A valid connection between hypoglycemia, fatigue and premature termination of exercise been firmly established and therefore carbohydrate loading is a proven form of boosting running endurance in prolonged events lasting more than two hours in duration. While there are various methods of carbo-loading, the process basically involves consuming large quantities of carbohydrate-rich food in order to saturate the bodys carbohydrate stores. It is proposed that with these increased energy stores, the competitor will be able to avoid exercise-induced hypoglycemia and continue exercising longer than if this saturation process had not occurred. This article aims to further explain how to perform carbohydrate loading and the reasoning behind its practice.
As previously mentioned in another article on this site the human body is able to store carbohydrates for energy use in the liver and the muscles in the form of a substance known as glycogen. This carbohydrate store is basically human starch and is able to be quickly broken down to fuel the muscles during high intensity exercise (muscle glycogen) and to maintain blood glucose levels (liver glycogen). In the unloaded/non-carbohydrate saturated state, an untrained individual consuming an average (45% carb.) diet is able to store approximately 100 grams of glycogen in the liver, whereas muscle is able to store about 280 grams. Remember also that muscle glycogen is committed to be used by muscle and cannot assist in maintaining blood sugar levels. Therefore should no additional carbohydrate be ingested during prolonged exercise, the task of maintaining blood glucose levels rests firmly on the livers glycogen stores and gluconeogenesis (the manufacturing of glucose from plasma amino acids). Oxidation of blood glucose at 70-80% VO2 max is about 1.0 g/min or about 60 g/hour. Therefore it can be predicted that even with full glycogen stores, a less conditioned athletes liver will be depleted of its carbohydrate within and hour and three quarters of continuous moderate intensity exercise. (Interestingly, the daily carbohydrate requirements of the brain and nervous system alone are enough to deplete the liver glycogen stores within 24 hours.) Once liver glycogen levels begin to drop and exercise continues the body becomes increasingly hypoglycemic (low blood sugar) mainly because blood glucose is depleted faster than it is replaced by gluconeogenesis. Professor Tim Noakes (see profile) considers liver glycogen depletion and subsequent hypoglycemia to be the primary factors affecting fatigue and performance during extended duration races and especially in instances where muscle glycogen levels are low as well.
The amount of additional carbohydrate that is able to be stored in the body is dependent on diet and athlete conditioning level. For an untrained individual consuming a high carbohydrate (75%) diet, glycogen stores may increase up to 130 g and 360 g for liver and muscle respectively for a total storage of 490g. For an athlete training on a daily basis consuming a normal (45% carb.) diet, glycogen levels approximate 55 g and 280 g for liver and muscle respectively yielding a total of 330 g. However, should this same well-conditioned athlete consume a high (75% carb.) diet, their total carbohydrate reserves may soar up to 880 g with approximately 160g stored in the liver and 720 g in the muscle. Clearly the conditioned athletes muscles are much more efficient at storing carbohydrates than those of his or her unconditioned competitor. In saturating the muscle by consuming of high levels of carbohydrate, the athlete automatically increases their time to hypoglycemic fatigue several fold.
Several methods for carbohydrate loading have been described in the literature. The most familiar method is the traditional glycogen stripping or carbohydrate-depletion/carbohydrate loading method. This method basically involves the athlete exercising to exhaustion the sixth day before a major competition and for the next three days consuming a high protein-fat, low carbohydrate (less than 10% total energy) diet. On day three the athlete again exercises to exhaustion but for the following three days consumes a high (90%) carbohydrate diet. The aim of this method is to severely deplete the glycogen reserves of the body to cause a super compensation effect in carbohydrate stores. Research has demonstrated however, that this glycogen stripping method may not in fact be necessary to achieve optimal carbohydrate saturation in well-trained individuals and that this super compensation effect may not even occur. Studies have demonstrated that athletes simply consuming a high (75%) carbohydrate diet for three days prior to competition resulted in carbohydrate stores comparable to those individuals who performed the glycogen stripping method. In addition, the amount of training performed before the start of the traditional regime has little effect on the resulting carbohydrate stores. Therefore, a well-conditioned athlete may need to do little more than consume a higher quantity of carbohydrates in the three days before competition to receive full benefit.
Optimal carbohydrate loading can be achieved if approximately 600g of carbohydrate is consumed daily for two to three days. It is probably of little matter if the extra carbohydrate is consumed as simple (glucose) or complex (starch) carbohydrate. Most carbohydrates are digested quickly and enter the bloodstream via the intestine much the same as if glucose had been ingested. Replenishment rates are higher immediately after exercise due to increased insulin sensitivity. The amount ingested should be about 50 to 80g starting immediately after exercise repeated 2 hourly and continuing for the first 6 hours. Full glycogen replenishment is usually achieved within 20 hours using this method; however the most rapid glycogen resynthesis is observed when glucose is infused directly into the bloodstream, yielding absolute peak muscle glycogen concentrations of near 800g (assuming approximately 20 kg of muscle) within about 8 hours. Full replenishment of glycogen after an extended event may take several days longer due to muscle damage resulting from repeated cycles of concentric and eccentric contractions.
With the benefits associated with carbohydrate loading it may be helpful to mention some possible disadvantages to following this procedure. Firstly, glycogen storage is associated with a concomitant storage of water. It is estimated that every gram of glycogen stored is associated with about 2.7 grams of water. Therefore, a well-conditioned athlete with total glycogen stores approaching 800g will find their body weight about 2kg heavier at the start of the race. This increased body weight will have implications on running economy and performance at least near the beginning of the event when energy reserves will be high. As the muscles and other organs progressively oxidize the glycogen stores during exercise, the stored water is again released into the body. This may in turn complicate the fluid requirements of the athlete, requiring them to consume less than a non-carbohydrate loaded competitor. The best advice for fluid replacement during prolonged exercise may be found on this site (see
How Much Should I Drink? [http://www.bossfitness.com/archive-nutrition.asp]) and in Lore of Running. A possible solution for water retention and weight gain is for the athlete to load to a lesser degree and ingest a carbohydrate/electrolyte enriched drink during exercise to help maintain blood glucose and electrolyte balance (consuming carbohydrate during an event in the fully loaded state is overkill and produces no additional benefit). Another drawback to carbohydrate loading if performed incorrectly is gastric/intestinal upset. Very large amounts of ingested carbohydrate can affect the osmolarity of the intestine. In other words, carbohydrates (especially simple/processed sugars) in the intestine draw water into the gut by osmosis affecting the water balance and may cause intestinal upset and diarrhea. As mentioned, an athlete should aim to consume about 600g a day preferably in multiple meals/sittings to avoid overloading the digestive capacities of the body.
In conclusion, this article has demonstrated the many benefits associated with carbohydrate loading. This process should be viewed as an effective and simple method for improving performance and endurance during extended duration exercise events. Increasing body carbohydrate stores before competition ensures sufficient energy to avoid hypoglycemic related fatigue and early termination of exercise. Simply consuming higher quantities of carbohydrate three days before competition may suffice for most athletes, however it is important to follow the loading regimen correctly to avoid intestinal upset. Exercise science is still exploring the significance and the relative contribution of the two sources of glycogen stores to exercise performance and further research will hopefully cast more light on connections relating to fatigue.
References and further reading: more information on carbohydrate loading and a detailed explanation of carbohydrate contributions during exercise can be found in Lore of Running a classic book in its fourth edition dedicated not only to running performance, but to cutting edge exercise physiology as well.