Your body’s engine is a hybrid, it can run off two different fuel sources: fat and carbohydrates. These fuels are stored in separate “gas tanks”:
FAT STORAGE: Adipose tissue (100,000 calories)
CARBOHYDRATE STORAGE: Muscles (500-2000 calories) + Liver (100-400 calories) + Blood Glucose (25-100 calories)
First a little bit of background.
In 1853 Claude Bernard discovered that the liver secretes your body’s main carbohydrate glucose by observing that the blood flowing out of the liver of fasted or meat fed animals contained glucose while the blood flowing into it contained little. When a cell takes in glucose from the blood it is modified by the process called oxidative phosphorylation (the major cell energy provider). This locks the glycogen molecule into the cell for future use. Only the liver can secrete glucose and it has the highest concentration of it. If you ever ate cooked liver, you know that it doesn’t taste sweet at all. That’s because the glucose in it stored as glycogen and glycogen does not taste sweet.
Liver contains 100 to 120 grams of glucose as glycogen. Compared to the liver, skeletal muscle contain less concentrated glycogen. However, because there is much more skeletal muscle than liver, we can they have much grated storage capacity. In total, your skeletal muscle contains 400 to 500 grams of glycogen. One gram of glucose produces 4 kilocalories of energy. Therefore the net energy stored in the liver is about 500 calories and about 1500 calories in skeletal muscle. Comparing trained and untrained individuals, more trained people are able to store more glycogen in their muscles and liver, however the difference is little. Average athlete might store 2000-2500 calories in total as usable carbohydrate energy in muscle and liver.
LIVER: 100-120g (500 calories)
MUSCLES: 400-500g (1500-2000 calories)
Carbs is preferred source of fuel for your body. They are easy to digest and provide fast energy. However due to the limited amount of them stored on board, you can easily deplete your carb storages just within 2 hours of moderate-intensity exercise.
75% of glycogen is used by the brain and central nervous system. For athletes, eating carbohydrates is like putting the octane in their engines. Consuming carbs will increase your blood glucose levels and help to fill your tanks before training or racing.
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. As mentioned before, your body can store a limited amount of glycogen before it gets converted to fat.
FAT or CARBOHYDRATES for FUEL?
Adequate glycogen storage in your body is the key for optimal performance. Exercising at low and moderate intensities your body uses both: muscle glycogen and fat. The higher exercise intensity, the more carbohydrates are required to fuel your engine. Below is an example of how your body shifts from using fat for fuel towards running purely on carbohydrates. This is my VO2 test, conducted in professional laboratory:
Red column on a left is my heart rate (HR), measured in beasts per minute (bpm). Two columns on the right show the carbohydrate (CH) and fat (F) utilization related to heart rate. What we see here is that at lower intensities (<111 bpm) my body works primarily on fat as fuel. However the higher intensity of the exercise becomes, the faster my heart beats, more carbohydrates are being burned. Approximately at 111 bpm my body’s engine works as a pure hybrid, utilizing about the same amount of fat and carbohydrates for fuel. As the intensity increases, less and less fat is being used to fuel the exercise, until my body shifts completely to using carbs for fuel at 163 bpm.
Maintaining optimal carbohydrate stores is essential not only for good performance, but for protecting your health. Your nervous system, and brain primarily use carbohydrates for fuel. When glycogen stores get depleted and blood sugar level is low your brain will do anything to stop you from continuing exercising. You “hit the wall”. Pack your bags buddy, cause you are not going anywhere at this point. Consider yourself lucky if you finish the race.
What restores glycogen?
When the body experiences glycogen stores depletion, it takes 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.
There are two commonly used methods among athletes to store large amounts of glycogen is through carb loading and “training low”. We will get into these methods more in depth later in a book.