggest that the recovery process could be even more rapid if the proteins are taken in together with the carbohydrates.
- Amateur athletes training up to 1h each day, independent on intensity, does not typically need to focus extra on making sure that they will be fully replenished until next workout (24h later).
However, during competition, this may be a time for even these athletes to pay a little bit extra attention to the carbohydrate intake before and after.
- There seems to be an upper limit of how much glycogen one and the same person can take up during a day and that limit seems to be around 10g of carbohydrates per kg body mass.
Fueling during workouts
- In the majority of cases, it’s highly beneficial to take in carbohydrates during workouts because it increases the access to glycogen for the muscles and hence makes it possible to maintain intensity throughout the entire workout.
Even if one aims to loose weight it is recommended to take in some carbohydrates during the workouts since it allows one to maintain intensity and also decreases hunger afterwards.
Typically, one should focus on the dietary aspects of loosing weight from 2h after the workouts and beyond.
Rapid fire questions
- What is the first thing you do as you wake up in the morning? During the last 6 months I have been doing pull ups since I think it is a good upper body exercise and it helps my swimming.
- What is your favorite book, blog or resource? When I read I almost always read non-fiction biographies (currently I read Apocalypse Never), but in recent time I have also really come to like watching plenty of youtube clips about sports science and/or swimming technique, I particularly like the GoSwim, Effortless Swimming YouTube channels.
- Who is somebody who has inspired you? My father, he wasn’t any ”special” person, but worked really hard his whole life to support his family and hence he has been a great inspirational source for me.
LINKS AND RESOURCES:
- Sports Science Insights - Dr. Murray's website
- Fundamentals of glycogen metabolism for coaches and athletes
- Musculoskeletal adaptations, “train low” strategies, and muscle fiber types with prof. John Hawley | EP#248
- High carbohydrate, low carbohydrate, or periodised carbohydrate intake with Louise Burke, PhD | EP#236
- Endurance sports nutrition: state of the art in 2019 with prof. John Hawley | EP#181