Whether solid or liquid food, food that enters the body via the mouth and stomach is not yet digested. Putting food or drink in the mouth does not mean that the energy it contains has reached the body. Gastrointestinal complaints sometimes indicate that some of the food is not or only insufficiently digested. This is particularly important to avoid in sports nutrition. This article aims to clarify how carbohydrates are absorbed and what level of absorption is physiologically possible.
The actual absorption of carbohydrates takes place in the small intestine after passing through the stomach. Long carbohydrate chains are split and broken down into simple sugars. These are absorbed by the cells of the small intestinal mucosa via sugar transporters (transport proteins), where they are partially converted and released into the bloodstream via further transporters. Although the function, tasks and mechanisms of action of the transport proteins in the body are not yet fully understood, research has nevertheless presented some important facts and observations.
If very high amounts of carbohydrates are consumed, the enzymes may not be sufficient to break down the carbohydrates and carbohydrates may continue to enter the large intestine undigested. Under certain circumstances, this can lead to severe digestive problems. This can also occur if the transporters are saturated. This means that there are more simple sugars than transporters. In lactose intolerance, i.e. an intolerance to milk sugar, the disaccharide lactose cannot be digested or cannot be fully digested due to a lack of or reduced production of the digestive enzyme lactase. In the case of fructose intolerance, it is the so-called GLUT5 transporters that are not present in sufficient quantities for absorption. Glucose is mainly absorbed via the sodium-dependent glucose transporter SLGT1 (sodium-glucose linked transporter). These transporters allow a maximum absorption of glucose from the small intestine of 1.0 to 1.1 grams per minute, which corresponds to a good 60 grams per hour. However, the muscles can process significantly larger amounts of glucose. If glucose is administered via blood infusion, it has been shown that almost all of the infused glucose of up to 3.0 g/min is oxidized by the muscle during exercise. The absorption capacity in the small intestine is a bottleneck for energy supply.
Some of the substances released from the small intestine into the blood are absorbed and processed by the liver. Glucose from the small intestine remains largely untouched by the liver and reaches the muscles via the blood. The liver itself can release glucose into the bloodstream and maintain the glucose concentration in the blood during increased consumption. It consumes and produces glucose at the same time. In the fasting state, the production pathway via gluconeogenesis dominates and leads to net glucose production. Glucose production in the liver is a complex process that results from the activation of glycogen breakdown (glycogenolysis) and gluconeogenesis. Both insulin and its counterpart glucagon control these processes. The ratio of the two hormones determines whether there is a switch from net glucose production to net glucose consumption. The oral intake of sufficient glucose increases the insulin level and the insulin:glucagon ratio. This slows down glucose production, allowing certain enzymes to be used for other metabolic processes. Therefore, the supply of glucose during exercise can partially or even completely maintain carbohydrate metabolism in the liver. Fructose taken together with glucose also has a positive effect on liver metabolism.
Both the intake of glucose and the intake of glucose together with fructose show positive effects with regard to liver metabolism. In contrast to glucose, which is used directly by all body cells, fructose is characterized by a specific, two-stage metabolism. Over 90 percent of orally ingested fructose is metabolized or converted in organs, particularly in the liver. These secondary substrates produced in the liver are then released again. Fructose ingested with food is therefore less immediately available to the skeletal muscle than glucose. Depending on the study, the release of energy sources such as lactate or glucose from the liver accounts for between 50 and 80 percent of fructose uptake during training and is sometimes dependent on individual liver metabolism. The training of the liver metabolism therefore also plays an important role in athletic performance. Around 30 to 55 percent of the fructose absorbed by the liver is converted into glucose. The liver converts a further 25 to 30 percent of the fructose absorbed into lactate. In general, fructose can also be broken down by the liver into fat and stored. However, this process is linked to the ratio of insulin to glucagon and hardly occurs under increased physical exertion.
As it is primarily the absorption process in the small intestine that limits carbohydrate oxidation, it is advisable to consume a combination of different carbohydrates that use different transporters for absorption. This means that not only the muscles, cardiovascular system and lungs need to be prepared for competitions, our digestive system and organs also need training. In order to prepare the body for the high energy intake during enormous exertion, a temporary intake of food with a high energy density and in particular a high carbohydrate intake is necessary. This concept is also known as “train the gut”. In the case of fructose in particular, a regular intake, e.g. by eating fruit, can enable tolerance of larger amounts of fructose, as this increases the number of GLUT5 fructose transporters in the intestine. The maximum amount of fructose absorbed by the mucous membrane of the small intestine is around 0.4 g/min, but is more individualized than for glucose. The number of GLUT5 transporters can also be increased in the short term and fluctuate over the course of the day. If you want to eat fructose-containing food during a race, you should therefore not only consume fructose in the days and weeks before the race, but also in the meal on the day of the race immediately before the race. For longer cycling marathons, this can be in the form of honey or jam, for example. With a high intake of glucose, the formation of an additional GLUT2 transporter can be observed, which can absorb additional glucose from the small intestine – even if only to a small extent. The joint intake of glucose and fructose increases the absorption capacity for both types of sugar. In general, all GLUT transporters are formed as required. The daily diet and the diet immediately before the competition therefore play an important role in competition nutrition.
Numerous studies have shown that carbohydrate intake and oxidation can be increased by combining different types of sugar. Compared to the high oxidation efficiency of exogenous glucose of 81 to 98 percent, the oxidation efficiency of fructose is lower. In most studies, the fructose oxidation efficiency was between 52 and 85 percent. The lower efficiency of fructose compared to glucose could, for example, be due to slower absorption in the small intestine and/or storage and metabolism of fructose in the liver. Fructose metabolism is closely linked to glucose metabolism. A small proportion of the fructose that is absorbed by the wall of the small intestine is already broken down into glucose there. The most important factor limiting exogenous glucose oxidation is absorption in the small intestine, which is improved or optimized when administered together with fructose. When glucose and fructose are combined, the oxidation rate of exogenous carbohydrates can increase by up to 65% compared to glucose alone. In the studies, increased carbohydrate oxidation through a combination of different types of sugar was associated with increased fluid intake and improved oxidation efficiency, which reduced the likelihood of gastrointestinal complaints. Studies have also shown that multiple transportable carbohydrates lead to less fatigue and better physical performance compared to a single carbohydrate. Some studies have shown that even a carbohydrate intake of 120 g/h is possible without problems by combining carbohydrates appropriately. Depending on individual preference, the intake could be from solid and/or liquid carbohydrate sources. The oxidation rates of the exogenous carbohydrates did not differ between the different food forms (solid/liquid).
Conclusion: A 2:1 ratio of glucose to fructose provides excellent energy availability combined with very good tolerance. In general, sports drinks and food should not only be consumed during the competition, but should be used during the training rides to acclimatize the body and for the optimum effect to improve performance.
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