What Carbohydrates are Difficult to Digest?

October 28, 2025 •

4 min read

While most carbohydrates are easily broken down by digestive enzymes, a significant fraction, including dietary fiber, passes through the body undigested. Understanding which carbohydrates are difficult to digest is key to managing gut health and avoiding discomfort.

Quick Summary

Certain carbohydrates like fiber, resistant starch, lactose, and FODMAPs resist digestion in the small intestine, undergoing fermentation by gut bacteria instead. This process can cause gas, bloating, and other digestive symptoms in sensitive individuals.

Key Points

Dietary fiber and resistant starch are difficult to digest, passing to the large intestine where they feed beneficial gut bacteria.
Lactose intolerance is caused by a deficiency of the lactase enzyme, leading to the fermentation of lactose in the colon.
FODMAPs are short-chain carbohydrates that can trigger IBS symptoms due to poor absorption and rapid fermentation in the gut.
Sugar alcohols, a type of polyol, are poorly absorbed and can cause digestive issues like gas and bloating, especially in larger quantities.
Cooking and cooling certain starches can increase their resistant starch content, which acts like fiber in the digestive system.
Managing difficult-to-digest carbs may involve a process of gradual introduction, mindful eating, or a temporary elimination diet under medical supervision.

The Science of Carb Digestion

Carbohydrates are the body's primary source of energy, but not all carbs are created equal when it comes to digestion. Carbohydrates can be broadly categorized into simple and complex carbs. Simple carbohydrates, such as glucose and sucrose, are quickly digested and absorbed, providing a rapid energy source. In contrast, complex carbohydrates, like starches, have more intricate chemical structures that take longer to break down. However, a specific subset of complex carbs is resistant to breakdown by human digestive enzymes altogether. Instead of being absorbed in the small intestine, they travel to the large intestine where they are fermented by gut bacteria. This fermentation process can be highly beneficial for gut health but can also lead to uncomfortable symptoms for some individuals.

The Primary Culprits: Indigestible Carbohydrates

Several types of carbohydrates are notoriously difficult for the human body to digest, each with its own unique mechanism and effect on the gut. For instance, dietary fiber, resistant starch, lactose, and FODMAPs all play a significant role in this digestive process.

Dietary Fiber

Dietary fiber is a type of carbohydrate that our bodies cannot break down, which is precisely what makes it so important for digestive health. Fiber comes in two main forms:

Soluble Fiber: This type dissolves in water to form a gel-like material, which can help lower cholesterol and blood sugar levels. Sources include oats, beans, and apples.
Insoluble Fiber: This fiber does not dissolve and acts as a 'bulking agent,' promoting the movement of waste through the digestive system and preventing constipation. Examples include wheat bran, nuts, and many vegetables. Since humans lack the necessary enzymes to digest fiber, it passes through the small intestine largely intact, reaching the large intestine where gut microbiota can ferment it.

Resistant Starch

Resistant starch is a type of dietary starch that resists digestion in the small intestine, passing through to the large bowel to feed beneficial bacteria. The amount of resistant starch in food can vary depending on its preparation. There are several types:

Type 1 (RS1): Found in whole grains, seeds, and legumes, this starch is physically inaccessible to enzymes.
Type 2 (RS2): Found in some raw foods, such as green bananas and raw potatoes, due to its tightly packed crystalline structure.
Type 3 (RS3): Formed when certain starchy foods like rice, potatoes, and pasta are cooked and then cooled.
Type 4 (RS4): Chemically modified starches used in processed foods.

Lactose (Disaccharide Intolerance)

Lactose is the primary carbohydrate found in milk and dairy products. Lactose intolerance occurs when the small intestine doesn't produce enough of the enzyme lactase to break down lactose into simpler sugars. When undigested lactose reaches the large intestine, gut bacteria ferment it, producing gas and causing symptoms like bloating, cramping, and diarrhea.

FODMAPs (Fermentable Oligosaccharides, Disaccharides, Monosaccharides, and Polyols)

FODMAPs are a group of short-chain carbohydrates that are poorly absorbed by the gut and rapidly fermented by gut bacteria. This can cause significant digestive distress for people with irritable bowel syndrome (IBS) or other digestive sensitivities. FODMAPs include:

Fructans and Galactans (Oligosaccharides): Found in foods like wheat, onions, garlic, and legumes.
Lactose (Disaccharides): In dairy products.
Excess Fructose (Monosaccharides): Found in certain fruits and honey.
Polyols (Sugar Alcohols): Present in some fruits, vegetables, and artificial sweeteners like sorbitol and mannitol.

Sugar Alcohols (Polyols)

While also part of the FODMAPs group, sugar alcohols are worth highlighting due to their use as artificial sweeteners in many 'sugar-free' products. Because they are not fully absorbed, consuming large amounts can cause gas, bloating, and diarrhea. Common examples include sorbitol, xylitol, and maltitol.

Comparison of Difficult-to-Digest Carbohydrates


Characteristic	Dietary Fiber	Resistant Starch	FODMAPs	Lactose (Intolerance)
Mechanism	Lack of digestive enzymes.	Structure resists enzymatic breakdown.	Poorly absorbed, quickly fermented.	Insufficient lactase enzyme.
Common Sources	Whole grains, vegetables, fruits.	Legumes, cooled starches, green bananas.	Wheat, legumes, onions, certain fruits, dairy.	Milk, yogurt, soft cheeses.
Primary Symptoms	Gas, bloating (when over-consumed).	Gas, bloating (especially new to diet).	Bloating, gas, pain, altered bowel habits.	Bloating, gas, cramping, diarrhea.
Gut Impact	Provides bulk, feeds beneficial bacteria.	Prebiotic effect, feeds beneficial bacteria.	Rapid fermentation causes gas and distress.	Bacterial fermentation causes gaseous symptoms.

Conclusion

For many, carbohydrates that are difficult to digest are a beneficial part of a healthy diet, playing a crucial role in gut health, blood sugar management, and satiety. However, for sensitive individuals, such as those with IBS or specific intolerances, these same carbohydrates—including fiber, resistant starch, lactose, and FODMAPs—can be a source of significant digestive discomfort. Understanding the specific types of hard-to-digest carbs and how they affect your body is the first step toward managing symptoms and improving your digestive well-being. By adjusting your intake of certain foods, you can harness the benefits of these carbs while minimizing negative side effects. For more detailed information on specific fibers and their effects, you can consult resources like Harvard's T.H. Chan School of Public Health.(https://nutritionsource.hsph.harvard.edu/carbohydrates/fiber/)

Frequently Asked Questions

The main reason is that the human body lacks the specific enzymes needed to break down certain carbohydrates, such as dietary fiber and some complex starches. This allows them to pass into the large intestine largely intact.

Not at all. In fact, many of these carbohydrates, like fiber and resistant starch, are essential for gut health as they feed beneficial gut bacteria. They are only problematic for individuals with specific sensitivities or intolerances.

FODMAPs are fermentable short-chain carbohydrates. People with Irritable Bowel Syndrome (IBS) often find relief from symptoms like bloating, gas, and pain by following a low-FODMAP diet under the guidance of a healthcare professional.

A simple way to get more resistant starch is by cooking starchy foods like potatoes, rice, or pasta and then allowing them to cool completely before eating. This process, known as retrogradation, increases their resistant starch content.

Sugar alcohols, or polyols, are low-calorie sweeteners used in many processed foods. They cause digestive upset because they are poorly absorbed in the small intestine, leading to fermentation and gas in the colon.

Increase your fiber intake gradually to allow your digestive system to adapt. Chewing food thoroughly and staying well-hydrated can also help improve digestion and minimize discomfort.

Symptoms of intolerance, like bloating and gas, are uncomfortable but not life-threatening. If you experience severe symptoms like consistent pain, significant diarrhea, or unexplained weight loss, you should consult a doctor to rule out more serious conditions like celiac disease or inflammatory bowel disease.