What Sugar Can Your Body Digest and How It Works

December 19, 2025 •

4 min read

According to the World Sugar Research Organisation, dietary carbohydrates are digested into glucose, fructose, and galactose, and absorbed into the blood in the small intestine. This process is crucial for providing the body with its primary energy source, but not all sugars are created equal in terms of how our bodies process them.

Quick Summary

The human body digests and absorbs different types of sugars, from simple monosaccharides like glucose and fructose to more complex disaccharides and starches. Specialized enzymes break down larger sugar molecules into their single-unit forms for absorption. Conditions like lactose intolerance and fructose malabsorption show how enzymatic deficiencies can prevent proper sugar digestion, affecting overall health.

Key Points

Complete Digestion: The body breaks down most dietary carbohydrates into simple sugar units (monosaccharides) like glucose, fructose, and galactose for absorption and energy.
Enzymes are Key: Specific enzymes like sucrase and lactase are required to break down more complex sugars (disaccharides) before they can be absorbed into the bloodstream.
Glucose is Primary Fuel: Glucose is the body's most important energy source; the liver converts other absorbed simple sugars into glucose to be used or stored.
Genetic Deficiencies: Some individuals have genetic disorders, such as congenital sucrase-isomaltase deficiency (CSID), that prevent them from producing the necessary enzymes for proper sugar digestion.
Dietary Fiber's Role: Fiber slows down sugar absorption, leading to a more stable blood sugar response, which is one reason whole foods with natural sugars are healthier than processed ones.
Malabsorption Issues: Insufficient lactase or a transporter deficiency for fructose can cause malabsorption, leading to uncomfortable symptoms like bloating, gas, and diarrhea.

Understanding the Basics of Sugar Digestion

The sugars and starches we consume are all carbohydrates, and our digestive system is a finely tuned machine designed to break them down into their simplest forms: monosaccharides. These single-unit sugar molecules are then absorbed into the bloodstream to be used as fuel. However, the journey from a sweet morsel of fruit or a piece of bread to usable energy varies significantly based on the sugar's chemical structure.

The Digestible Sugars: Monosaccharides and Disaccharides

Monosaccharides are the simplest forms of sugar and include glucose, fructose, and galactose. Because they are already in their single-unit form, they do not require any enzymatic breakdown and can be absorbed directly into the bloodstream from the small intestine.

Glucose: Often called "blood sugar," glucose is the body's preferred and most efficient energy source. All other digestible carbohydrates are eventually converted into glucose.
Fructose: Known as "fruit sugar," fructose is also a monosaccharide found in fruits, vegetables, and honey. It is absorbed directly but is primarily metabolized by the liver, which converts it into glucose before it can be used for energy.
Galactose: This monosaccharide is mainly found in milk and dairy products, primarily as a component of the disaccharide lactose. The liver also converts absorbed galactose into glucose for use.

Disaccharides are composed of two monosaccharides linked together. They must be broken down by specific enzymes in the small intestine before they can be absorbed.

Sucrose: Commonly known as table sugar, sucrose consists of one glucose and one fructose molecule. The enzyme sucrase breaks it down into its two components.
Lactose: Found in milk and dairy, lactose is made of one glucose and one galactose molecule. The enzyme lactase is responsible for its digestion.
Maltose: Often called "malt sugar," maltose is found in grains like barley and consists of two glucose molecules. The enzyme maltase breaks it down.

The Role of Enzymes in Sugar Digestion

The digestive process for carbohydrates begins in the mouth with salivary amylase, but the bulk of digestion occurs in the small intestine. Here, enzymes produced by the intestinal wall and pancreas target specific sugar bonds.

Amylase: Pancreatic amylase continues the work of salivary amylase, breaking down starches (long chains of glucose) into smaller sugar units like maltose.
Sucrase: Splits sucrose into glucose and fructose.
Lactase: Breaks down lactose into glucose and galactose.
Maltase: Converts maltose into two glucose molecules.

When Sugar Digestion Goes Wrong

For some individuals, the digestive process for certain sugars can be impaired, leading to various gastrointestinal symptoms. These conditions are typically caused by a deficiency in the specific enzymes needed for digestion.

Lactose Intolerance: A common condition resulting from the small intestine producing insufficient lactase. Undigested lactose ferments in the colon, causing gas, bloating, and diarrhea.
Fructose Malabsorption: A restricted transport capacity of the protein GLUT5 in the small intestine can lead to poor absorption of fructose. This can result in bloating, flatulence, and abdominal pain as bacteria ferment the unabsorbed sugar.
Congenital Sucrase-Isomaltase Deficiency (CSID): A rare genetic disorder where the body cannot properly break down sucrose and maltose due to a missing or inactive sucrase-isomaltase enzyme.

Comparison of Common Digestible Sugars


Feature	Glucose	Fructose	Sucrose	Lactose
Classification	Monosaccharide	Monosaccharide	Disaccharide	Disaccharide
Source	Grains, fruits, vegetables	Fruits, honey, high-fructose corn syrup	Table sugar, fruits, vegetables	Milk, dairy products
Digestion	Absorbed directly; no enzymatic breakdown needed	Absorbed directly; no enzymatic breakdown needed	Requires sucrase to break down into glucose and fructose	Requires lactase to break down into glucose and galactose
Metabolism	Main energy source; used by all cells	Primarily processed by the liver and converted to glucose	Converted into glucose and fructose and then metabolized	Converted into glucose and galactose and then metabolized
Blood Sugar Impact	Raises blood sugar quickly, stimulates insulin	Gradual blood sugar rise; less immediate insulin impact	Affects blood sugar via its component monosaccharides	Can be tolerated differently depending on lactase levels

How Fiber Affects Sugar Digestion

Dietary fiber, a type of polysaccharide, is not digested or absorbed by the human body. Instead, it passes relatively untouched through the digestive system and is either fermented by colonic bacteria or eliminated. The presence of fiber, such as in whole fruits and vegetables, slows the absorption of sugars, leading to a more gradual increase in blood sugar levels. This is one of the key reasons why the sugars in whole foods are digested differently than those from processed foods.

Conclusion

In summary, the human body is equipped to digest a variety of sugars, primarily breaking down complex carbohydrates into the absorbable monosaccharides: glucose, fructose, and galactose. Glucose is the body's main fuel, while fructose and galactose are converted into glucose in the liver. The efficiency and completeness of this process depend on the availability of specific digestive enzymes like sucrase and lactase. Genetic or acquired deficiencies in these enzymes can lead to conditions that impair digestion, such as lactose intolerance or congenital sucrase-isomaltase deficiency. Understanding these mechanisms helps explain why the sugar in whole foods is processed differently and generally healthier than the refined sugars in processed products, and highlights the importance of gut health for proper nutrient absorption.

Outbound link: For more in-depth information on the human digestive process, including carbohydrates, see the Physiology, Carbohydrates article on the National Institutes of Health (NIH) bookshelf at ncbi.nlm.nih.gov/books/NBK459280.

Frequently Asked Questions

The simplest sugars for the body to digest are monosaccharides like glucose and fructose because they are single sugar units that do not need to be broken down further by enzymes and can be absorbed directly.

Many people with lactose intolerance can still consume some dairy products, especially those with lower lactose levels like aged cheeses or yogurt. The severity of intolerance varies, and some can also use lactase enzyme supplements to help digest milk sugar.

If the body cannot digest a sugar, it passes through the small intestine and into the colon. There, bacteria ferment the undigested sugar, producing gases, and causing symptoms like bloating, gas, and diarrhea.

Yes. While both contain digestible sugars, the fiber in whole fruit slows down the absorption of its natural sugars. Table sugar is refined, and its component sugars (glucose and fructose) are absorbed more rapidly, potentially causing a quicker blood sugar spike.

Honey contains both glucose and fructose as monosaccharides, which are absorbed directly. Table sugar (sucrose) is a disaccharide that must first be broken down by an enzyme into glucose and fructose before absorption. Both are ultimately used similarly by the body.

The liver plays a crucial role in metabolizing absorbed sugars. It converts fructose and galactose into glucose, which is the body's primary energy fuel. It also stores excess glucose as glycogen for later use.

Glucose provides the quickest source of energy because it is a monosaccharide that is absorbed directly into the bloodstream and can be used immediately by cells. It raises blood sugar levels faster than more complex sugars.