Other non-monosaccharides must be digested as well prior to absorption of their respective monomers. Glucose polymers and maltodextrins are digested in a manner similar to that described for amylose. Sucrose is digested to the monosaccharides glucose and fructose primarily by the enzyme sucrase, which is secreted at the brush border of the small intestine. Lactose is digested to its complementary monosaccharides by the enzyme lactase, which is also secreted at the brush border of the small intestine. If inadequate amounts of lactase are produced by the body to appropriately digest this sugar, lactose intolerance occurs, which makes it difficult for an individual to tolerate large (or sometimes even small) amounts of lactose. As described above for maltose that is produced from the breakdown of starches, dietary maltose is primarily digested to form glucose monomers by the brush border enzymes maltase-glucoamylase and sucrase. Although a less common dietary constituent, the disaccharide trehalose must be digested to two molecules of free glucose by the enzyme trehalase. Therefore, the final digestive products of dietary carbohydrates are typically glucose, fructose, and galactose, which can be absorbed by cells of the small intestine. Glucose and galactose are absorbed along with sodium by a common transporter referred to as sodium-glucose cotransporter 1 (SGLT1). This is an active transport process in which energy is provided for the exit of Naby the NaK-ATPase. Glucose is exported from the enterocyte across the basolateral membrane by the GLUT2 transporter for entry into the portal circulation by which the hexoses make their way to the liver. Fructose is absorbed into the gut cell by a facilitated diffusion process involving the transporter GLUT5 and also exported across the basolateral membrane by GLUT2. This absorption mechanism is highly saturable, meaning that the rate of fructose absorption is limited, which produces absorption at a relatively slow rate. Because it is absorbed more slowly than other monosaccharides, particularly when consumed in the absence of other carbohydrates, it produces a lower glycemic response, which in turn produces a lower insulinemic response. This slower absorption has been considered a cause for concern for athletes, because large amounts of fructose residing in the gut for an extended period of time can produce gastrointestinal distress including cramping and diarrhea. However, as described later, foods producing a lower glycemic response may be beneficial for performance as pre-exercise feedings when compared with those that produce a higher glycemic response.In contrast, recent research has demonstrated that because fructose is absorbed by a transporter different from that which allows for glucose absorption, when consumed with a source of glucose, total exogenous carbohydrate bioavailability during exercise may be enhanced. In summary, when a carbohydrate is ingested, it must be broken down by digestion to its simplest form prior to absorption, as the gut prefers to allow only the monosaccharides into the bloodstream. The blood circulating around the intestinal tract is directed first to the liver. The liver is instrumental in converting non-glucose saccharides into glucose derivatives that can either be metabolized further by the liver or released to the general circulation.
The vast majority of carbohydrate that is found within the blood is glucose, which provides a source of energy to most tissues of the body and is especially preferred by the red blood cells and tissues of the central nervous system. Furthermore, much of the energy produced within the skeletal and heart muscle is from glucose. When energy production is not required, glucose can be stored as glycogen within the body for future use, particularly in the muscle cells and liver.
