Why can't sucrose be digested in the body?

December 18, 2025 •

4 min read

Over 50% of adults with chronic gastrointestinal symptoms have some form of disaccharidase deficiency. This crucial enzyme is missing or defective, which explains why can't sucrose be digested in the body without being broken down first. Instead, it must be hydrolyzed into simpler sugars for absorption.

Quick Summary

Sucrose cannot be absorbed directly because it is a disaccharide. The small intestine requires the enzyme sucrase to break it down into absorbable glucose and fructose. A deficiency in this enzyme leads to malabsorption and GI symptoms.

Key Points

Sucrase is Required: The body cannot directly absorb sucrose; it must first be broken down by the enzyme sucrase in the small intestine.
Sucrose is a Disaccharide: Sucrose is a complex sugar composed of two simple sugars, glucose and fructose, which are too large for direct absorption.
Deficiency Causes Malabsorption: A lack of the sucrase enzyme leads to sucrose malabsorption, where the sugar passes undigested to the large intestine.
Bacteria Cause Symptoms: Gut bacteria ferment the undigested sucrose, producing gas and causing water to enter the bowels, which leads to bloating and diarrhea.
Deficiency Can Be Genetic or Acquired: The deficiency can be a congenital genetic condition (CSID) or an acquired problem resulting from intestinal damage due to infections or diseases like celiac or Crohn's.
Management is Possible: Dietary restrictions and enzyme replacement therapy (ERT) are common strategies to manage sucrose intolerance and alleviate symptoms.

The Basic Science of Sugar Digestion

To understand why the body can't digest sucrose as is, it's essential to grasp the fundamental process of carbohydrate digestion. Most carbohydrates are polysaccharides (long chains of sugar molecules) or disaccharides (two sugar molecules linked together). For the body to use them for energy, these larger molecules must be broken down into single sugar units, called monosaccharides. Only monosaccharides, such as glucose and fructose, are small enough to be absorbed through the intestinal walls and enter the bloodstream.

Sucrose, or common table sugar, is a disaccharide made up of one glucose molecule and one fructose molecule bonded together. While other carbohydrates begin to break down in the mouth, the digestion of sucrose is specifically dependent on an enzyme located much further along the digestive tract.

The Crucial Role of the Sucrase Enzyme

At the final stage of carbohydrate digestion, a critical enzyme called sucrase performs the specific task of breaking down sucrose.

Where it works: Sucrase is located in the small intestine, specifically on the 'brush border'—the surface of the intestinal lining composed of millions of tiny, finger-like projections called microvilli. This location provides a large surface area for efficient nutrient absorption.
How it works: Sucrase acts as a biological catalyst to hydrolyze the glycosidic bond connecting the glucose and fructose units in sucrose. This hydrolysis reaction uses a water molecule to split the bond, releasing the individual monosaccharides, glucose and fructose.
High specificity: Like most enzymes, sucrase is highly specific and can only break down sucrose. It cannot digest other disaccharides like lactose or complex starches. This specificity is why a problem with just one enzyme can cause a distinct dietary intolerance.

Without sufficient and functional sucrase, the body lacks the specific tool required to dismantle the sucrose molecule. As a result, the entire disaccharide passes through the small intestine intact.

What Happens When Sucrose Isn't Digested?

When sucrose isn't broken down and absorbed in the small intestine, it continues its journey to the large intestine, or colon. This undigested sugar creates a cascade of physiological effects that result in unpleasant gastrointestinal symptoms.

Fermentation by gut bacteria: The colon contains a vast population of bacteria (the gut microbiota) that feed on fermentable carbohydrates. When undigested sucrose reaches the colon, these bacteria feast on it through a process called fermentation.
Gas production: A byproduct of this bacterial fermentation is the production of various gases, including hydrogen and methane. This accumulation of gas leads to abdominal bloating, cramping, and flatulence.
Osmotic effect: The high concentration of undigested sugar in the large intestine creates an osmotic effect, drawing excess water into the bowel. This influx of water leads to watery stools and diarrhea.

Comparison of Sucrose and Lactose Intolerance


Feature	Sucrose Intolerance (CSID)	Lactose Intolerance
Problem	Deficiency of the sucrase enzyme.	Deficiency of the lactase enzyme.
Indigestible Sugar	Sucrose (table sugar, fruit sugar).	Lactose (milk sugar).
Monosaccharides	Glucose and Fructose.	Glucose and Galactose.
Cause	Genetic mutation of the SI gene (CSID) or secondary intestinal damage.	Genetic predisposition or intestinal damage from illness/medication.
Prevalence	Fairly rare genetically, but more common secondary forms exist.	Very common, affecting around 65% of the world's population.
Symptoms	Abdominal pain, bloating, gas, and diarrhea upon consuming sucrose.	Similar symptoms to sucrose intolerance, triggered by consuming dairy products.

Causes of Sucrase Deficiency

Sucrase deficiency is the primary reason the body struggles to digest sucrose, and it can arise from a couple of sources:

Congenital Sucrase-Isomaltase Deficiency (CSID): This is a rare, inherited metabolic disorder caused by a genetic mutation in the SI gene that directs the production of the sucrase-isomaltase enzyme. Individuals with CSID are born with either a complete absence or significantly reduced function of this enzyme. Symptoms typically appear in infancy when sucrose is introduced into the diet.
Acquired Sucrose Intolerance: More common than the congenital form, this occurs when the intestinal lining is damaged, temporarily reducing sucrase enzyme levels. Conditions that can cause this damage include:
- Gastrointestinal infections
- Celiac disease
- Crohn's disease
- Small Intestinal Bacterial Overgrowth (SIBO)
- Certain medications

Symptoms and Management

Symptoms of sucrose intolerance are a direct result of the undigested sugar and vary in severity depending on the amount consumed and the level of enzyme deficiency.

Key symptoms
- Abdominal pain and cramping
- Bloating and gas
- Diarrhea
- Nausea and vomiting
Management strategies
- Dietary modification: The cornerstone of treatment involves restricting or avoiding foods high in sucrose and certain starches. This requires careful label reading to identify hidden sugars.
- Enzyme replacement therapy (ERT): For individuals with CSID, a prescription oral enzyme replacement like sacrosidase can be taken with meals to aid in sucrose digestion. This provides the missing enzyme to break down the sugar.
- Treating the underlying cause: For acquired intolerance, resolving the primary medical condition (like an infection or celiac disease) can lead to the restoration of normal sucrase function over time.

Conclusion

The inability to digest sucrose directly stems from the body's lack of a specific digestive tool—the sucrase enzyme. As a disaccharide, sucrose is too large to be absorbed into the bloodstream and must first be broken down into its simple sugar components. When sucrase is absent, as in the rare genetic condition CSID, or temporarily reduced due to intestinal damage, the unabsorbed sugar ferments in the colon, causing a range of uncomfortable and disruptive symptoms. Proper diagnosis and management, often involving dietary changes and enzyme therapy, can effectively alleviate these issues. For more information on congenital sucrase-isomaltase deficiency, consult resources like the National Institutes of Health.

Frequently Asked Questions

Sucrose is a disaccharide, or a double sugar, made of glucose and fructose. It is too large to be absorbed directly into the bloodstream and must be broken down into its simpler, single sugar components to be utilized for energy.

The enzymatic digestion of sucrose takes place in the small intestine. The enzyme responsible, sucrase, is located on the brush border of the intestinal lining, where it can break down the sugar before absorption.

A deficiency in the sucrase enzyme means the body cannot break down sucrose adequately. The undigested sugar then travels to the large intestine, causing gastrointestinal symptoms.

Symptoms of sucrose intolerance include abdominal pain, bloating, excessive gas, and diarrhea. These digestive problems typically occur shortly after consuming foods or drinks containing sucrose.

No, while they cause similar digestive symptoms, they are distinct conditions caused by a deficiency in different enzymes. Sucrose intolerance is due to a lack of sucrase, while lactose intolerance is caused by a lack of lactase.

CSID is a rare genetic disorder caused by mutations in the SI gene. This gene provides instructions for making the sucrase-isomaltase enzyme, and its mutation results in the body being unable to produce a functional enzyme.

While there is no cure, sucrose intolerance can be managed effectively. Treatment options include dietary modifications (reducing sucrose intake) and, for CSID, prescription enzyme replacement therapy (ERT) with sacrosidase.