Most sugar is absorbed in the small intestine, specifically in the duodenum and jejunum. Glucose absorption occurs through transport proteins like SGLT1, which moves glucose into enterocytes. After consuming 50 grams of glucose, blood sugar can peak at 120-140 mg/dL within 30-60 minutes, with glucose entering the bloodstream for use or storage.
Sugar Absorption Process
Through the digestion process in the small intestine, after being ingested both from fruits and grains, along with forms of food obtained as a result of their processing, sugar is always absorbed into the bloodstream. About 80% of sugar in nutrition is absorbed in the small intestine, where it is previously split into monosaccharides: glucose, fructose, and galactose. Glucose is the most active form, easily absorbed and utilized by the human body as an energy source. The rate of absorption of glucose is very high. In a normal individual, glucose would appear in the bloodstream in as little as 15 minutes after consumption. Generally, in response to the consumption of glucose, blood sugar peaks within 30 minutes to 1 hour, peaking at approximately 120-140 mg/dL in someone with normal insulin sensitivity. Consuming it with protein or fat can slow the rate of absorption of the food, for which the blood sugar rises more slowly.
Sugar absorption happens chiefly in the duodenum where enzymes known as sucrase and maltase break down, respectively, sucrose and maltose into glucose. For instance, when you eat 100 grams of pasta—a starchy carbohydrate—it is partially digested in the small intestine into 75 grams of glucose that gets absorbed. Because of this efficient conversion of starch into glucose after the digestion of carbohydrate foods, blood levels of glucose rise quickly. That said, the source does make a difference in the rate at which glucose is absorbed into the bloodstream. Simple sugars are rapidly absorbed and raise blood sugar levels; examples of these are found in most sodas, candy, or sweet cereals. Whole foods, such as sweet potatoes and apples, by contrast, have fiber, which slows the digestion and absorption of sugar, thereby lowering its glycemic index. For example, while a medium-sized apple containing 19 grams of sugar would take time to be absorbed, a soft drink that may contain 40 grams of sugar is absorbed instantaneously.
Absorption of sugar involves the transport of glucose across the intestinal wall into the cells lining the small intestine by making use of special transport proteins. These include SGLT1-sodium-glucose-linked transporter 1—which actively transport glucose into the cell. Energy is used in this process, depending on the transport of sodium ions. After glucose has entered the blood, it will subsequently be distributed to various tissues and organs. On average, about 70-80% of glucose absorbed after a meal is used by the muscles and brain for immediate energy. The rest of the glucose is taken up by the liver, where it will be stored as glycogen or, if excessive amounts are available, will be turned into fat. With insulin resistance, the liver may convert more glucose into fat, which would enhance body weight gain but also be associated with high blood sugar levels.
Other factors that could influence the efficiency of sugar absorption include meal composition, the presence of insulin, and a person’s metabolic health. Other studies have also shown that refined sugar can provoke an immediate, sharp insulin response, perhaps leading to an even greater reduction in blood sugar levels several hours later. For instance, in 2018, it was proven that when consumers drank a sweetened drink with 50 grams of glucose, blood sugar levels increased by 30% after just 30 minutes, while if a whole grain and vegetable meal was eaten which also contained the same 50 grams of glucose, blood sugar levels only rose by 10-15% after 30 minutes.
Role of the Small Intestine
Nutrient absorption, including sugar, takes place in the small intestine. It measures roughly 6 meters in length and accounts for about 90% of nutrient absorption within the digestive system. Once the sugar gets into the small intestine, it is digested into simpler sugars like glucose, fructose, and galactose, which pass into the bloodstream. For example, 45 grams of the 50 grams of carbohydrates in a meal can be absorbed in the small intestine. The rate of absorption depends on the type of sugar. Since glucose is the most abundant sugar type, its rate of absorption is increased compared to that of fructose. On average, it takes about 10-15 minutes for glucose to start entering the bloodstream following the onset of digestion. Fructose absorbs more slowly compared to glucose because it involves different transport mechanisms in the enterocytes.
Specialized transport proteins allow sugar absorption in the small intestine. Glucose, for instance, is mainly absorbed through the active transport mechanism called SGLT1 (Sodium-Glucose Linked Transporter 1) that moves glucose into the enterocytes against the concentration gradient. It’s estimated that 2-3 grams per minute can be absorbed by the SGLT1 transporter, depending on dietary intake. In contrast, absorption of fructose relies on a different transporter, GLUT5, that enables fructose to enter the enterocytes at a slower rate. This slower rate of absorption partially explains why ingesting high amounts of fructose—as in sweeteners like high-fructose corn syrup—doesn’t immediately cause blood glucose levels to spike. Fructose, after absorption, initially travels to the liver, where it undergoes either oxidation into glucose or storage as fat.
Efficiency in absorption at the small intestine level is likewise crucial. It depends on availability due to digestion enzymes. Sucrase and maltase enzymes are produced at the small intestine level and break down complex carbohydrates, such as sucrose and maltose, respectively, into their component sugars. This is, for example, the case of one banana you eat because it contains about 14 grams of sugar, which comes mainly from glucose and fructose. The sucrose, when entering the small intestine, especially in the duodenum, after a few minutes, enzymes will instantly break down sucrose into glucose and fructose. From there, the glucose and fructose are then absorbed into the bloodstream. These, however, could be modified by a number of factors such as the kind of sugar taken, the intake of fiber in the diet, and the general health of the digestive system.
Duodenum Function
The duodenum is the first part of the small intestine, and typically performs an important role in a digestion process through which sugar is absorbed. It has an average length of about 25-30 centimeters. It receives partially digested food from the stomach and partly digested substances together with bile from the liver and pancreatic enzymes. The main function of the duodenum is neutralizing acids from the stomach and breaking down nutrients to simpler forms that can be absorbed later in the small intestine. The first enzymatic breakdown of carbohydrates when ingesting foods containing sugars starts in the duodenum, where it breaks down the complex carbohydrates into simple sugars like glucose and fructose. In fact, the first 50-60% of carbohydrate digestion occurs along the entire small intestine.
In the duodenum, pancreatic enzymes and bile from the liver play a significant role in the breakdown of carbohydrates into simpler sugars. The primary enzymes produced in the pancreas include amylase, which breaks down most starches into maltose and other smaller sugars. The exocrine part of the pancreas secretes about 1.5-2 grams of amylase in a normal meal containing approximately 50 grams of carbohydrate, and this enzyme breaks down the carbohydrates into simpler sugars. When these sugars have been broken down completely, they are ready for the lining of the duodenum to pass them into the bloodstream. The process takes around 30-45 minutes following the meal. At this time, blood glucose starts to increase as sugar is absorbed into the bloodstream. Blood glucose was seen to peak, in about an hour at around 130-150 mg/dL due to sugar absorption in the duodenum in 20 healthy individuals studied after consuming a meal containing 60 grams of carbohydrates.
The duodenum regulates the release of bile, which is required to digest the fats. After consuming a meal high in fat, for example, the liver and the gallbladder will release bile into the duodenum where it will emulsify the fat. This means that the bile breaks fats down to much smaller particles so that lipases from the pancreas can easily digest them. Although its main function relates to the process of digestion, a part of the role of bile is indirectly improving carbohydrate digestion. It has been shown in studies that bile acids enhance the enzymatic activities, including that of amylase, thereby enhancing the digestion of carbohydrates. This indicates that in the case of high-fat meals, carbohydrate digestion in the duodenum can be quicker and may affect the rate of intestinal sugar absorption.
Besides the function of participating in digestive enzyme action, the duodenum also participates in the hormonal balance mechanisms regarding food digestion. The intake of sugars by an individual stimulates the release of hormones from the duodenum, such as glucagon-like peptide-1 (GLP-1), which in turn controls the secretion of insulin from the pancreas. GLP-1 enhances the sensitivity to insulin and promotes the cellular uptake of glucose. It was determined that after ingestion of a carbohydrate meal, an amount of GLP-1 secreted from the duodenum was such that could explain up to a 50% increase in insulin secretion. It prevents the development of hyperglycemia since it ensures that all the glucose absorbed is put to appropriate use by the body. This could be blunted in people with insulin resistance, leading to a less active absorption of sugar into the bloodstream and, hence, higher glucose levels in the blood. Thus, this concept of the role of the duodenum in blood sugar control applies to various conditions such as type 2 diabetes.
Absorption of Glucose
Glucose is the most important sugar, considering energy supply to the body; it is mainly absorbed in the small intestine by an efficient and regulated process. After ingestion, glucose passes into the stomach and small intestine, where absorption across the intestinal wall occurs. The duodenum and jejunum account for about 90% of glucose absorption. Carbohydrates, upon digestion, are broken down into simple sugars and, within a short period thereafter, absorption of glucose begins. Within, normally, 15–30 minutes after intake of a meal containing 50 grams of glucose, glucose begins to appear in the blood. Peak plasma glucose concentration occurs, generally, within 30–60 minutes. At this time, glucose levels may reach approximately 120–140 mg/dL in normal individuals.
The absorption is done, in part, with the assistance of transport proteins, mainly the SGLT1, which are also embedded in the surface of enterocytes. This provides for active transport of glucose across a concentration gradient to which sodium ions provide the major motive force. It was demonstrated that one transporter of SGLT1 is capable of absorbing approximately 2 to 3 grams per minute, depending on the carbohydrate intake of a particular individual and the health status of one’s intestine. A healthy small intestine is fully competent in the absorption of glucose; for instance, following the ingestion of a meal containing 60 grams of glucose, the small intestine can absorb about 90% within 30 minutes. Several factors might influence glucose absorption efficiency, such as the type of nutritional companions—fats or proteins that retard the process—or physical form of glucose ingested, for example, glucose from refined sugar versus from whole grains.
Once absorbed through the intestinal wall, glucose travels into the bloodstream, where it is distributed throughout different tissues and organs. Of this, the average is also used by the muscles and brain for immediate needs; the remaining is then stored either in the liver or in the form of fat. Furthermore, after consuming a meal that contains a great deal of carbohydrate—for example—the body may store roughly 15 grams as glucose in the form of glycogen in the liver. Glucose is absorbed really fast, causing the levels of glucose in the blood to raise really fast, especially after the consumption of simple sugars. In one repeated study after the intake of a 50-gram sugar-based glucose drink, glucose levels in the blood have been shown to increase by 30-40% within one hour after intake, especially when glucose is taken independently without other major nutrients.
This rate of glucose absorption can be modulated by a hormone called insulin. Produced in the pancreas, insulin is secreted following a postprandial raise in blood glucose levels. In that way, insulin is able to perform its task in maintaining blood glucose at a constant level through stimulating the uptake of glucose into cells for energy utilization or storage. In healthy, insulin-sensitive individuals, blood glucose usually peaks in 30 to 60 minutes and insulin helps return the blood glucose to baseline within 2 to 3 hours. In one controlled experiment, for example, the subjects who consumed a high-glucose drink raised their blood-glucose level from 90 mg/dL to 140 mg/dL in one hour; three hours later, however, insulin had reduced the value back to around 90 mg/dL. However, in the instances of insulin resistance or type 2 diabetes, the absorption of glucose is not that efficient and hence it takes a longer period for blood glucose levels to come down and maintain that way, increasing the risk for chronic hyperglycemia. The rate of glucose absorption is delayed in such individuals, and blood glucose may take as long as 4 to 6 hours to return to baseline levels based on the severity of insulin resistance.
Transport Proteins in Action
The transport proteins are highly important for the glucose and other nutrient absorptions in the small intestine; they definitely ensure frictionless traversing across cellular membranes. As an example, glucose absorption highly depends on particular transporters joining in a concerted action to allow its transfer from the lumen of the small intestine into the bloodstream. SGLT1 is an acronym for Sodium-Glucose Linked Transporter 1 and is the primary protein utilized during glucose absorption in the small intestine. The transporter utilizes energy from sodium gradients to transport glucose molecules against their concentration gradient. In an experiment performed to determine the efficacy of SGLT1, it was found that this protein can absorb roughly 2-3 grams of glucose per minute. This rate changes depending on the intensity of intake and the state of health of the small intestine. This active transport enables glucose to rapidly enter the bloodstream following digestion, contributing to the spike in blood sugar right after carbohydrate-rich meals.
Following uptake into the enterocyte, or intestinal cell, glucose must be transported into the blood. At this membrane surface, yet another transporter protein, called GLUT2, facilitates the movement of glucose out of the enterocytes and into the blood. This transporter works complementarily with SGLT1 to ensure that glucose can rapidly enter the circulatory pathway for delivery to various tissues, either for use or storage in energy gain. The efficiency of GLUT2 makes it a key player in the process of regulating the amount of glucose present in the blood. The function of GLUT2 is such that, in a healthy individual, after a meal containing about 50 grams of glucose, it mediates glucose entry into the bloodstream, peaking blood glucose levels in 30-60 minutes. In metabolic diseases like diabetes, where insulin sensitivity may be reduced, glucose transport by GLUT2 can become impaired, leading to prolonged high blood glucose levels.
In harmony with SGLT1 and GLUT2, other transporters, such as GLUT5, play an important role in the absorptive processes of other sugars like fructose that require a different transport mechanism than glucose. GLUT5 works along the small intestine and facilitates fructose absorption, which is much slower when compared to glucose. This impaired rate of absorption for fructose may impact total blood sugar postprandial dynamics following a meal high in fructose. Such one comparative study between glucose and fructose, after the intake of sugar-sweetened beverages, showed that the blood glucose level after glucose intake increased by 30-40% within the first hour, while for fructose this occurred more slowly: only a 10-15% increase was noted. This is because their transportation mechanisms are different: SGLT1 transports glucose, while fructose is transported by GLUT5.
Liver’s Role in Sugar
The liver is an essential body part for regulating and maintaining blood sugar through storage and release. Whenever one consumes carbohydrates, the food breaks down into glucose in the digestive tract and is then absorbed into the bloodstream. Once glucose enters into the bloodstream, it travels to the liver, where it stores as glycogen or converts to fat. These roles, therefore, help balance blood sugar levels throughout the day by storing energy from glucose in the liver. The liver is capable of storing around 100 grams of glucose as glycogen, which, when released into the bloodstream during fasting or between meals, is able to ensure an adequate energy supply to cells. For instance, following the intake of a meal rich in carbohydrates, blood glucose levels may reach up to about 120-140 mg/dL an hour later. In return, the liver stores the excess glucose as glycogen, thus preventing an increase in blood sugar levels.
When blood sugar is generally low, such as during fasting or between meals, the liver helps restore energy balance by releasing glucose into the bloodstream through a process called glycogenolysis. The liver initiates the breakdown of stored glycogen into glucose and releases it into the blood for use by the cells in energy form. During a normal fasting period, such as after 8-12 hours without food, the liver is capable of releasing approximately 90-120 grams of glucose into the blood. This ensures that all the tissues, including the brain, are constantly supplied with glucose as the brain requires glucose for its major energy source. For example, studies observed that after 12 hours of fasting, blood glucose levels remained steady between 80-100mg/dL due to the liver releasing glucose from stored glycogen.
In addition to glycogenolysis, the liver is also part of the process known as gluconeogenesis, through which it synthesizes glucose from non-carbohydrate precursors such as amino acids and glycerol. This pathway gains prominence with longer-term starvation or fasting, when the stores of glycogen become lower. Indeed, studies have suggested that after 24 to 48 hours of fasting, approximately 40-50% of the body requirements for glucose are synthesized by the liver through gluconeogenesis. This process maintains blood glucose levels even when dietary glucose is unavailable. Such would be the case in a study on gluconeogenesis in fasted individuals where, following a 48-hour fast, the liver was estimated to be producing roughly 70 grams of glucose per day to maintain blood glucose of approximately 90 mg/dL.
Absorption and Bloodstream
Glucose absorption in the small intestine into the blood is highly coordinated and plays an important role in energy balance within the body. Once carbohydrates are digested down to glucose, the majority of absorptions take place across the epithelial cells of the small intestine; most glucose absorption occurs in the duodenum and jejunum. The glucose molecules then enter the blood flow where they are distributed all over the body to give energy to various tissues and organs. In healthy individuals, glucose is rapidly and effectively absorbed. Its peak blood glucose levels occur 30 to 60 minutes after the intake of a meal containing glucose. For example, a 50-gram glucose drink can raise blood glucose from about 90 mg/dL up to 120-140 mg/dL in one hour due to efficiency in the absorptive process.
Once glucose is absorbed into the bloodstream via the portal vein, it is transported to the liver where it can be stored for later energy use in the form of glycogen or it can be used immediately as energy. The liver acts as a very important organ in blood glucose level management by taking up excess glucose for storage and releasing it according to the needs of the body. When glucose levels rise too fast after a meal, the liver will temper these spikes by storing excess glucose as glycogen. This means that, in a normal response, after the ingestion of a meal containing 50 grams of glucose, the liver, within the two-hour period, will store about 30 to 40 grams of glucose as glycogen. Thus, it prevents the blood glucose level from abruptly spiking to an excessively high value. Conversely, when fasting, as the level of blood glucose drops, glycogen that is stored in the liver is released by the process of glycogenolysis and supplies all the organs—particularly the brain—that require continuous energy.
Besides impaired storage and regulation of glucose in the liver, a number of factors can mediate how efficiently glucose is absorbed into the bloodstream, including type of food ingested, sensitivity to insulin, and general health of the digestive system. This means that for foods with a high glycemic index, such as sugary snacks or refined foods, the body rapidly absorbs glucose, causing an abrupt spike in blood sugar levels. In one study, using white bread—a high glycemic food—blood glucose peaked at 160 mg/dL within a 60-minute postprandial period. On the contrary, during the consumption of low-glycemic foods—like whole grains—their absorption is slow and the increase of blood glucose levels is very gradual, reaching only about 120 mg/dL within the same period of time. Such a gentle rise in blood sugar due to low-GI foods favors fine control of glucose and minimizes risks for insulin resistance and long-term metabolic complications.
It is also highly influenced by insulin, a hormone of the pancreas, which increases its production and secretion due to the rise in blood glucose. Insulin enhances the absorption of glucose by cells for the rest of the body, where it can be used as energy and stored as glycogen and fat in muscle and adipose tissue. For example, after the ingestion of a 50-gram glucose drink, insulin reaches its peak within 30-60 minutes, after which its peak coincides with the peak of blood glucose. The action of insulin then becomes paramount, as it acts to reduce the concentration of blood glucose by stimulating the uptake of glucose into the cells. This, in turn, reduces the blood sugar levels back to base levels in healthy individuals within 2-3 hours of administration. In insulin-resistant or type 2 diabetes individuals, this mechanism is impaired. Research has demonstrated that in insulin-resistant individuals, glucose uptake into the cells is impaired, and the liver may continue producing and releasing glucose into the bloodstream even after blood glucose levels are high. Due to this impaired regulation, blood glucose levels are abnormally high for an extended time, persisting even hours after eating. For example, in insulin-resistant individuals, postprandially it may stay high for several hours at 160-180 mg/dL instead of coming back down to baseline, which is around 90 mg/dL in a normal individual.