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.
Reading time ~12-mins (2500-words)
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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.
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.
How can you eat carbohydrates after exercise to replenish glycogen?
Sports drinks, bars, and gels that provide sugary mixtures of glucose (or maltodextrin) and fructose can indeed be used, but they are not nutrient dense and are not usually what many people crave at the end of a work-out.
So, what about some real food?
Some fun evidence shows that following exercise-induced glycogen depletion, muscle glycogen levels and endurance performance (20 km time-trial) are equally restored with either fast food (hamburgers, coke and fries) or sports nutrition products (Gatorade, Clif Bloks, Cytomax, and Power Bars) when matched for total calories and grams of carbs and protein. Like I said: fun. I love that research subjects came to a prestigious physiology lab and were told to exercise and then eat burgers. I do admit that in 2007, I went through a phase I called “long run dirty Sunday”, which involved driving to a different fast food outlet each Sunday night. I did not do this in the interest of “recovery”. Instead, it was prompted by the fact that I had just moved to the USA and was surrounded by an abundance of dirty food outlets and no cultural barriers to stop me. I still bloody love a burger. Anyway, before I head too far off-piste, I do not advise smashing a KFC family bucket after any of your workouts. Neither do I suggest always reaching for sports nutrition products (which I agree can sometimes be more convenient).
Fast food and sports nutrition products are not ideal since they are not nutrient dense—they do not provide a range of food groups nor a range of vitamins and minerals. Instead, try choosing nutrient dense whole-foods. This way, you can achieve your daily healthy eating goal while also achieving your between-session recovery nutritional goals. And, while doing so, you will learn that the “magical” glucose and fructose sugars are also found in very many of the commonly eaten foods—fruits are abundant in sucrose and its break-down products, glucose and fructose, and all carbohydrate-containing foods are ultimately digested to glucose.
When choosing whole-food sources, eating carbohydrates to replenish your muscle glycogen doesn’t have to mean becoming a spaghetti fiend. I am always surprised how many folks solely associate pasta with carbo-loading—there are lots of whole-food carbohydrate sources, including bread, rice, corn, potatoes, sweet potatoes, couscous, quinoa, fruits (bananas) and vegetables (carrots), etc. For example,
One 55-gram Powerbar energy bar provides
~200 kcals, including ~40 grams of carbs.
One-cup (100 grams) of dry oats + ½ cup (125 mL) low-fat milk + 1 medium banana + ½ tbsp honey provides
~495 kcals, including ~95 grams of carbs.
Two-cups (140 grams) of spaghetti + 2 tablespoons pesto provides
~370 kcals, including ~45 grams of carbs.
To help choose nutrient dense foods, ones that you like to eat, while ensuring they are meeting your recovery nutrition carbohydrate goals, there are a couple of very useful (and free) tools. FoodNutritionTable.com is an excellent resource for finding the nutritional value of different foods, as is the USDA FoodData Central; while Nutritionix host a user-friendly diet analysis tool with a database of grocery-, restaurant-, and common-foods from several countries.
What can you add to your training toolbox?
You can now bolster your recovery toolbox with knowledge of how the timing, dose, and type of carbohydrate can affect your “between-session” glycogen resynthesis rate and restoration of performance. But, carbohydrate-rich snacks, meals, or drinks need to be integrated with your other recovery nutrition goals, like aiding muscle protein synthesis and maintaining hydration. These goals apply to both of our hypothetical athletes, Stone Gossard and Deborah Anne Dyer.
Because of her high training load and high training frequency, Deborah, the 2:25 marathoner, makes purposeful glycogen-replenishment decisions soon after every session, especially on days when she trains twice and her work-outs are separated by just a few hours. Since she weighs 50 kg, Deborah aims for 250 to 500 grams of total carbohydrate intake every day (equivalent to 5 to 10 g/kg/day, based on current sports nutrition guidelines). On her heavy-load “2-a-day” training days, Deborah aims to consume ~60 grams of carbohydrate per hour between sessions (1.2 g/kg/h) and she does so using a mix of whole foods and carbohydrate-containing sports nutrition products. On the contrary, Stone, who weighs 75 kg and trains just three times per week, which includes a 5 km “parkrun” race every Saturday, is fine with eating ~375 grams of carbohydrate distributed across every day (equivalent to ~5g/kg/day). He does not stress about immediate post-exercise feeding or dosing and types of carbohydrate in the post-exercise window since he usually has 48 to 72-hours until his next session. I intend these fictional examples to relate to your own needs, helping you learn how to tailor your nutritional strategies accordingly.
As I will discuss in a future post, some training adaptations can be enhanced by occasionally doing sessions with low carbohydrate availability, which reduces muscle glycogen levels. This type of training can be put to good use but it is best achieved within a well-managed training programme. Why? Well, because it can easily be “eff dup” and because every ying has a yang—glycogen-depletion lowers both endurance and strength performance. For example, three-weeks of a low-carbohydrate diet (<50 g/day) worsened 10 km time trial finish times in elite athletes when compared to a high-carbohydrate diet (8.6 g/kg/day), and just two-days of a carbohydrate-restricted diet (1.2 g/kg/day) following glycogen depleting exercise decreased back squat reps to failure at 80% of 1RM, in trained lifters.
More coming soon on low carbohydrate diets and fat adaptation but for a deeper dive into glycogen in the context of athletic performance, I can recommend reading the “Fundamentals of glycogen metabolism”, a review pitched for athletes and coaches, or Postexercise muscle glycogen resynthesis in humans”, a review published by Louise Burke, Luc van Loon, and John Hawley, three of the world's leading experts on the topic.
Thanks for joining me for another “session” and, until next time, keep eating smart...
Disclaimer: I occasionally mention brands and products but it is important to know that I am not sponsored by or receiving advertisement royalties from anyone. I have conducted biomedical research for which I have received research money from publicly-funded national research councils and medical charities, and also from private companies, including Novo Nordisk Foundation, AstraZeneca, Amylin, the A.P. Møller Foundation, and the Augustinus Foundation. I have also consulted for Boost Treadmills and Gu Energy on their grant applications. These companies had no control over the research design, data analysis, or publication outcomes of my work. Any recommendations I make are, and always will be, based on my own views and opinions shaped by the evidence available. The information I provide is not medical advice. Before making any changes to your habits of daily living based on any information I provide, always ensure it is safe for you to do so and consult your doctor if you are unsure.
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About the author:
I am Thomas Solomon and I'm passionate about relaying scientific information to the masses and helping folks meet their fitness and performance goals. I hold a BSc in Biochemistry and a PhD in Exercise Science and am an ACSM-certified Exercise Physiologist and Personal Trainer, a VDOT-certified Distance running coach, and a Registered Nutritionist. Since 2002, I have conducted biomedical research in exercise and nutrition and have taught and led university courses in biochemistry, molecular medicine, and exercise physiology. My work is published in over 80 peer-reviewed medical journal publications and I have delivered more than 50 conference presentations & invited talks at universities and medical societies. I have coached and provided training plans for truck-loads of athletes and regular folk, have competed at a high level in running, cycling, and obstacle course racing, and continue to run, lift, and climb as often as possible. To stay on top of scientific developments, I participate in journal clubs and peer-review papers for medical journals, and I invest every Friday in reading what new delights have spawned onto PubMed. In my spare time, I hunt for phenomenal mountain views to capture through the lens, boulder problems to solve, and for new craft beers to drink with the goal of sending my gustatory system into a hullabaloo.
One 55-gram Powerbar energy bar provides 
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