The energy required to power the human body begins with the consumption of food, and the subsequent extraction by the body of the carbohydrate-based sugars, known as glucose and glycogen. The manufacture, storage, and utilization of these sugar compounds for the energy needs of the body is an intricate and multidimensional process.
Carbohydrates are compounds formed from carbon, hydrogen, and oxygen molecules. Carbohydrates are divided into two general groupings: simple carbohydrates and complex carbohydrates. Simple carbohydrates are the simple chemical structures of monosaccharides, or single sugars, such as glucose and fructose. Complex carbohydrates are composed of complex sugars known as polysaccharides, of which glycogen is the most prominent example.
Foods are divided for nutritional purposes into three basic groups: carbohydrates, fats, and proteins. Each of these food groups is a primary source for different materials essential to the growth, development, and maintenance of the human body; individual foods may contain varying amounts of each of these groups.
Seventy-five percent of the energy derived and stored by the body from carbohydrates is ultimately used by the body to sustain brain function; the balance is divided between muscle function and red blood cell production, essential to the transport of oxygen. The carbohydrate food group is composed of foods that are derived from plants (such as whole grain cereals and their byproducts such as breads and pastas); green vegetables, fruits, and dairy products are also rich in carbohydrates. Proteins, which are used primarily by the body to build and repair muscle tissue, are found in meat, soy products, dairy products, and some nuts. Fats, which the body converts into fatty acids, are essential to the absorption of a number of fat-soluble vitamins that are critical to body health, such as vitamins A, D, and E.
Physical health will generally be maintained with a diet that comprises from 60% to 65% carbohydrates, 12-15% proteins, and less than 30% fat. When the intake of carbohydrates exceeds that which can be stored and converted to energy as glycogen or glucose, the body will store the excess carbohydrates as fat, often leading to weight gain.
The body extracts carbohydrates from food sources through a process known as hydrolysis, whereby the warm fluids, commencing with the saliva in the mouth and concluding with the action of the small intestine, break down the carbohydrates in the food into glucose. As it is a simple sugar, glucose is able to be transported through the wall of the small intestine to be stored by the body in the liver.
Once processed, glucose will take one of three pathways into the body. Irrespective of the route through which glucose is directed in the body, it will be metabolized into energy in the same fashion. While it is commonly stated that the body "burns" its stored carbohydrates, the actual chemical process has an additional component. No matter where the glucose is stored, when it is used it creates a compound known as adenosine triphosphate (ATP), which is the actual energy source within the body.
The first and most direct route into the body for recently converted glucose from the small intestine is the bloodstream, where glucose is immediately available to be converted into ATP, in combination with the oxygen received into the bloodstream from the cardiorespiratory system.
The second repository for glucose is the skeletal muscle system. Glucose is converted into its storage form, glycogen, which is a long string of single sugars stored as a starch, a complex sugar. Once stored in the muscles, glycogen cannot be released into the bloodstream, but will be utilized as fuel to produce ATP by the muscle itself.
The most important storage mechanism of processed glucose is performed by the liver. As the largest organ in the body, the liver performs a number of purifying and metabolic functions within the body, one of which is to store glucose in its glycogen form. The liver is capable of containing up to 10% of its volume in glycogen, in contrast to the 1% storage by volume carried on in the skeletal muscles. The liver both releases glycogen when it is needed for energy production, as well as regulates the amount of glucose present in the blood, critical to health (known as the blood sugar level).
The process by which liver glycogen is converted into blood glucose is related to the actions of the pancreas, which monitors blood glucose levels. When the pancreas determines that blood glucose levels are too low, causing a condition known as hypoglycemia, the pancreas produces a hormone, glucagen, to stimulate a release of stored glycogen from the liver, in the form of glucose, into the blood to restore balance. When blood glucose levels are too high, which is, conversely, a hyperglycemic condition, the pancreas releases the hormone insulin to stimulate the liver to release less glucose. Impaired insulin production in the pancreas is the essence of the condition known as diabetes.
When the production of insulin in the pancreas becomes impaired, careful attention must be paid to blood glucose levels, which may be tested on a frequent basis. Regulation of diet, including attention to the amount of carbohydrate consumed, is essential to the maintenance of healthy blood glucose levels. Exercise, although it may place pressure on blood glucose levels through the body's use of carbohydrate stores to produce ATP, is believed to be an important tool in counteracting the serious potential impacts of a diabetic condition on the cardiovascular system.
see also Carbohydrates; Cardiovascular system; Glycogen level in muscles; Lactic acid and performance.
By Melodie Anne Updated December 12, 2018
During digestion, your body breaks down carbohydrates from the food you eat and converts them to glucose, or blood sugar. You need glucose to provide fuel to every cell throughout your body. Brain cells require adequate amounts of glucose so that they can send and receive messages. Your body uses the glucose it needs right away and stores the rest as glycogen in your liver and muscles. Glycogen is a polysaccharide that your system quickly converts back to glucose when carbohydrates are not immediately available from food. Since glycogen is your body's backup source of fuel, it is essential to eat certain foods to keep adequate levels of glycogen in your system.
Fresh fruits are full of simple carbohydrates in the form of fruit sugar, or fructose. These types of carbs digest rather quickly in one step in your small intestine. Enzymatic juices convert simple carbohydrates into glucose, and it absorbs directly through cell walls. A 4-ounce apple or banana, 1/2 cup of pineapple chunks, a 1 1/4-cup serving of raspberries or a 3-ounce serving of grapes provides 15 grams of simple carbs to help boost your glycogen stores. As an added bonus, fresh fruits are also loaded with fiber to help keep you satisfied for a while after you eat.
Starch is a complex type of carbohydrate that starts breaking down in your mouth. As with simple carbs, starch eventually absorbs as glucose. Saliva breaks complex starch molecules into maltose, which is a simple carb. Once maltose hits your small intestine, it instantly converts into glucose and goes right into your bloodstream. Starchy vegetables, such as potatoes, squash and sweet potatoes, are full of starch. A 1/2-cup, cooked serving of any of these veggies offers 15 grams of complex starch carbohydrates. Your digestive tract will quickly convert the starch it needs and hang on to the rest as glycogen.
One slice of whole-wheat bread, 1/3 cup of wheat pasta or 1/3 cup of brown rice each provide 15 grams of carbohydrates. Most of the carbohydrates are complex, but these foods may offer some simple sugar carbohydrates as well. Opting for whole-grain foods in place of white or processed varieties ensures that you get adequate fiber to help with digestion. This is important because normal digestion is essential for carbohydrate conversion and getting glucose into your blood to store as glycogen.
Most of your diet needs to come from carbohydrates to keep enough glycogen in your system. Between 45 and 65 percent of your total calories should come from carbs, explains the Centers for Disease Control and Prevention. Based on an average 2,000-calorie diet, you need 225 to 325 daily grams of carbs. Since glycogen is a backup source of fuel, it is imperative to eat certain foods to build up glycogen stores if you are planning an intense workout. This way, you'll have plenty of energy to support your high activity level. If you are an athlete, you need as much as 55 to 60 percent of your calories to come from carbohydrates to build up adequate glycogen stores.
Glycogen is a form of carbohydrate that your body stores in your muscles and liver. During exercise, especially high-intensity exercise, muscles tap into this storage form of sugar to produce ATP, the energy currency muscles need to contract. Glycogen is constructed of long chains of glucose held together by bonds called glycosidic bonds.
Muscles and liver are the primary organs that make glycogen from glucose and store it for later use, although your kidneys and intestines do so to a lesser degree. Your liver has the unique ability to break down glycogen and release the glucose into the bloodstream when your body runs low on glucose. Your muscles, however, lack the enzyme needed to do this.
Why is glycogen important? Muscle glycogen is a major fuel source your body taps in to during exercise, especially high-intensity exercise. During low to moderate-intensity exercise, your muscles primarily use fat as fuel, but as exercise intensity increases, you can’t mobilize fat quickly enough to meet your body’s energy demands. So, muscle cells break down stored glycogen to glucose and glucose is used to make ATP, the energy currency that muscles need during exercise.
As mentioned, glycogen is a preferred fuel source during high-intensity exercise. Once you reach an exercise intensity above 70% of V02 max, your body preferentially burns carbs in the form of glycogen over fat, although it continues to burn fat as well. At most times, you’re using a mixture of carbs and fat as a fuel source and, to a much lesser degree, protein. But, without adequate muscle glycogen, performance during high-intensity exercise, especially explosive, power moves, may be compromised.
So, if you have a high-intensity workout planned, you want enough glycogen stored in your muscles to maximize your performance. How much glycogen can your body save up and store to use during exercise and how much can you boost your glycogen stores in preparation for exercise?
Glycogen Storage
Unless you’re fasting or unless you’ve just completed a long and exhausting exercise session, your muscles already contain varying amounts of glycogen. How much? A well-nourished person that weighs 80 kilograms stores, on average, about 500 grams of glycogen, although you can boost this even more by eating a carbohydrate-rich diet. Since each gram of glucose can be converted to 4 kilocalories of usable energy, this equates to 2,000 calories of stored energy. Of this, around 400 grams, or 1,600 calories, are in your muscles and about 100 grams, or 400 calories of glycogen, are stored in your liver.
So, muscles store their own fuel in the form of glycogen and the liver provides an additional source of glycogen that can be converted to glucose. In fact, your body can store a fair amount of glycogen, enough to fuel a 20 to 30 mile run at a moderate intensity, but there is a limit to how much glycogen you can store. Glycogen holds onto water and water has weight. If you could store unlimited quantities of glycogen, you would eventually weigh too much to get around! Once glycogen stores are maximized, extra macronutrients, mainly carbohydrates and fat, are stored as fat. You have an almost unlimited ability to store fat. That’s not what you wanted to hear, right?
The Role of Diet in Glycogen Storage
How much glycogen is in your muscles and liver at any one time is closely related to your diet and how much-stored glycogen you burn off through exercise. If you eat a high carbohydrate diet and consume enough calories, you maximize your glycogen stores. Yet, fasting for as little as a day can greatly reduce your muscle and liver stores of glycogen. When this happens, your body is dependent on fat as a fuel source, as well as ketone bodies produced by the rapid breakdown of fat.
On the other hand, eating a very high carbohydrate diet for a few days will almost double your body’s baseline glycogen stores to the point that they reach maximum capacity. Studies show most people can store a maximum of 15 grams of glycogen per kilogram of body mass. So, an 80-kilogram person can hoard a maximum of around 1,200 kilograms of glycogen under optimal conditions. That’s not insignificant since 1,200 grams of glycogen is equivalent to 4,800 calories of energy.
Exercise, of course, depletes glycogen stores and how quickly you deplete it depends on the intensity of your workouts. As mentioned, you might be able to run a distance of 30 miles at a moderate intensity before you empty your glycogen stores. But, if you do a very high-intensity workout, you could drain your muscle glycogen stores in as little as an hour. Of course, this depends on how full your glycogen stores were when you started.
Some people try to deplete their glycogen stores by fasting or exercise prior to indulging in a big meal, for example, on Thanksgiving Day. When your glycogen stores drop, insulin sensitivity increases. This makes it possible, in theory, to consume more carbohydrates without gaining body fat. The carbohydrates will go toward replenishing your muscle and liver glycogen stores. That’s not necessarily a healthy approach though.
Carb Loading
Athletes manipulate their glycogen stores as well by carb loading before a big event. Since we know glycogen stores help boost performance during vigorous exercise and reduce fatigue, someone participating in a race wants as much glycogen stored in their muscles as possible. To do this, they first try to lower their muscle glycogen stores about a week before the event through exercise and by eating a low-carb diet. This boosts levels of an enzyme called glycogen synthase that synthesizes new glycogen. Then, 3 days before the event, they switch to a high-carb diet of around 60% carbs daily to maximize glycogen stores before a race. Studies show this works only for intense aerobic activities that are at least an hour in duration. There’s little benefit to using this technique for shorter races and events. So, boosting muscle glycogen offers some benefit for intense, longer duration exercise.
The Bottom Line
Now, you know why glycogen is so important for optimal exercise performance during high-intensity exercise of longer duration. You don’t have an unlimited ability to build up glycogen, but you can stockpile it by carb loading before an event.
References:
Exercise Physiology. Nutrition, Energy, and Human Performance. Eighth edition. McArdle, Katch, and Katch. (2015) J Physiol. 2013 Sep 15; 591(Pt 18): 4405–4413.
Effects of Carb Loading on High-Performance Athletics. Ali Mueller, Amelia Reek, Josh Schantzen.