Significant glycogen depletion occurs during endurance exercise that exceeds 90 minutes (such as marathon running). When glycogen stores drop to critically low levels, high-intensity exercise cannot be maintained. In practical terms, the athlete is exhausted and must either stop exercising or drastically reduce the pace.
However, glycogen depletion may also be a gradual process, occurring over repeated days of heavy training, in which muscle glycogen breakdown exceeds its replacement. The same process may occur during high-intensity exercise that is repeated several times during competition or training. How long does it take to replenish glycogen sores? It depends on the amount of carbohydrates in our diet, and on the timing of their consumption. Generally, it takes 24-48 hours to fully replenish glycogen, which is quite a bit longer than it takes to use it up.
Chart 1: The influence of dietary carbohydrates on muscle glycogen stores during repeated days of training. In a classic study, glycogen synthesis was compared on a 40% vs. 70% carbohydrate diet during repeated days of 2-hour workouts (Chart 1).
On the low carbohydrate diet: the muscle glycogen stores dropped lower with each successive day of training. After several days of the diet and exercise regimen, the athletes had low muscle glycogen stores, and could not exercise at even a moderate intensity.
On the high carbohydrate diet: the athletes enjoyed nearly maximal repletion of the muscle glycogen stores after the strenuous training, allowing them to continue the heavy training.
This study and others, suggest that athletes who fail to consume enough carbohydrates on a daily basis while training, will likely decrease endurance as well as exercise performance.
Other studies showed dose-related glycogen storage (i.e. a direct relation between the amount of carbohydrates consumed, and the amount of glycogen stored in the muscle). Three diets were consumed by athletes: 15% carbs, 55% carbs and 55-65% carbs. The highest carb-diet yielded 4 times more accumulated glycogen compared to the lowest carb-diet. When these athletes were subjected to “all-out” exercise, their exercise times were proportional to the amount of glycogen stored before the test (Chart 2).
Insulin, Carbohydrates and Fat
Our body derives energy from 3 main food sources: Carbohydrate, protein and fat. In general, carbohydrates are our main fuel for exercise, though the lower the intensity or longer the duration of exercise, the more we will rely on fat; the predominant role for proteins is to provide the building blocks for muscle and other tissues, but during endurance exercise, up to 15% of energy may come from protein; fats provide a concentrated source of energy and are the second most important contributor to energy demands during exercise, behind carbohydrates.
During exercise, blood glucose levels increase, while blood insulin levels fall. This occurs due to the exercise-induced rise in catabolic hormones (hormones that induce the breaking-down of fuels for energy), which inhibit the release of insulin from the pancreas. In other words, during exercise, the body doesn’t need insulin for glucose mobilization, and therefore its’ secretion is suppressed. As a result, glucose is released from the liver to the bloodstream, and fat is oxidized for energy. The higher the intensity of your exercise, the more insulin is suppressed.
Endurance training also causes several major adaptations in the muscles to increase fat utilization. First, endurance training increases the number of capillaries in the trained muscles, so that the muscles receive more blood and oxygen. Second, endurance training increases the ability of the muscle to burn fat (activating specific enzymes). Third, endurance training increases tissue insulin sensitivity, which means the muscles need less insulin to allow glucose in, resulting in less insulin in the blood. In endurance athletes, the claim that a high carbohydrate diet promotes greater body fat storage through activiation of insulin is also, therefore, unfounded.
In conclusion, carbohydrate, and not fat, is the preferred energy source during exercise at or above 70% of VO2max (maximal oxygen consumption) – the intensity at which most endurance athletes train and compete. Fat supplies a secondary source of energy, but becomes more important the longer the duration of exercise. In fact, after several hours of endurance work, fat may supply up to 80% of calories. Even at this stage, however, fat still ‘burns in a carbohydrate flame’. This means that one must still be mindful of adequate carbohydrate ingestion even at the late stages of a prolonged endurance event when fat usage is maximal.
The Importance of Glycogen
Think of glycogen as the “wick” in a candle and our fat stores as the “wax”. If we have no glycogen, we cannot sustain a flame and therefore cannot burn energy even with lots of stored fat available. Similar to a candle, our body stores of glycogen (the wick) are very small in comparison to fat stores (the wax), but so very essential. In other words, we cannot burn fat without the presence of at least small amounts of glycogen. Therefore, glycogen has to be present for aerobic exercise to continue.
However, glycogen is not only a fuel source for muscles, but also the food source for our brain. Although the muscle can store glucose and burn fat, the brain does neither, but rather feeds on glucose, supplied to it from the blood stream. Since the brain has no fuel storage capacity, it is extremely sensitive to fluctuating glucose levels in the blood. It is well known that depleted blood glucose causes athletes to “hit the wall”, or to “bonk” or “crash”, as athletes with low blood sugar tend to perform poorly because the under-fueled brain limits muscular function and mental drive.
What is Glycogen?
Chemically, glycogen is a branched glucose polymer, i.e. a type of starch. About 300 g of glycogen is stored in the muscles, though this amount can increase fivefold with physical training coupled with proper nutrition. Muscle glycogen is the primary energy source during exercise. The glycogen store in human liver is about 90 g and is involved in the hormonal control of the blood sugar. Because glycogen is a carbohydrate, it contains water, making it a large and weighty molecule. Such a large molecule is unsustainable for long-term energy storage (as opposed to fat). The 70 Kg "reference man" stores only an 18-hour resting fuel supply as glycogen (or 90 minutes of exercise), compared to a 2 months' supply of fat (or 4 marathons back to back).