Uploaded July 2024 | Updated September 2026, 2 weeks ago
Like a car, the human body is a machine that requires the combustion of fuel for energy. Instead of gasoline, we burn fats, carbohydrates and to a lesser extent, proteins. The major source of energy, particularly during high-intensity exercise, is glucose. This is available from sugars or starch in the diet, but some glucose is also stored in muscles and the liver as glycogen, a polymer composed of glucose units linked in a long chain. Whenever glucose is needed, glycogen is broken down and glucose is released. The amount of glycogen stored in the body varies depending on the carbohydrate content of the diet. On average it is about 500 grams, although “carbohydrate loading,” meaning an emphasis on eating carbs like pasta can increase it to over 1000 grams.
During a marathon, glycogen stores have to be maintained and that requires the intake of about 60-90 grams of carbs per hour. There is no point in trying to increase this because the body cannot absorb more than this amount of carbohydrates. Traditionally, carbs have been ingested in the form of gels or drinks that contain a mix of glucose and fructose; in the body fructose ultimately converts to glucose. The stomach, however, rebels when it senses a high dose of the glucose-fructose mix entering and tries to eliminate excess by vomiting. Also, the carbs serve as food for bacteria in the gut that then produce gas as a byproduct of their metabolism. This results in bloating and belching, neither of which is welcome during a marathon.
One way of reducing the gastrointestinal distress caused by the influx of carbohydrates into the stomach is to speed up the gastric emptying time (GET), the time it takes for stomach contents to pass into the intestine. This can be done by, instead of simple sugars, using complex carbohydrates like maltodextrin, cyclodextrin or highly branched cyclic dextrin (HBCD) that are composed of a large number of glucose units linked in various patterns. All these complex carbs can be produced from corn starch by the action of bacterial enzymes. Alpha-amylase from Bacillus subtilis breaks down the starch into short chains of glucose that are then combined into large cyclic structures by an enzyme from Bacillus stearothrmophilus.
The rate at which molecules pass from the stomach into the intestine depends not on their size, but on their concentration, known as the osmolality. Maltodextrin or HBCD contain many glucose units per molecule, so not as many molecules of these are needed to deliver the same amount of glucose as a glucose or fructose solution. The reduced osmolality of a maltodextrin or HBCD solution means it will pass more quickly into the intestine, reducing the risk of gastric upset. Once in the intestine, these large molecules are degraded by enzymes at a constant rate, supplying glucose to the bloodstream for delivery to muscles.
Endurance events benefit from a steady infusion of glucose into the bloodstream which is just what HBCD does. Indeed, mice fed HBCD exhibit greater endurance in swimming tests than mice given glucose, an observation that was corroborated in a small study of human swimmers. The benefits, however, depend on the carbohydrate content of the diet. If there is a sufficient intake of carbohydrates, supplementation with HBCD may not offer much benefit. Although the evidence of improved performance is not overwhelming, most serious marathoners now use HBCD during their runs. It seems that when it comes to molecules that can enhance performance, size matters.
Like a car, the human body is a machine that requires the combustion of fuel for energy. Instead of gasoline, we burn fats, carbohydrates and to a lesser extent, proteins. The major source of energy, particularly during high-intensity exercise, is glucose. This is available from sugars or starch in the diet, but some glucose is also stored in muscles and the liver as glycogen, a polymer composed of glucose units linked in a long chain. Whenever glucose is needed, glycogen is broken down and glucose is released. The amount of glycogen stored in the body varies depending on the carbohydrate content of the diet. On average it is about 500 grams, although “carbohydrate loading,” meaning an emphasis on eating carbs like pasta can increase it to over 1000 grams.
During a marathon, glycogen stores have to be maintained and that requires the intake of about 60-90 grams of carbs per hour. There is no point in trying to increase this because the body cannot absorb more than this amount of carbohydrates. Traditionally, carbs have been ingested in the form of gels or drinks that contain a mix of glucose and fructose; in the body fructose ultimately converts to glucose. The stomach, however, rebels when it senses a high dose of the glucose-fructose mix entering and tries to eliminate excess by vomiting. Also, the carbs serve as food for bacteria in the gut that then produce gas as a byproduct of their metabolism. This results in bloating and belching, neither of which is welcome during a marathon.
One way of reducing the gastrointestinal distress caused by the influx of carbohydrates into the stomach is to speed up the gastric emptying time (GET), the time it takes for stomach contents to pass into the intestine. This can be done by, instead of simple sugars, using complex carbohydrates like maltodextrin, cyclodextrin or highly branched cyclic dextrin (HBCD) that are composed of a large number of glucose units linked in various patterns. All these complex carbs can be produced from corn starch by the action of bacterial enzymes. Alpha-amylase from Bacillus subtilis breaks down the starch into short chains of glucose that are then combined into large cyclic structures by an enzyme from Bacillus stearothrmophilus.
The rate at which molecules pass from the stomach into the intestine depends not on their size, but on their concentration, known as the osmolality. Maltodextrin or HBCD contain many glucose units per molecule, so not as many molecules of these are needed to deliver the same amount of glucose as a glucose or fructose solution. The reduced osmolality of a maltodextrin or HBCD solution means it will pass more quickly into the intestine, reducing the risk of gastric upset. Once in the intestine, these large molecules are degraded by enzymes at a constant rate, supplying glucose to the bloodstream for delivery to muscles.
Endurance events benefit from a steady infusion of glucose into the bloodstream which is just what HBCD does. Indeed, mice fed HBCD exhibit greater endurance in swimming tests than mice given glucose, an observation that was corroborated in a small study of human swimmers. The benefits, however, depend on the carbohydrate content of the diet. If there is a sufficient intake of carbohydrates, supplementation with HBCD may not offer much benefit. Although the evidence of improved performance is not overwhelming, most serious marathoners now use HBCD during their runs. It seems that when it comes to molecules that can enhance performance, size matters.
