Digestion and Absorption of Carbohydrates in Human Body

Carbohydrates and saccharides can simply be defined as polyhydroxy aldehyde or ketones and their derivatives. These are important sources of energy for living organisms as well as a means by which chemical energy can be stored.

Table of Contents

Major Dietary Carbohydrates

They are categorized into three main groups based on their number of sugar units, namely monosaccharides, disaccharides, and polysaccharides.

Monosaccharide (Simple Sugars)

These are fundamental units of carbohydrates. These are simple sugars with the general chemical structure of C₆H₁₂O₆.

Glucose
Glucose (C₆H₁₂O₆) is the main energy source for most organisms and especially for the brain, and is the most abundant carbohydrate in the body. Plants make it by photosynthesis, and we obtain their stored solar energy by eating plant foods.

Galactose

Galactose is the same as glucose, but with a different position of one hydroxyl group. Thus, it is less stable, and the liver quickly changes it into glucose. The major source of galactose is lactose in milk.

Fructose

Fructose is a 5-carbon ring sugar with the same molecular formula as glucose and is naturally present in fruits, honey, sugarcane, and high fructose corn syrup; it is mostly processed by the liver and not directly utilized by most cells.

Disaccharide

Compound sugars that consist of two monosaccharides with the elimination of a water molecule, with the general chemical structure C₁₂H₂₂O₁₁.

Sucrose (table sugar)

It is a combination of glucose and fructose. Sugarcane and sugar beets contain this. It is widely used as a sweetening agent.

Lactose (milk sugar)

It is a blend of glucose and galactose. Limited digestion crashes are noted in most adults due to low levels of lactase in the intestine, which leads to lactose intolerance.

Maltose (malt sugar)

The malt sugar is made up of two glucose residues and is found in the grain germination process, and results as an intermediate form during the process of starch hydrolysis.

Polysaccharides

These are the most complex carbohydrates that consist of long chains of monosaccharides linked by glycosidic bonds.

Starch ((C₆H₁₀O₅)n)

It is a plant storage form of glucose. Major sources of foods like potatoes, rice, bread, and other cereals.

Glycogen

It is the glucose Storage form in animals, and the main deposits are in the liver (for blood glucose regulation) and muscle (as a rapid fuel for activity).

Cellulose

A Structural glucose polymer in plant cell walls. Humans cannot digest its bonds, so it acts as an insoluble dietary fiber instead of an energy source.

Figure: Digestion and Absorption of Carbohydrates. Image Source: Djésia Arnone et al. 2022.

Digestion of Carbohydrates in the Mouth: The Role of Salivary Amylase

Mechanical and chemical digestion of carbohydrates starts right in the mouth.
As soon as food is taken in, it undergoes mastication
Mastication stimulates the salivary gland (especially the parotid glands) to release saliva containing the enzyme salivary α-amylase.

Action of salivary amylase

The primary function is to hydrolyze internal α-1,4-glycosidic linkages in starch (amylose and amylopectin) into smaller molecules such as maltose, malt triose, and dextrin as the mechanical breakdown of particles by teeth persists and food is mixed with saliva.
It needs a chlorine ion for its activation and a pH of 6.7.
About five percent of starches get broken down in the mouth.

The Stomach Environment: Why Carbohydrate Digestion Pauses?

When carbohydrates reach the stomach, no further chemical breakdown occurs.
In the stomach, the acidic pH (≈0.8–3.5) inactivates salivary α-amylase, and because gastric juice contains virtually no carbohydrate-splitting enzymes, carbohydrate digestion temporarily stops.
Only mechanical mixing continues until chyme enters the duodenum, where pancreatic amylase resumes starch hydrolysis.

Pancreatic Alpha-Amylase: Major Hydrolysis in the Small Intestine

Most of the chemical digestion of carbohydrates occurs in the small intestine.
As acidic chyme from the stomach enters the duodenum, the pancreas releases pancreatic juice through a duct.
Pancreatic amylase is released from acinar cells into the small intestine in concert with other enzymes under the stimulus of secretin and CCK, and continues the process of carbohydrate digestion.
Chyme mixes with alkaline pancreatic juice, which contains pancreatic amylase and bicarbonate. The juice neutralizes gastric acid and provides a near-neutral pH (about 6–7), optimal for amylase activity.
Amylase targets the α-1,4 bonds of complex carbohydrates and is unable to break terminal bonds or α-1,6 bonds.
Pancreatic amylase, similar in action to salivary amylase, resumes starch digestion by hydrolyzing dextrin and other starch fragments to smaller oligosaccharides (3–10 glucose units) and disaccharides such as maltose and malt triose, thereby completing most luminal starch breakdown before brush-border enzymes act.

Brush Border Enzymes: Final Breakdown at the Mucosal Lining

In the small intestine, brush border enzymes on the microvilli carry out the final step of carbohydrate digestion by converting disaccharides and small oligosaccharides into monosaccharides right at the absorptive surface.
The synthesis and further glycosylation of brush border enzymes occur in the endoplasmic reticulum and Golgi apparatus, respectively, of enterocytes and are shipped to their workstation on the gut’s inner surface.
These enzymes, known collectively as disaccharidases, are sucrase, maltase, and lactase.
Sucrase breaks sucrose into glucose and fructose molecules.
Maltase breaks the bond between the two glucose units of maltose, and lactase breaks the bond between galactose and glucose.

Mechanisms of Absorption: SGLT1 and GLUT Transporters

Absorption of monosaccharides is mainly mediated by Na+-D-glucose cotransporter SGLT1 and the facilitative transporters GLUT2 and GLUT5.

SGLT-1 (sodium glucose co transporter)

SGLT-1 is most abundant in the duodenum and jejunum and is responsible for active transport of glucose and galactose across the brush border membrane.
It is an energy-requiring process that requires sodium ions and a transport protein.
A SGLT-1 binds both glucose and sodium at separate sites and transports them into cells.
The sodium is transported down the gradient, and glucose is transported against the concentration gradient.

GLUT2

It is found on the basolateral membrane (BLM) of enterocytes.
The main function is to facilitate the diffusion of glucose, galactose, and fructose from the enterocyte into the portal circulation.
Acts as a facilitated transporter—moves monosaccharides down their concentration gradient without energy input.

GLUT5

It is found on the apical brush border membrane throughout the small intestine.
Also present on the basolateral membrane together with GLUT2 to transport fructose to the portal circulation.
It specializes in D-fructose transport by facilitating diffusion (no energy required).

Absorption of Fructose vs. Glucose and Galactose

Glucose and galactose are absorbed together at the brush border by the sodium–glucose cotransporter SGLT1, which uses the inward sodium gradient to drive these sugars into the enterocyte even against their own concentration gradient.
After this active uptake, both glucose and galactose leave the cell via GLUT2 on the basolateral membrane and enter the portal circulation.
Galactose is converted rapidly to glucose in the liver.
Fructose, in contrast, enters the enterocyte through GLUT5 in the brush border by facilitated diffusion down its concentration gradient, without sodium coupling or direct energy use.
It then exits across the basolateral membrane mainly via GLUT2 into the portal vein, where it is taken up and metabolized extensively by the liver before much of it reaches the systemic circulation.

Factors That Influence Carbohydrate Absorption Efficiency

The type of carbohydrate

Different types of carbohydrates are broken down and absorbed at different rates.
For example, simple sugars like glucose and fructose are rapidly absorbed, while complex carbohydrates like starch take longer to break down and absorb.

The presence of dietary fiber

Fiber slows down the rate of carbohydrate absorption by adding bulk to the digested food, delaying its movement through the small intestine.

pH Environment

Pancreatic amylase requires a near-neutral pH (6–7). Excess stomach acid or low bicarbonate levels can turn off these enzymes, halting digestion.

The presence of enzymes

Enzymes in the small intestine, such as amylase, play a crucial role in breaking down complex carbohydrates into simpler forms that can be absorbed more easily.

The Fate of Indigestible Carbohydrates: Fiber and Colonic Fermentation

Almost all carbohydrates, except for dietary fiber and resistant starches, are efficiently digested and absorbed into the body.
In the large intestine, remaining indigestible carbohydrates are broken down by enzymes released by bacteria.
Bacterial digestion of these slow-releasing carbohydrates produces short-chain fatty acids such as acetate, propionate, butyrate, and some gases.
SCFAs, which are Short-chain fatty acids, are used by bacteria for energy and growth, eliminated in feces, or absorbed into colonic cells, with a small amount transported to the liver.

Clinical Relevance: Lactose Intolerance and Carbohydrate Malabsorption

Lactose intolerance

This is a condition in which there is a deficiency of the enzyme lactase.
The enzyme is responsible for hydrolyzing lactose to glucose and galactose.
Due to this, lactose gets accumulated in the gut, where it is a substrate for bacterial fermentation in the large intestine with the production of H2 and CO2 gases and low molecular weight acids like acetic acid, propionic acid, and butyric acid, which are osmotically active.
Accumulation of gases results in abdominal cramps and flatulence, where osmotically active products draw water from intestinal cells into the lumen, resulting in diarrhea and dehydration.

Carbohydrate malabsorption

This condition arises when the small intestine cannot metabolize or take in the sugars and starches in the diet properly.
It can lead to the development of various symptoms like constipation, diarrhea, gas, and pain in the belly.
The primary types of disorders that cause carbohydrate malabsorption are the inherited disorders in brush-border enzymes or transport mechanisms, which usually affect one specific carbohydrate, such as autosomal recessive lactase deficiency, sucrase-isomaltase deficiency, and SGLT1 sodium/glucose co-transporter deficiency.
Secondary forms are a result of various conditions related to the structure or function of the pancreas and small intestine, which lead to the incorrect absorption of several carbohydrates and other nutrients, like mucosal diseases, for instance, celiac disease or Crohn’s disease, loss of the mucosal surface area, small intestinal bacterial overgrowth, radiation injury, and drug/toxin injury.

Glycemic Response: How Absorption Rate Impacts Blood Sugar Levels

The effect of food on blood sugar following its intake is called glycemic response. It does not only depend on the amount of carbohydrates an individual consumes, but also on the speed at which they enter the blood.
Rapidly digestible, rapidly absorbed carbs (e.g., refined starches, sugary drinks) result in a sudden influx of glucose in the blood, leading to sharp postprandial glucose and insulin spikes.
Slowly digestible starch, resistant starch, and fiber‑rich, minimally processed foods retards gastric emptying and limit enzyme access, resulting in a slower introduction of glucose into plasma, giving a lower, wider glycemic curve and reduced insulin demand.

Conclusion

The primary goal of carbohydrate digestion is to break polysaccharides and disaccharides into monosaccharides, which can be absorbed into the bloodstream.
This process involves the coordinated action of enzymes found in saliva, pancreatic secretions, and along the intestinal brush border. Through their combined work, dietary carbohydrates are hydrolyzed into absorbable forms.
Once broken down, glucose and galactose are absorbed into intestinal cells using a sodium-dependent cotransporter, whereas fructose enters through facilitated diffusion.
After absorption, all these monosaccharides are transported to the liver via the portal vein, where they are processed for either immediate energy use or storage.
Several factors influence how efficiently carbohydrates are absorbed, including the acidity (pH) of the intestinal lumen, the presence and activity of digestive enzymes, and the nature of the carbohydrate source.
Humans lack the enzymes necessary for enzymatic digestion; because of this, fiber in food is not broken down in the digestive system. However, gut microbes in the large intestine ferment some dietary fiber.
Clinically relevant malabsorption syndromes and altered glycemic responses are caused by defects in enzymes or transporters.

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