Nutrition Diet: What sugar cannot be digested?

October 22, 2025 •

5 min read

According to the NIH, up to 65% of people worldwide have some degree of lactose intolerance, which is a prime example of a sugar the body cannot fully digest. Understanding what sugar cannot be digested is a crucial step toward better managing digestive health and overall well-being. It's not just about what you eat, but how your body processes it.

Quick Summary

This article explores the various types of sugars and complex carbohydrates, such as dietary fiber and FODMAPs, that the human body cannot fully break down. It details the reasons behind this inability, including enzyme deficiencies and complex molecular structures, and explains how these undigested components can affect digestive health and gut function.

Key Points

Cellulose is Indigestible: The human body lacks the enzyme cellulase to break down cellulose, a type of fiber, which is why it passes through largely intact.
Enzyme Deficiencies Cause Intolerance: Conditions like lactose intolerance and congenital sucrase-isomaltase deficiency occur due to a lack of the specific enzymes needed to digest certain disaccharides.
FODMAPs Trigger Digestive Symptoms: Fermentable Oligosaccharides, Disaccharides, Monosaccharides, and Polyols (FODMAPs) are poorly absorbed and fermented by gut bacteria, causing gas and bloating in sensitive individuals.
Gut Bacteria Ferment Indigestible Sugars: Any sugars that are not digested in the small intestine travel to the colon, where gut microbes ferment them, producing gas and beneficial short-chain fatty acids.
Dietary Fiber Is Crucial: Though indigestible, fiber provides vital bulk and acts as a prebiotic, feeding beneficial bacteria and supporting bowel health.
Not All People React the Same: The tolerance for specific indigestible carbohydrates varies greatly among individuals, and sensitivity often depends on genetic factors, gut health, and underlying conditions like IBS.

Understanding Indigestible Carbohydrates

Not all carbohydrates are created equal, especially when it comes to digestion. While simple sugars like glucose and fructose are easily and rapidly absorbed, a significant portion of carbohydrates, both simple and complex, can resist digestion in the small intestine. This indigestibility is primarily due to the human body lacking the specific enzymes required to break the unique chemical bonds that hold these sugar molecules together.

The Body's Digestive Machinery

In a typical digestive process, the body uses enzymes to break down food into smaller, absorbable molecules. For most digestible carbohydrates, this begins in the mouth with salivary amylase and continues in the small intestine with pancreatic enzymes and intestinal mucosal enzymes like lactase and sucrase-isomaltase. The resulting simple sugars are then absorbed into the bloodstream. However, when certain carbohydrates reach the small intestine and encounter no matching enzyme, they pass onward to the large intestine.

The Role of Gut Bacteria

Once in the large intestine, the undigested carbohydrates become a feast for the trillions of bacteria that make up the gut microbiome. This bacterial fermentation process is not inherently bad; it is, in fact, a crucial part of gut health. Fermentation produces beneficial byproducts called short-chain fatty acids (SCFAs), which nourish the cells lining the colon and have systemic health benefits. However, this same process also produces gases like hydrogen and methane, which can lead to bloating, cramping, and discomfort in sensitive individuals, especially those with conditions like Irritable Bowel Syndrome (IBS).

Dietary Fiber: The Most Common Indigestible Carb

Dietary fiber is a well-known example of a carbohydrate that cannot be digested by humans. Fiber is primarily composed of long-chain sugar molecules, and the human digestive system lacks the enzymes needed to break them down. There are two main types of dietary fiber, both of which are important for a healthy diet:

Soluble Fiber: This type of fiber dissolves in water and forms a gel-like substance in the gut. It can help lower cholesterol and blood sugar levels. Examples include oats, peas, beans, and apples.
Insoluble Fiber: This fiber does not dissolve in water and adds bulk to stool, promoting regular bowel movements. Sources include whole wheat products, nuts, and leafy greens.

Carbohydrate Malabsorption and Intolerances

For some individuals, even specific, less complex sugars can be poorly absorbed due to enzyme deficiencies. These conditions are distinct from a food allergy, which involves an immune response.

Lactose Intolerance Lactose, the sugar in milk, is a disaccharide made of glucose and galactose. It requires the enzyme lactase to be broken down. Primary lactose intolerance, the most common type, occurs when the body's production of lactase naturally declines after childhood. Symptoms include gas, bloating, and diarrhea after consuming dairy.

Congenital Sucrase-Isomaltase Deficiency (CSID) This is a rare genetic disorder where the body cannot properly break down sucrose (table sugar) and maltose due to a deficiency of the sucrase-isomaltase enzyme. Symptoms, which can include chronic diarrhea, bloating, and abdominal pain, often appear when an infant begins consuming solid foods containing these sugars.

FODMAPs: A Group of Problematic Sugars

FODMAP is an acronym for Fermentable Oligosaccharides, Disaccharides, Monosaccharides, and Polyols. These are short-chain carbohydrates that are poorly absorbed and can trigger digestive issues in sensitive people, especially those with IBS. This group includes:

Oligosaccharides: Fructans (found in wheat, onions, garlic) and GOS (found in legumes).
Disaccharides: Lactose (in dairy), if an individual is intolerant.
Monosaccharides: Excess fructose (found in honey, apples).
Polyols: Sugar alcohols like sorbitol and mannitol (in some fruits and artificial sweeteners).

Comparative Look at Digestible vs. Indigestible Sugars

To illustrate the key differences, here is a comparison of common carbohydrate types:


Carbohydrate Type	Example	Digestible by Human Enzymes?	Reason for Indigestion (if applicable)	Effect on the Body
Monosaccharide	Glucose	Yes	Absorbed directly; no digestion needed.	Primary energy source; rapid absorption.
Disaccharide	Lactose	Sometimes	Requires lactase enzyme; deficient in lactose intolerance.	Digested to glucose and galactose; fermented in colon if not.
Disaccharide	Sucrose	Sometimes	Requires sucrase-isomaltase; deficient in CSID.	Digested to glucose and fructose; fermented if enzyme is lacking.
Polysaccharide	Starch	Yes	Broken down by amylase.	Energy source, slower release than simple sugars.
Polysaccharide	Cellulose (Fiber)	No	Human body lacks necessary enzymes for beta-glycosidic bonds.	Provides bulk, aids bowel regularity.
Oligosaccharide	Fructan (FODMAP)	No	Not broken down in the small intestine due to bond type.	Fermented by gut bacteria in the colon.
Polyol	Sorbitol	No	Poorly absorbed in the small intestine.	Fermented in the colon, attracts water, laxative effect.

Managing a Diet with Indigestible Sugars

For most people, a balanced diet including both digestible and indigestible carbohydrates is essential for good health. However, if you experience digestive distress, understanding your specific triggers is key. Keeping a food and symptom diary can help identify problematic foods. For those with diagnosed intolerances or IBS, working with a healthcare provider or a registered dietitian is recommended to create a personalized nutrition diet plan, such as a low-FODMAP diet. In some cases, enzyme supplements can help manage specific intolerances. It is important to note that many indigestible carbohydrates are beneficial prebiotics that feed healthy gut bacteria, so full avoidance is not usually the best long-term strategy. The goal is to find a balance that supports both digestive comfort and a healthy gut microbiome.

Conclusion: The Bigger Picture of Your Nutrition Diet

Ultimately, understanding what sugar cannot be digested by your body provides crucial insight into managing your health. From dietary fiber that promotes bowel regularity to complex FODMAPs that can cause discomfort for some, these indigestible carbohydrates play a significant role in our diet. Rather than viewing all indigestible sugars negatively, recognizing their unique roles and individual tolerances allows for a more informed approach to nutrition. By identifying your personal triggers and managing your intake, you can harness the benefits of these foods while minimizing digestive issues, leading to a healthier gut and a more comfortable life. For a deeper dive into the science behind digestive health, explore the National Institutes of Health's research on the topic(https://pmc.ncbi.nlm.nih.gov/articles/PMC4632417/).

Frequently Asked Questions

Humans cannot digest cellulose because they lack the necessary enzymes, specifically cellulase, to break the unique beta-glycosidic bonds that link its glucose monomers.

Lactose is the most common sugar that people cannot digest, a condition known as lactose intolerance, which is caused by a deficiency of the lactase enzyme.

FODMAPs are a group of short-chain carbohydrates, including some oligosaccharides, that are poorly absorbed in the gut and are often referred to as indigestible or fermentable sugars.

Indigestible sugars pass through the small intestine largely intact and are fermented by bacteria in the large intestine. This process produces gas and short-chain fatty acids.

Yes, although it's rare. A genetic disorder called Congenital Sucrase-Isomaltase Deficiency (CSID) prevents the digestion of sucrose and maltose due to a lack of the sucrase-isomaltase enzyme.

Keeping a food and symptom diary, potentially with the guidance of a doctor or registered dietitian, can help you identify patterns and potential food triggers. In some cases, specific breath tests or an elimination diet may be recommended.

Yes, polyols, or sugar alcohols, such as sorbitol and mannitol, are poorly absorbed in the small intestine. They are fermented by gut bacteria and can cause digestive symptoms.

No, many indigestible carbohydrates, particularly dietary fiber and prebiotics like oligosaccharides, are beneficial for gut health by feeding healthy bacteria and producing nourishing SCFAs.

Fermentation occurs in all individuals, but it only causes significant issues in people with a sensitive gut, such as those with IBS. The level of discomfort varies widely.

Yes, some infants are born with congenital forms of carbohydrate malabsorption, such as CSID or glucose-galactose malabsorption, which present with severe digestive symptoms soon after birth.