Glycogen: The Best Fuel for Your Muscles

Glycogen – Key Points:

  • Your muscles and your liver are your two main stores of glycogen. You fill them up by eating carbohydrates.
  • Your body prefers to use muscle glycogen to fuel intense workouts and other demanding physical work. Liver glycogen keeps your blood sugar stable when you don’t eat, like when you’re sleeping.
  • The average person can store a little more than 500 grams of glycogen in the muscles and 100 grams in the liver. If you exercise regularly, you increase that capacity.
  • If you’re not a high-level athlete, you replenish your glycogen stores with your regular diet without having to plan or time anything.
  • High-level endurance athletes might need up to 10 grams of carbohydrates per kilogram of body weight per day to fill up their glycogen stores in time for the next workout.
  • The amount of carbohydrates you eat is more important than details like type, glycemic index level, and if you get them from solids or liquids.
  • If you eat or drink carbs during long workouts, you have glycogen in your muscles left late in the training session. That allows you to train harder and longer.

Glycogen is your muscle’s favorite fuel! Even though you can use pretty much anything you eat to fuel them, only glycogen will do if you put them to challenging work. After reading this article, you’ll know what glycogen is and how your body utilizes it during workouts, as well as the importance of muscle glycogen for optimal performance.

What Is Glycogen?

Glycogen is a large polysaccharide with many branches, as illustrated in the picture below. Polysaccharides are carbohydrates made up of simple sugars. Glycogen, in particular, is made up of many molecules of the monosaccharide glucose.

A cross-sectional view of glycogen.

You store the carbohydrates you your body don’t need as immediate energy as glycogen, mainly in your muscles and liver. The average person carries around about 600 grams of glycogen when those two stores are filled and combined.1 That number is only an estimate. It depends on many factors, like how much muscle you have, what your diet looks like, your fitness level, and your exercise habits.

Your muscles prefer to use glycogen as fuel when it’s available. During prolonged or intensive physical work, you break down glycogen particles. That frees up glucose molecules, which you then oxidize into ATP-molecules necessary for muscle contractions.2 ATP stands for adenosine triphosphate, the primary energy source for almost all processes in your cells.

Every gram of glycogen retains 3 grams of water, give or take.3 We’re talking about water inside your muscles, water weight that makes you both look and perform better. It doesn’t make you look bloated or puffy. However, you can gain or lose quite a lot of weight, fast, from water weight, depending on your activity and diet, when you fill up and empty your muscle glycogen. An athlete able to store 800 grams of muscle glycogen has 2.4 kilograms of body weight in the form of glycogen + water right there.

Liver Glycogen

The average liver weighs 1,500 grams. Between 6–10%, or 100–120 grams, is glycogen, when the liver is full, after some hefty meals and rest.

After a carbohydrate-rich meal, your blood sugar levels increase. Your pancreas responds to this rise in blood sugar by releasing insulin, helping your liver cells to absorb the glucose. The insulin also activates enzymes that convert glucose to glycogen. One such enzyme is called glycogen synthase. Once both glucose- and insulin levels are high enough, glycogen synthase ties the glucose molecules together in the form of glycogen chains.4

Eventually, your blood sugar and insulin levels taper off and go back to fasting levels, and glycogen synthesis follows suit. If your blood sugar gets low, your pancreas reacts again. Low blood sugar is hazardous, and your body won’t allow it to fall below tightly regulated levels unless you have diabetes or another medical condition that prevents proper blood glucose control. The pancreas releases glucagon, a hormone that tells your liver to break down glycogen. This way, you free up the glucose molecules, which soon appear in your blood.

What’s the practical relevance of these processes? It means that regardless of what you eat, or even if you eat, your liver and the glycogen you store in it keeps your blood sugar within predetermined and tightly regulated intervals. When your blood sugar levels rise, your liver stores the excess sugar as glycogen, and when they fall, your liver releases that glucose again, keeping your blood sugar where it should be. Four grams of glucose circulates in your blood. Your blood sugar drops when you fast, like during the night when you’re catching some Zs and when you exert yourself physically. When that happens, your liver steps in and makes sure your blood sugar level goes back to those four grams, with the help of its glycogen stores.

glycogen
The liver of a well-fed rat under an electron microscope. The dark areas are glycogen granules.
glycogen
The liver of a starved rat, after 24 hours without food. Now, the glycogen is all but used up.

Of all your organs, your brain is the one using most of your blood sugar. Your brain metabolizes around 60% of the glucose in your blood.5 US authorities base their recommendations of 130 grams of dietary carbohydrate per day upon the brain’s needs.6

Under normal circumstances, your brain is entirely dependent on glucose as the energy source for ATP production. When things are not normal, it adapts and learns to use other substrates as fuel. However, as long as you eat a mixed diet, it very much prefers glucose. One example of an abnormal condition is when you eat a ketogenic diet. That doesn’t mean that abnormal things in general or ketogenic diets, in particular, are automatically bad. It just means that they force your brain to adapt to other fuel sources than the one it prefers.

Muscle Glycogen

While glycogen can make up a tenth of your liver’s weight, your muscle weight is only 1–2% glycogen. However, your muscles weigh a lot more than your liver, which means that the total amount of glycogen you store in it is much more significant.

The average adult can store a little more than half a kilogram of muscle glycogen. Intense or extended physical activity empties these stores, but not below 10% of your max muscle glycogen level.7

You more fit you are, and the more muscle mass you have, the more glycogen your muscles can hold. The glycogen stores of an elite-level endurance athlete are much larger than those of a couch potato. If you spend your days at a desk and your nights in bed, you don’t need much muscle glycogen. Your body adapts your stores according to your needs. If your muscles need a lot of high-quality fuel readily available, your body makes sure you can store it, too. A high-level cross-country skier might be able to hold twice as much muscle glycogen as the average person.8 There is little to no research on strength-training individuals, but the same principles should apply. For example, a 100-kilogram bodybuilder is probably capable of storing massive amounts of muscle glycogen.

When exercising at 60–65% of your maximal oxygen consumption or above, your muscles rely on glycogen as their primary fuel. Your muscles oxidize stored glycogen, turning it into the ATP molecules they need to contract. The higher your intensity, the more motor units associated with fast muscle fibers you activate. Low-intensity exercise relies more on fatty acids as fuel. However, fat isn’t good enough once you ramp up the intensity. Your muscles require carbohydrates in the form of muscle glycogen and blood glucose for peak performance.9 10 

In other words: the harder you train, the more you rely on muscle glycogen and blood glucose to fuel your workout. Fatty acids in your muscles and your blood won’t cut it.

Other Places You Store Glycogen

Even though your liver and your muscles are your two primary glycogen deposits, you also store smaller amounts inside your red and white blood cells, in your kidneys, and the glial cells of your central nervous system. Pregnant women hold a certain amount of glycogen in the uterus as a glucose source for the embryo: a nifty little mechanism that grants the fetus access to sugar during the early parts of the pregnancy.11

Carbohydrates, Glycogen, and Performance

For more than 100 years, scientists have documented the importance of carbohydrates for athletic performance.12 By observing marathon runners, researchers during the 1920s determined an association between blood glucose levels and fatigue.13 They also found that eating plenty of carbohydrates before a race allowed the runners to perform better. And when they consumed sugar-based candy during the marathon, they fended off fatigue effectively.14

Glycogen was discovered way back in 1858.15 However, it took more than 100 years before Swedish scientists analyzed muscle biopsies from athletes and concluded that muscle glycogen levels directly influence physical performance.

Today, we know that a carbohydrate-rich diet is an integral part of the athlete’s strategy to perform his or her best and recover quickly from training sessions. Even though more recent research shows that you can train both hard and effectively with a low-carb diet, or even entirely without carbs, you’ll have to look hard to find elite-level athletes who don’t rely on carbohydrates to perform and recover.

How Much Glycogen Do You Store In Your Body?

The amount of glycogen stored in your liver and muscles depends on what you do and how much you eat. When you exercise or perform some other type of physical labor, you fuel that activity with glycogen from both the liver and muscles, depleting your reserves. You drain your liver glycogen to keep your blood sugar stable and use muscle glycogen to fuel your muscle contractions.

The longer and more intensely you exercise, the faster you’ll empty your glycogen reserves. If you eat plenty of carbohydrates and enough calories, you replenish those reserves in between workouts, allowing you to perform as good or better next time you exercise. If you exercise regularly, your body adapts and enables you to store more carbohydrates as muscle glycogen.

A casual, non-overweight endurance athlete needs between 500 and 600 grams of carbs a day to replenish and maintain high glycogen stores after a workout.16 A diet providing 60% of the total number of calories usually covers all such requirements.

Muscle glycogen concentrations are usually measured in millimoles per kilogram, either as wet weight or dry weight. Dry weight means that you subtract the water in the tissue from the result, and those values are 4.325 times higher than the values for wet weight.

The muscle glycogen concentration of an untrained, average-weight, and well-fed individual is somewhere between 80–120 mmol/kilogram wet weight.17 The concentration of muscle glycogen in a well-fed and rested athlete is usually around 150 mmol/kilogram wet weight. After a few days of eating many carbs and relaxing, the muscle glycogen concentration of high-level endurance athletes can go above normal levels, so-called supercompensation, up to 200 mmol/kilogram wet weight.

Glycogen Use During Exercise

During low-intensity workouts, you don’t break down a lot of glycogen per minute of exercise. You don’t need such a powerful fuel to perform at that intensity, simply put. Fueling with fat is enough, and you have plenty of that to last a long time. However, when you work as hard as you can, it’s the opposite: fat no longer cuts it, and your muscles immediately turn to glycogen to fuel your efforts. You can release up to 20 times more glucose per minute from your glycogen stores during high-intensity exercise.

When you exercise with high intensity or long duration, you more or less empty your muscle glycogen stores.18 When this happens, your muscle cells can no longer produce ATP at a high enough rate. You reach the point of fatigue and have to lower your training intensity to continue. You might even have to stop completely – you “hit the wall.” The more muscle glycogen you’re able to hold, the longer you can exercise at a high intensity.

If you regularly train without eating enough carbohydrates, your muscle glycogen levels keep diminishing, day by day, and you might not be able to perform to your usual standards.

Replenishing Your Glycogen Stores

During exercise, you drain your glycogen stores. Not entirely, but a long and hard workout will deplete the working muscles’ glycogen. If you want to fill those stores again, you have to eat or drink enough carbs in time for the next training session.

After a workout or any other glycogen-depleting physical activity, your muscle cells are more sensitive than usual to any carbohydrates you eat. They are ready to soak up all the carbs you give them and store them as glycogen to perform better next workout.

At this time, your insulin sensitivity is as good as it gets, the capacity of your muscle cells to absorb glucose is improved, and your levels of the enzyme glycogen synthase are high. Eating or drinking carbs now stimulates insulin release and glycogen synthesis. Because your muscle cells are wide open and ready to receive the sugar you give them, you can shovel a lot of glycogen into them in a short amount of time.19

Endurance Training

The best way to utilize this high sensitivity and improved capacity to store glycogen is to eat carbohydrate-rich foods both often and in large amounts during the hours after the depleting workout. One gram of carbohydrate per kilogram of body weight per hour lets you store as much glycogen as possible inside your muscles.

That’s a lot of carbs. Too much for the casual athlete, in most cases. Let’s say you weigh 80 kilograms. That would mean 300 grams of carbs over 4 hours. Eating can be fun, but shoveling food into your mouth when you are already uncomfortably full isn’t. 

While these amounts are unnecessary, maybe even detrimental, for the casual athlete, they might be essential for high-level athletes who train every day or even several times per day. If they don’t get enough carbs daily, they won’t replenish their muscle glycogen within 24 hours.

Even professional athletes might find themselves hard-pressed to consume such amounts. Optimal laboratory numbers and results aren’t always optimal in every way in a real-life scenario.

One scenario where you benefit from pounding the carbs would be if you compete in multiple events over the same day. Or during an especially tough training period, if you have several training sessions every day on the agenda and repeatedly empty your glycogen reserves. If you have 5–6 hours to replenish your muscle glycogen as much as possible, you need to eat or drink 1 gram of carbohydrate per kilogram of body weight per hour during those hours. Don’t expect to do it with boiled potatoes and broccoli. Here’s where concentrated carbohydrate sources, including liquids, are a boon.

If you have 24 hours, you need around 10 grams of carbohydrates per kilogram of body weight during that time if you want to fill your glycogen stores again after depleting them.20 Preferably, you want multiple smaller carb intakes rather than a couple of large ones. That’s the most effective strategy to ensure efficient glycogen synthesis.

Also, you can’t just focus on carbs. You also need plenty of calories.21 You can’t eat fewer calories than you burn if you want to restore your depleted glycogen. It won’t happen even if your carbohydrate intake itself is large enough. Enough energy is vital in this process. As if that wasn’t enough, you need enough protein to repair and build muscle- and organ protein, as well as fat for your hormones and cell health.

Let’s go back to the 80-kilogram example person. Not only does he have to eat large amounts of carbs, but he also needs a lot of calories. The carbs he needs to restore his muscle glycogen adds up to 3,200 calories all by themselves. Add to that 2 x 80 grams in the form of protein, and you have another 640 calories right there. Plus 350 to 700 calories from fats. As you can see, you end up with more than 4,000 calories for an average-weight athlete.

My point isn’t that eating 4,000+ calories is weird or wrong. It’s not. It can be necessary. The fact is that you need to eat a lot if you train a lot, if your goal is performance rather than weight loss, and it’s not always something enjoyable. That number of calories is not too much for an endurance athlete, but many feel that it means an uncomfortable amount of food daily. 

Adapt your carbohydrate intake to your training schedule. Days you rest, train less or not as intensely, you probably don’t need as many carbs as when you go to bed physically exhausted.

To ensure as fast and complete glycogen replenishment as possible, you need to satisfy the following four criteria:22 23 24

  • Large amounts of carbohydrates
  • Frequent carbohydrate-rich meals
  • High glycemic index
  • Reach at least caloric balance

Strength Training

If your focus is lifting, you have it easier.

Strength training burns through a considerable amount of muscle glycogen per minute. However, “normal” strength training sets do not last for minutes on end, and more importantly, you don’t usually lift continuously for an hour without rest. A strength training session is most often split into short periods of near-maximal and depleting efforts and rest periods, unlike endurance training. In aerobic exercise, the same muscles contract without pause for a long time, thereby using more muscle glycogen.

Most strength programs won’t have you training the same muscle groups day in and day out without rest days in between. You might train chest and triceps one day, legs the next, and then finish off your split with a back and biceps workout. You deplete muscle glycogen locally, meaning in the working muscles. Glycogen stores in your other muscles remain intact. In other words, and using the above example, you’re in no hurry to replenish chest muscle glycogen since leg day is up next, and those glycogen stores are unaffected by your bench presses.

If you’re not training the same muscle groups again within 24 hours, you’re in no hurry at all. Any kind of average diet will replenish your glycogen stores in time for your next workout, as long as you eat enough calories. You don’t need to consider anything special, like carb-loading or timing your carb intake down to the minute or gram if your training mainly consists of lifting weights.

The one exception might be the type of whole-body training gaining popularity in recent years, where you train the entire body every day, day after day, five days in a row. If that’s your jam, you might benefit from planning your carbohydrate intakes to make sure you eat enough carbs after training and time those intakes to your workouts.

The Best Carbs to Load Your Muscles With Glycogen

Which carbohydrate sources are the best?

The ones you like the best and find it easy to eat enough of. Only high-level endurance athletes need to pick and choose between specific carbohydrate sources depending on which ones give the most efficient glycogen synthesis.

As long as you’re not depleting the same muscles more often than every 24 hours, the most important dietary factors are the total amount of carbs you eat, along with your energy intake. Those two things determine if you will replenish your muscle glycogen, not if you get your carbs from pasta, pancakes, or sponge cake.

glycogen
Pretty much any type of carbohyrdate-rich foods will fill up your glycogen stores effectively.

That said, fructose is not as useful for muscle glycogen storage as glucose or starchy carbs. Your liver has first dibs on the fructose you eat.25 You only need to give this any thought if you have no more than a few hours to fill your muscles with as much glycogen as possible. That’s when you need to ensure that as many grams of carbohydrate as possible end up as muscle glycogen. Of course, you need to replenish lost liver glycogen as well, but your liver isn’t picky. It doesn’t need fructose and can use glucose and starchy carbs just fine.

Since regular sugar is 50% fructose, choosing glucose- and starch-based carbohydrate sources is a good idea if you’re in a hurry to load your muscles with glycogen. Again, you don’t need to fiddle with different carbohydrate sources unless you’re an elite athlete in a hurry to restore glycogen and peak performance. Otherwise, it doesn’t matter.

You might have heard that you should avoid fat after a workout since a fat-rich meal slows down gastric emptying and delays your uptake of carbohydrates and protein, and therefore glycogen synthesis as well. In a real-life scenario, post-workout fat or no post-workout fat does not seem to make any difference.26 In other words, you don’t have to choke down dry rice to replenish your glycogen stores. You can safely drench it in a fatty sauce if you like.

Solid foods and liquids fill your glycogen reserves equally well, as long as you get the same amount of carbs. If you’re in a real hurry, drinking carbs is often easier than loading up on hefty pasta meals.27 If you’re not in a hurry, it’s probably a waste of calories to drink many sugary beverages without any real benefits.

High or Low Glycemic Index?

Glycemic Index, or GI, is a measure and ranking of how much the carbohydrates you eat affect your blood sugar. High GI foods make your blood glucose levels spike rapidly, which tells your pancreas to release insulin to keep your blood sugar in check. Low GI foods don’t affect your blood sugar as much in a short time and don’t require as much insulin.

When the Glycemic Index was a new and fresh concept, many believed we had found the holy grail to control body fat and body weight. Today, we know that glycemic index is pretty much irrelevant and that energy balance trumps glycemic index every day of the week. The Glycemic Index is a valuable tool for diabetics, but when it comes to body fat and body weight, it’s a non-issue rather than a grail. Also, low or high GI meals before a training session don’t seem to alter performance.28

The Glycemic Index of your carbohydrates influences how fast you store them as glycogen, though. If you eat primarily high GI foods the day after a depleting workout, your blood sugar and insulin levels spike. That leads to an up to 25% faster and larger glycogen synthesis compared to low GI carbs.29

Is this something the casual athlete needs to consider? Probably not. Neither does it matter when you have more than 24 hours between workouts. In those cases, the total amount of carbs you eat is much more critical. But again, if you need to replenish muscle glycogen in a hurry, you need all the tricks in the book, and that’s when opting for high GI foods is beneficial.

Other Methods to Improve Glycogen Synthesis

By eating your carbs with other nutrients or together with certain supplements, you can speed up glycogen synthesis a bit. Some athletes need up to 10 grams of carbs per kilogram of body weight per day to replenish their muscle glycogen completely. Since you also need to rest from training and eat plenty of calories, it can be tough to get enough. Any help along the way from other nutrients and chemicals might be welcome.

Protein

Everyone knows carbohydrates release insulin. However, so does protein, in some cases just as much. Whey protein is the insulin champ, making your pancreas release just as much insulin as the same caloric amount of sugar or white bread.30 The hours after a workout, you can store more glycogen inside your muscles if you combine carbs with protein.31 32 That way, you can get away with a doable 0.4 grams of carbohydrates per kilogram and bodyweight per hour and still load up your muscles with glycogen effectively.

A brand new meta-analysis reviewing the available research concludes that adding protein only leads to better glycogen synthesis if it also means you get more calories.33 Remember that you don’t lose any glycogen synthesis by replacing some of the carbs with protein. That opens up for more varied post-exercise meals.

Another advantage of eating protein after a training session is, of course, building muscle. Whether your exercise of choice is running or lifting weights, you can’t repair the muscle fibers your break down or build new muscle mass properly until you eat or drink some protein.

Why not kill two birds with one stone by combining carbs and protein in your post-workout meal? You get the best of both worlds, for recovery and for building muscle. Not only do you fill up your muscles with glycogen faster and more effectively, but you also start building muscle as soon as possible. You don’t need the carbs to build muscle, but you can’t replace the glycogen you just used up during the workout without them.

Carbs + protein replenish your muscle glycogen faster than carbs alone.

Creatine

The jury is still out on the exact mechanisms, but several studies show that creatine helps you store more glycogen in your muscles after training.34 35 Another example of combining carbohydrates with something that helps you build strength. And you recover better as a bonus.

Caffeine

A cup of coffee or ten along with your post-workout carbs give your glycogen synthesis a helping hand.36 37 It’ll boost your glycogen synthesis rate, compared to the same amount of carbohydrates on their own. Unfortunately, you might need a whopping 8 mg of caffeine per kilogram of body weight to do the trick. That’s a lot of caffeine, more than some can handle. You might get side-effects like dizziness, tremors, and nausea along with the effects you want. You probably have to use caffeine supplements rather than regular coffee if you don’t want to drink more than a liter of coffee in one go after your workouts. If you work out afternoons or evenings, you might find yourself looking forward to a sleepless night if you take half a gram or more of caffeine a few hours before going to bed, regardless of source.

Caffeine does not help save your stored muscle glycogen for later during your training sessions.38 Caffeine boosts your exercise performance through other mechanisms, but don’t expect longer-lasting glycogen reserves by gulping java before a workout.

Carbs Before a Workout: A Good Idea?

The last century’s research mainly focused on carbohydrate loading and increasing glycogen storage capability. Sooner or later, such a narrow field of study is exhausted. After the 1970s, exercise scientists started looking at the effects of carbohydrate intake before and during exercise and competition instead.

If you eat a meal with many carbs 3–4 hours before a workout, you improve your performance compared to exercising in the fasted state.39 40 In that short time, you can jam a bit more glycogen into your muscles and liver before you start exercising. You can increase your glycogen stores up to 15% in those few hours.41

That’s only possible if you chose high glycemic carbohydrates, though. Low GI carb sources don’t cut it here. Also, you use a lot more muscle glycogen as fuel during your exercise if you eat high GI foods rather than low GI foods.

You “burn” more fat during your training session if you load up on low GI carbs. That way, you have more muscle glycogen left until the later stages of your training session. Performance-wise, things might even out regardless of carbohydrate choice. Using more or less fat as fuel during a workout is not associated with losing body fat or body weight. It’s just a matter of what kind of energy your body prefers to use during various parts of your training sessions and possibly allow you to perform better. As for losing body fat over time, look no further than good old and boring calories in vs. calories out. Burning more or fewer carbs or fat during a particular workout is not a thing for weight control or losing fat.

Studies that suggest a performance benefit from eating carbohydrates before training look at endurance training. There are a few investigating possible effects on strength training performance, but they aren’t as optimistic. Most of them find little to no impact on how much you can lift, and it doesn’t seem likely that pre-exercise carbohydrate meals do much of anything for your performance in the gym.42 43

If endurance training is your thing, you gain a performance benefit by eating a carbohydrate-rich meal a few hours before working out. Also, the more carbs you eat, the better, at least up to a specific limit. Research shows that 2.2 grams of carbohydrates per kilogram of bodyweight improve performance compared to 1.1 grams per kilogram when ingested an hour before training.44 Both amounts are better than exercising fasted, though, for improved performance.

Before a training session, current carb intake recommendations suggest that a meal providing 1–4 grams per kilogram of bodyweight 3–4 hours ahead of the workout could be a good idea for peak performance.45

Carbohydrates During Workouts

Even if you have prepared by filling up your glycogen stores, your can improve your exercise performance even more if you eat or drink carbs during the training session itself.46

Several mechanisms drive this performance boost.

You rely less on muscle glycogen if you provide carbs during exercise. That means that you’ll have more of your glycogen stores left later on and can keep your training intensity up for longer.47 48

Also, you prevent liver glycogen depletion.49 By doing so, you have liver glycogen left at the tail end of the workout, when you would otherwise be running out. You have less liver glycogen than muscle glycogen, and if you have liver glycogen left, you maintain your blood sugar better, helping your muscles oxidize more carbohydrates for energy.

Together, these effects allow you to train longer and harder by having some carbs ready to use during your training sessions.

If you want to use many carbohydrates from outside sources during a workout, you need to trick your intestines. Early studies showed that you absorb about 1 gram of glucose or maltodextrin per minute from your small intestine, then you saturate the transporters that move sugars through plasma membranes. Recent research demonstrates that the combination of glucose + fructose or maltodextrin + fructose lets you absorb and utilize up to 1.8 grams of carbohydrates per minute. 50

On average, 30–60 grams of carbohydrates per hour is suitable if your workout lasts for 60–150 minutes. If you exercise even longer, you might benefit from up to 90 grams per hour.51 52

It matters little if you choose solid foods like raisins or rice cakes to snack on or prefer drinking functional beverages like sports drinks. Commercial energy gels and home-made sugary lemonade works, too, as do any combination of these. The important thing is how your stomach reacts to carbs during a training session. A hundred grams of raisins improve your performance just as much as an expensive commercial energy gel but might also force you to jump into a shrub to relieve yourself halfway through your workout. Your stomach often has a mind of its own, so you’ll have to find out what it likes the best by trial and error. Competition day is not the time to try something new. Handle that during not-so-important training sessions. When it counts, you want to use something you know your stomach tolerates without issues.

As usual, the research on the subject is endurance training-oriented. If you’re primarily interested in performing better in the gym, science does not have any clear answers for you. A couple of studies show improved performance when it comes to strength training, too.53 Others suggest that carbs during or immediately before a strength training session do little for performance, even though they spare stored muscle glycogen.54 55 In theory, high-volume strength training should benefit more from this practice, but at the moment, we don’t have enough high-quality evidence to say yay or nay. Feel free to give it the old college try. Consuming carbohydrates during a workout likely has no adverse effects unless you’re trying to cut down on calories.

Carbohydrate Mouth Rinse

Carb intake during exercise improves your performance even when the training session is shorter than an hour, even though your muscle glycogen should not be a limiting factor.56 That brings us to a new and exciting field of research, namely carbohydrate mouth rinsing. You might not need to store carbs as glycogen or even digest them to benefit exercise performance. Rinsing your mouth with a carbohydrate-rich liquid for 10 seconds every five to ten minutes during a workout seems to affect the central nervous system and your performance positively, even if you spit instead of swallowing.

Most mouth rinse-studies use cycling as the exercise of choice. One meta-analysis found that carbohydrate mouth rinses improve cycling power, but that this does not translate into decreased time to complete a cycling time trial.57 However, one year later, a new study demonstrated just that.58

We don’t know what drives this performance-enhancing effect yet, but it does not seem to be dependent on a sweet taste like you might think.59 Maybe receptors in the oral cavity sense the presence of carbohydrates and activate your nervous system and the parts of your brain responsible for triggering your reward system and recruiting muscle fibers, even without any sweetness.

How is this relevant to glycogen? You see, carbohydrate mouth rinsing is more effective if you exercise during a fast or when you eat a carbohydrate-restricted diet . When your muscle glycogen levels are low.60 That means that you could benefit from the carbohydrate mouth rinse tactic if you “train low, compete high,” meaning deliberately training with low or depleted glycogen levels.

Speaking of “train low, compete high,” let’s take a look at that concept.

Train Low, Compete High

“Train low, compete high” is a training method where you perform most of your workouts without enough carbs available to perform your best. That might sound counter-intuitive, but it seems to force your muscles to adapt to the situation, leading to better results in the long run. That’s the “compete high” part: when it counts, you load up on carbs, and the combination of your adapted muscles and the sudden influx of a high-quality energy source lets you perform above your previous best. The method has scientific support, especially if we’re talking about endurance training.

Endurance Training

You improve your exercise capacity when your muscles adapt to the demands you put on them. Adaptations include things like enhanced fat oxidation, angiogenesis (the process of creating new blood vessels from existing ones), and a larger mitochondrial mass. Almost all the ATP, the primary energy source for your cells, is manufactured inside your mitochondria: the larger your mitochondrial mass, the more effective your ATP production. The result is your muscles producing more energy if we’re talking about exercise and training in particular.

Signals from your working muscles control these effects. When your muscles contract, like during a training session, a cascade of signals activates or shuts down metabolic pathways, controlling gene expressions and protein turnover.

Many decades of exercise physiology research, beginning in the early 1900s, show us methods to provide exercising muscle with as much carbohydrate as possible, before, during, and after workouts. A plentiful supply of carbs is key to optimal performance.

At the same time, more recent research suggests that your training results might improve if you regularly train without that plentiful supply of carbs. You rob your muscles of their preferred fuel and force them to adapt to lesser sources.

Both short-term and long-term studies show more significant enzyme activity and more mitochondrial protein content if you regularly train with “too little” available carbohydrate. You also get a more effective fatty acid turnover in your muscles and your entire body. The glycogen content in your muscles and how much carbs you eat add to these effects. That kind of carbohydrate restriction can improve your performance and training capacity over time.61 62

The term “train low, compete high” refers to the fact that you utilize carbohydrate periodization to get the desired effects. In other words, you train without a lot of carbs leading up to a competition or an important event, and then you make sure you load your muscles with glycogen and eat plenty of carbs when it counts. That way, you combine the greater training adaptations from carbohydrate restriction with the benefits of carbohydrate loading, giving you the best possible performance when you want it and need it the most.

Remember that “train low” does not mean “train zero.” Don’t misinterpret the concept and eradicate all carbohydrate sources unless you, for some other reason, eat a ketogenic diet. If restricting carbohydrates means better results, no carbohydrates do not mean even better results. That could have the opposite effects, leading to low energy availability, fatigue, and even loss of muscle mass and depressed immune functions.

Training Before Breakfast

Training without enough carbohydrates might be something for elite athletes who need optimal results at all costs to consider. However, the casual athlete likely finds that method of training less than fun. And fun is integral to regular exercise habits.

An easier way to accomplish “train low” and reap similar benefits is to exercise before eating breakfast. You don’t run out of muscle glycogen during an overnight fast. Your liver, on the other hand, won’t have too much glycogen left. It has been busy keeping your blood sugar stable while you were snoring.

According to several studies, this milder variant of “train low, compete high” is adequate for most. You force your body to use more fat as fuel during your workouts, increase the activity of enzymes controlling muscle glucose uptake, improve fat oxidation, and optimize mitochondrial function, compared to always loading up on carbs before your training sessions.63 64 65 

Exercising before breakfast like this leads to similar training adaptations in the long run as more dedicated carbohydrate restriction.66

What About Strength Training?

As usual, strength training research is less abundant, and that research tends to be ambiguous. There is no consensus yet.

Muscle Protein Synthesis and Breakdown

Most available research suggests that you don’t need to eat many carbs or make sure your glycogen levels are full to build muscle. You get the same anabolic effects and stimulate muscle protein synthesis just as well regardless.67 68 69 On the other hand, these studies do not provide evidence for any advantages if you “train low” either.

Other studies speculate that not eating enough carbs could have negative consequences.

The anabolic response to a strength training session is mainly dependent on signaling mechanisms and metabolic pathways, just like endurance training. However, the two different types of exercise activate different pathways. One of the most powerful ones for building muscle is the so-called mTOR-complex. Signaling pathways activated by low energy availability and depleted glycogen reserves inhibit mTOR.70 71 In other words, weight training with low muscle glycogen levels could also mean not gaining as much strength and muscle mass from your efforts.

Muscle protein synthesis is the essential part of the muscle protein balance for building muscle mass. It’s not the only one, though. Muscle protein breakdown also factors in.

Research from Swedish scientists suggests more significant muscle breakdown if you train with depleted muscle glycogen.72 Also, as you probably already know, carbohydrates release insulin. Insulin, in turn, reduces muscle breakdown and improves nutrient uptake in your muscles. Cut down on carbs, and your insulin levels drop. In theory, that might mean that you break down more muscle mass and provide your tired muscles with fewer nutrients with your post-workout meal.

Keep in mind that these are theoretical effects. Protein also releases plenty of insulin. You always want to make sure your muscles have access to protein when you’ve lifted weights if you want yo build muscle. The protein in and of itself does not reduce muscle breakdown, but the protein intake’s insulin response does. Also, a very moderate insulin release reduces muscle breakdown maximally, and a normal-sized protein intake is enough for that insulin release.73

Performance

Beyond all that, you have to consider a couple of other things: performance and your immune system.

When you lift weights, you primarily use muscle glycogen to fuel your efforts. Though you don’t use up as much as when you perform aerobic training, you still rely heavily on your glycogen reserves, even at a relatively low intensity and volume. One study reported that five sets of leg extension at 60% of 1RM depleted thigh muscle glycogen by 40%. Another study found ten reps of biceps curls reduced muscle glycogen in the biceps muscle by 13%. Three such sets used up 25% of the glycogen stores in the muscle.74

As you know by now, carbohydrates before an endurance training session might improve your performance and work capacity. If the same goes for strength training is unclear, even though your muscles rely on their glycogen stores to lift weights.

Several studies suggest that you can handle a higher training volume if you eat carbohydrates before hitting the weights.75 76

On the other hand, several other studies show that strength training performance does not benefit from extra carbs.77 78

In other words, we can’t draw any definite conclusions about the importance of carbohydrates to perform in the gym from current evidence. However, that allows you to conduct some unscientific experiments on your own. If you prefer a low-carb diet and feel that you perform just as well without loading up on bread and pasta, maybe it doesn’t matter what participants in various studies report. The same applies if you notice the opposite, that you perform better in a carb-loaded state.

Immune Function

Training with more or less depleted glycogen levels and generally low carbohydrate availability lead to more stress hormones. Markers indicating immune function are also negatively affected.79

Possibly, that could increase your risk of catching colds and other infections if you train with too few carbohydrates long-term. Even though always exercising with a low carbohydrate availability might depress your immune system, the milder version of carb restriction, training before breakfast, does not seem to have any negative effects in this regard.80

Summary and Conclusions

  • The average person stores between 500–700 grams of glycogen in the muscles and around 100 grams in the liver. If you are a big person, carry around a lot of muscle mass, or are more fit than the ordinary person, your capacity to store glycogen increases.
  • While training with a low- to moderate intensity, you don’t require much glycogen to fuel your exercise. Fat is good enough. At higher intensities, your muscles switch to using an increasing amount of glycogen. As your glycogen levels deplete, you fatigue and start performing worse.
  • If you only have 24 hours to restore your muscle glycogen following a workout, you better hurry! You have to cram down around 10 grams of carbohydrates per kilogram of bodyweight in that time to make it. Also, you have to eat at least as many calories as your burn during that day.
  • Glycogen synthesis is more effective if you eat several smaller carbohydrate-rich meals after a workout rather than loading up on one or two hefty ones. One gram of carbohydrate per kilogram of body weight and hour during the hours following a training session optimizes glycogen restoration.
  • You speed up your glycogen synthesis if you eat protein along with your carbs. Caffeine and creatine also cooperate with the carbohydrates you eat or drink, speeding up the rate at which you store glycogen in your muscles.
  • Eating 1–4 grams of carbs per kilogram of bodyweight 3–4 hours before a training session likely improves your endurance performance. As for strength training, the jury is out. There is not enough evidence to recommend carb-loading for a weight training session if you don’t like it, but there is likely no harm in doing so either. Eat before a workout if it feels good, but you can train on an empty stomach if you prefer.
  • If you eat or drink some form of carbohydrates during long workouts, you save your liver and muscle glycogen for later, allowing you to perform at a higher level for a longer time.
  • Training with low amounts of muscle glycogen can improve your results over time, at least if we’re talking about endurance training. Again, as for strength training, research is lacking, but it might be detrimental to go into a lifting session without decent levels of muscle glycogen. That could increase muscle breakdown and impair anabolic signaling.
  • Prolonged or intense workouts with low muscle glycogen might be tough on your immune system, making you more susceptible to catching a cold.

As always, it’s a good idea to base your diet on relatively unprocessed foods and making nutritious dietary choices. However, if you have very high carbohydrate requirements, you might find it hard to eat enough, if that base consists of boiled potatoes and broccoli. Also, you might get too much fiber, making your stomach unhappy. If that’s the case, don’t shy away from refined carbs like white bread, pancakes, pasta, and even the occasional candy bar and sugar-sweetened beverage to get the amount you need. When you burn thousands of calories exercising, you don’t need to fear the sugar demon.

If you’ve read the article, you might have noticed the recommendation of vast amounts of carbohydrates. That mainly concerns endurance athletes training at a pretty high level. Seeing as you empty glycogen locally, in the working muscles, ask yourself: do you deplete the same muscles every day, or even several times a day? Then you need that amount of carbs to restore your glycogen levels. If not, those kinds of intakes are probably too much.

If you’re a casual athlete, jogging a few miles per week, you don’t need that much. In the long run, it would likely make you fat rather than a high-performing athlete. The same goes for weight training. A workout in the gym burns up a lot of muscle glycogen during the sets themselves, but the total amount required to get you through a training session does not amount to that much.

However, even if you are not a high-level endurance athlete, your training benefits from glycogen and filled muscles. The difference is that you don’t have to plan and think about carbohydrate timing and loading. An average mixed diet works just fine for your needs.

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