Recovery nutrition:
Eating carbohydrate replenishes muscle glycogen.

Thomas Solomon, PhD.
22nd August 2020.

Having already discussed the “post-exercise window” and summarised the importance and nuances of carbohydrate and protein timing between your sessions, I now delve into the science that has led to the current knowledge. So, for the nerds among you, buckle up and join me on a journey into the role of carbohydrates in your recovery nutrition.
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Glycogen is an energy store found in many tissues of the body. It is a branched polymer of glucose molecules that are ready to be cleaved and metabolised to produce energy (ATP). In the context of exercise, glycogen stores in your liver and your muscles provide a vital supply of glucose for keeping you moving forward. In the early 1900s, a classic “test tube” muscle physiology experiment by Fletcher and Hopkins showed that isolated frog muscle could resynthesise glycogen from lactate following contractions in the complete absence of oxygen (truly anaerobic conditions). Outside the test tube, in living animals, mice also resynthesise glycogen following exercise-induced glycogen depletion in the complete absence of re-feeding. This exciting phenomenon has now been extended to fish, amphibians, reptiles, and other mammals, including our canine and equine brethren. While evidence shows that human muscle has some capacity for this mechanism, our post-exercise glycogen resynthesis in the absence of re-feeding is very small and the phenomenon is not supported by all studies.

Since the late 1960s, thanks to Eric Hultman and Jonas Bergström (inventor of the needle muscle biopsy method), we have known that endurance exercise depletes muscle glycogen levels in humans and that glucose provision during exercise reduces the rate of glycogen depletion. The same is true for resistance exercise. Since the early 1970s, we’ve also known that repeated daily hard training (including running) sequentially lowers muscle glycogen. We also know that the magnitude of glycogen depletion is determined by the duration and the intensity of your session, your cardiorespiratory fitness (VO2max), your pre-session glycogen level, and whether or not you ate carbohydrate during the session, all of which are biological events that were excellently reviewed by Will Hopkins and Jose Areta in Sports Medicine meta-analysis.
Exercise depletes muscle glycogen. Thomas Solomon at Veohtu.
Figure re-constructed using data from Costill et al. (1971) J Appl Physiol.
When no food is eaten after exercise, us humans can synthesise some muscle glycogen from lactate and amino acids but we are not well-equipped to be masters of this biological incredibility. And so, carbohydrate feeding is our best tool for glycogen resynthesis and the more we feed the greater the rate of resynthesis. Even if we were well equipped to restore glycogen without feeding, shuttling amino acids away from protein building in the post-exercise period is unwise as it will encourage negative protein balance, making you weak and slow. Furthermore, following a bout of glycogen-depleting exercise, there is significant loss of amino acids from leg muscle during exercise, indicative of net protein degradation. Avoid getting yourself into this predicament too often, especially since low-carbohydrate availability during intense training increases protein intake requirements in endurance runners.

What is the optimal timing of post-exercise carbohydrate ingestion for replenishing glycogen?

To resynthesise muscle glycogen, carbohydrate must be eaten and digested, then glucose must be absorbed in the intestine, delivered to the muscle, taken up into muscle cells and stored as glycogen. Therefore, potential rate-limiting steps include the intestinal absorption of glucose, muscle glucose uptake, and glycogen synthesis. After exercise, when the gut is empty, ingested carbohydrates can easily be digested and moved into the intestine to be absorbed. After exercise, blood flow is increased and muscle capillaries are dilated, so glucose can easily be delivered to the muscle. During, and for about 2-hours after exercise, glucose uptake into muscle is increased because more glucose transporters are embedded in the muscle cell membranes to allow more glucose to enter the cell from the blood. For a few hours after exercise, the activity of glycogen synthase, the enzyme that stores glucose as glycogen, is increased. And, for up to 24-hours after exercise, the muscle cell is more sensitive to insulin, a hormone that is released when you eat food and tells muscles to allowing more glucose to enter the muscle cells. So, glucose delivery to muscle is always possible but glycogen synthesis is highest in the first couple of hours after your session.

What is the optimal dose (or rate) of post-exercise carbohydrate ingestion for replenishing glycogen?

When no food is eaten after exercise, the rate of glycogen resynthesis is very low (1 to 2 mmol/kg wet weight/hour). Note: “wet weight” is the weight of muscle that has not been freeze-dried and therefore still contains water. With carbohydrate ingestion this can be increased to 5 to 10 mmol/kg wet weight/hour and the rate of resynthesis is proportional to the amount of carbohydrate ingested… up to a certain point.

The increase in muscle glycogen synthesis rates during the 4 to 6-hour recovery period after a workout is relative to the amount of carbohydrate ingested during that period. The experimental model that is used to determine this involves a glycogen-depleting work-out (often combined with a low-carbohydrate diet) followed by different feeding strategies after the workout, and another exercise test later in the day. During these tests, muscle biopsies are taken at regular intervals so that glycogen can be measured (this is a method I have used a tonne in my own research). The evidence to date (recently reviewed by Javier Gonzalez and James Betts at the University of Bath) tells us that the muscle glycogen resynthesis rate following glycogen depletion maxes out when carbohydrate ingestion reaches about 1.2 to 1.4 g/kg bodyweight(BW)/hour—pushing carbohydrate intake beyond this dose causes no additional benefit. This observation is supported by evidence that running performance is also restored by carbohydrate ingestion but plateaus at an ingestion rate of approximately 1.2 grams of carbohydrate per kg bodyweight per hour within the 4-hours following glycogen depletion.

So, if your between-session nutritional “urgency” is high, consume up to 1.2 grams of carbohydrate per kg bodyweight per hour before your second session of the day to restore glycogen and get your legs ready to go again. Noting that this amount forms part of the recommended daily carbohydrate intake of 5 -10 g/kg/day for athlete’s engaged in moderate to heavy training loads. While most prior studies have examined glycogen repletion (and/or restoration of exercise performance) within a 4 to 6-hour window following glycogen depletion, it is important to remember that glycogen synthesis does not cease to exist beyond that 4 to 6-hour window. In fact, total carbohydrate intake over the 24-hour period following a run is positively associated with increases in glycogen repletion up to at least 650 grams (approx 2600 kcal) of total carbohydrate intake over those 24-hours.
Carbohydrate timing and dosing for restoring muscle glycogen after exercise. Thomas Solomon at Veohtu.
Data extracted from Alghannam, Gonzalez, and Betts (2018) Nutrients.

What is the optimal type of post-exercise carbohydrate ingestion for replenishing glycogen?

We consume carbohydrates in two forms: starch and simple sugars. Fibre is an indigestible starch that does not contribute to energy production, so when we talk about carbohydrate as a fuel, we refer to the “available” carbohydrate that is digested and absorbed. We eat starch in vegetables and (whole)grains, and it must be digested to simple sugars (like glucose, fructose, and galactose) before it can be absorbed in the intestine. We also eat simple sugars, like glucose and fructose, in fruits and veg and in grains, table sugar, energy drinks, candy, and cakes, and so on. As you can see, all carbohydrate-containing foods release simple sugars into blood—these sugars can be used to synthesise glycogen either directly (glucose) or indirectly (fructose, which can be converted to glucose in the liver). But, do different types of carbohydrate influence glycogen resynthesis rates? Of course they do.

Insulin is a hormone that increases muscle glucose uptake and glycogen synthesis. Just one bout of exercise increases the magnitude of the effect insulin has on these functions in muscle—this is known as an exercise-induced increase in insulin sensitivity, a phenomenon that was first demonstrated in humans in the 1970s and something I have studied in detail during my academic career. Since simple sugars cause an insulin response and since ingested simple sugars are more rapidly absorbed compared to starchy carbs, post-exercise sugar ingestion more profoundly increases glycogen resynthesis than starch ingestion. Therefore, high glycemic index (GI) foods (ones that cause a higher blood glucose and insulin response) typically result in a higher glycogen resynthesis response immediately after exercise. But, again, don’t forget about the bigger picture - adequate total carbohydrate intake over a 24-hour period can restore muscle glycogen no matter whether it comes from low-GI or high-GI foods or liquid vs. solid foods.

On a more granular level, different types of simple sugars also influence glycogen resynthesis. For example, glucose ingestion stimulates a greater glycogen synthesis rate than fructose ingestion while combined glucose and fructose (or fructose combined with maltodextrin, a tasteless long-chain glucose polymer) increases the rate of glucose appearance in the blood (i.e. has better intestinal absorption) and exerts an additive effect on glycogen resynthesis. For shorter bouts (30-mins) of running or cycling, combined glucose (or maltodextrin) and fructose feeding restores endurance performance better than glucose (or maltodextrin) alone. However, for longer bouts (90-mins), subsequent exercise performance within 4-hours of glycogen depleting exercise is equally restored with glucose or glucose+fructose ingestion during the between-session period. To summarise: when you train twice a day, combined glucose and fructose (or maltodextrin and fructose) in the post-exercise period may be more advantageous than glucose alone BUT total daily carbohydrate ingestion is always more important than stressing over the type of carbohydrate (or sugar) for restoring glycogen and restoring performance.