Glucose is the gasoline to your hard-working body machine. If you fill your vehicle up with the good stuff—premium gasoline in the form of nutrient dense whole-foods—your body will operate efficiently like a fancy hybrid car, not one of those standard minivans.
Why a hybrid? Well, what’s impressive is that your body doesn’t just know how to use fuel, it also knows how to store fuel to keep you moving throughout the day without having to stop and fill your tank repeatedly. It burns glucose(the fuel) and stores it as glycogen for later.
What’s the difference between glucose and glycogen?
Glycogen is a more complex version of glucose, called a polysaccharide (poly = many, saccharide = sugar). When our body needs a quick boost of energy or when we aren’t getting enough glucose from food, glycogen depletion occurs as your body uses up all the stored fuel, giving you no way to get where you want to go.
How your body stores glucose for longer drives
To understand glycogen depletion, let’s first explain how your body reserves energy (glucose) for emergencies. To clue you into the importance of glucose, seventy-five percent of glycogen (stored glucose) is used by the brain and central nervous system. It’s no wonder why we get hangry!
Glycogen is either created directly from food (glycogen synthesis) or through an indirect pathway (gluconeogenesis). When you eat a meal with carbohydrates, your body releases insulin, which takes glucose from the blood for energy into the cells. When the body gets excess fuel, the glucose molecules are linked together in a chain, producing longer units, called glycogen.
Glycogen has a max level of storage before it gets converted to fat. Storage levels depend on your body and factors such as activity level, sex, and muscle mass. Your body can store about 1,600-2,800 calories of carbohydrates in the form of glycogen in your muscles, liver, red blood cells and kidneys before it is converted to fat.
What is glycogen depletion?
When we don’t have enough glucose in our diet, or we use it up during exercise, we tap into our body’s precious reserves. When our blood sugar levels drop lower than normal, glycogen is released by the liver then broken down so the glucose can be shared. This complex mechanism, called glycogenolysis, helps balance blood sugar levels. Once all the stored glycogen is depleted, you will feel tired, fatigued, and your exercise performance will suffer.
The glycogen that is stored in our muscles is for “locals only.” In other words, once it’s stored in muscle, it’s not capable of being transported to other areas of the body to provide fuel. Instead, it must be used at the site. When your body can’t push for one more rep, this likely means that the glycogen has been depleted in those muscles.
How long does it take to deplete glycogen stores?
The type, intensity, and duration of exercise can impact how long it takes to exhaust glycogen stores. Here are some general ranges:
Daily living activities: 12-22 hours
Low to moderate intensity exercise (distance running): 90-120 minutes
High-intensity exercise (HIIT training): 20 minutes
When you “hit a wall” during your workout, this is partially due to glycogen depletion. Typical symptoms are an extreme loss of energy and fatigue. Glucose also generates ATP which is the ultimate body fuel source. Small amounts are present in the muscles at any given time and are used in anaerobic (lack of oxygen) exercise lasting less than 10 seconds (i.e., a quick sprint) or up to 30 seconds for an athlete. Other than this, ATP must be made through glycogenolysis.
What does glycogen do for muscles?
Glycogen is the main energy-giver during exercise. The better your body can store glycogen, the better it will be able to complete physical tasks. If low levels of glycogen available in the body, you won’t be able to work out at a high intensity, and the duration of your training session will be limited. Multiple studies show that fatigue, a decrease in performance, and symptoms of overtraining are related to low-carb diets that cause glycogen depletion.
What restores glycogen?
When the body experiences depletion of stores, it will take about 24 hours to refuel (i.e., to ingest, digest, and convert food into glycogen). It is understood that carb-containing foods will help replenish stores the most efficiently. When food is digested, glucose is created. The pancreas recognizes this and produces insulin, a hormone that regulates the amount of glucose present in the bloodstream. Any glucose that isn’t used at this time is directed to the liver to be stored as glycogen.
To carb load or not to carb load
One way that athletes store large amounts of glycogen is through carb loading. This is when carbohydrate-rich meals are consumed before an event. While this method may provide fuel, it has fallen out of favor because of side effects of excess water weight and digestion problems (I speak from personal experience). If you’re training for an event, such as a marathon, experiment with any diet changes weeks—even months—before the event to make sure they work for you.
A different method used by some athletes is to reduce carbs during training. This results in a reduction of glycogen and triggers the body to use fat stores for fuel instead. It’s not advisable to try a new workout routine if you are new to a low-carb diet like the ketogenic diet (keto for short). You can end up with flu-like symptoms such as grogginess or an upset stomach. Go slow and try not to make too many changes at once.
Before you try any extreme diet, such as the keto high-fat, low-carb one, we recommend you do your research and consult a physician.
More about keto
When switching to a low-carb diet, glycogen stores are depleted. This results in fatigue, low energy, sluggishness, and mental dullness. Once the body renews its glycogen stores, energy and mental clarity are restored. It can take up to several months for an athlete to adjust to a low-carb and high-fat or keto diet before they finally experience positive metabolic changes and muscle glycogen.
Starting a low-carb diet will result in initial weight loss. This happens because when glycogen is stored in your muscle, its bound to water. However, the quick depletion of glycogen and rapid water weight loss will eventually cause weight to return. Keep in mind that these weight fluctuations are water, not fat.
Eating very low-carb is not ideal for high-intensity exercise because the anaerobic system relies on breaking down glucose for energy. Since diets like the ketogenic diet burn fat instead of glucose for fuel, that quick glucose-based energy is not available. During high-intensity exercise, the body shifts to use glycogen as fuel regardless of your carb intake. That means, if you consume fewer carbs, your body will have less energy to work with therefore compromising your training.
Keto-curious? Find out more about the ketogenic diet and see if it's really worth all the the hype.
That was a lot of science-backed information. Here it is in short:
Glucose is the primary source of energy for the body and brain
Glycogen is the name for stored glucose
Glycogen depletion happens when we run out of glycogen stores because of lack of food or intense exercise
Glycogen in the muscles can only be used by that muscle
Glycogen stored in the liver can be used throughout the body
Carb-containing foods restore glycogen most effectively
When following a low-carb diet, your body needs time to adjust to a new fuel source
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