Can carbs cause diarrhea? A guide to understanding carbohydrate malabsorption

October 11, 2025 •

4 min read

According to the MSD Manuals, carbohydrate intolerance is a widespread condition causing symptoms like diarrhea, bloating, and gas in sensitive individuals. This guide delves into the digestive processes to explain why certain carbohydrates can indeed cause diarrhea and how various underlying factors contribute to this issue.

Quick Summary

Carbohydrate malabsorption leads to diarrhea when undigested sugars ferment in the colon, creating gas and drawing excess water into the intestines. This often affects individuals with specific food sensitivities like lactose intolerance, fructose malabsorption, and irritable bowel syndrome.

Key Points

Mechanism: Undigested carbs reach the large intestine and are fermented by bacteria, producing gas and drawing water into the bowel, causing osmotic diarrhea.
FODMAPs: High-FODMAP foods, which include certain fruits, vegetables, grains, and sweeteners, are poorly absorbed and a common trigger for diarrhea, especially in individuals with IBS.
Specific Intolerances: Common specific issues include lactose intolerance (dairy sugar) and fructose malabsorption (fruit sugar), which are diagnosed and managed with dietary changes.
Underlying Conditions: Diarrhea from carbs can be a symptom of more serious conditions like celiac disease or diabetic neuropathy, requiring specific medical management.
Diagnosis and Management: Diagnosis often involves a hydrogen breath test, while treatment focuses on dietary elimination (such as a low-FODMAP diet), enzyme supplements, and treating any underlying conditions.

The Science Behind Carbohydrates and Diarrhea

The short answer is yes, carbohydrates can cause diarrhea in some individuals, particularly those with an underlying intolerance or digestive condition. When your body has trouble digesting and absorbing certain carbohydrates, these undigested sugars move into the large intestine. In the colon, gut bacteria ferment these carbohydrates, which produces gas and creates an osmotic load that pulls extra water into the bowel. This combination of increased water and gas can lead to the classic symptoms of carbohydrate intolerance: diarrhea, bloating, and flatulence.

Common Carbohydrate Triggers for Diarrhea

Not all carbohydrates are created equal when it comes to causing digestive upset. Several specific types are notorious for triggering symptoms in sensitive individuals.

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

FODMAPs are a group of short-chain carbohydrates that are poorly absorbed by many people. When they reach the large intestine, gut bacteria quickly ferment them, causing the release of hydrogen and methane gas. This draws water into the bowel and can cause significant digestive distress, including diarrhea. A low-FODMAP diet is a common and effective strategy for managing symptoms in people with Irritable Bowel Syndrome (IBS). For a list of high-FODMAP foods, see the referenced web document.

Lactose Intolerance

Lactose is a disaccharide sugar found in milk and dairy products. Lactose intolerance occurs due to a deficiency of the lactase enzyme, which is needed to break down lactose into simpler sugars for absorption. When undigested lactose reaches the colon, it ferments and causes gas, bloating, and osmotic diarrhea. The severity of symptoms can vary depending on the degree of lactase deficiency and the amount of lactose consumed.

Fructose Malabsorption

Fructose is a simple sugar found in fruits, honey, and high-fructose corn syrup. For some people, the system that absorbs fructose in the small intestine has a limited capacity, especially when the amount of fructose exceeds the amount of glucose. This can lead to malabsorption, with unabsorbed fructose entering the large intestine and causing symptoms similar to lactose intolerance.

Celiac Disease

Celiac disease is an autoimmune condition where the body reacts negatively to gluten, a protein found in wheat, barley, and rye. The immune response damages the lining of the small intestine, leading to nutrient malabsorption and, frequently, chronic diarrhea. While celiac disease is not a carbohydrate intolerance, the gluten protein is found in many carbohydrate-rich foods, and the resulting intestinal damage causes a wide range of malabsorptive symptoms.

Starch Malabsorption

In rare cases, individuals can have a congenital sucrase-isomaltase deficiency, affecting their ability to digest sucrose and starch properly. This leads to starch malabsorption, with undigested starches fermenting in the colon and causing gastrointestinal distress.

Carbohydrate Intolerance vs. Food Allergy

It's important to distinguish between carbohydrate intolerance and a food allergy. An intolerance is a digestive issue caused by the body's inability to process a food or ingredient. A food allergy is an immune system response that can cause more severe, potentially life-threatening reactions. While both can cause digestive upset, the mechanisms are different. For example, lactose intolerance is a carbohydrate intolerance, whereas a milk protein allergy is a true food allergy. Symptoms of carbohydrate intolerance typically involve bloating, gas, and diarrhea, while allergies can involve hives, wheezing, and swelling.

Comparison of Common Carbohydrate Intolerances and Digestive Conditions


Condition	Primary Carbohydrate Culprit	Root Cause	Key Symptoms (often overlapping)
Lactose Intolerance	Lactose (dairy sugar)	Insufficient lactase enzyme production	Diarrhea, bloating, gas, abdominal pain
Fructose Malabsorption	Fructose (fruit sugar, HFCS)	Limited fructose absorption capacity in small intestine	Diarrhea, gas, bloating
IBS (Diarrhea-predominant)	FODMAPs (various)	Highly sensitive gut, motility issues	Diarrhea, abdominal pain, bloating triggered by FODMAPs
Celiac Disease	Gluten (in wheat, barley, rye)	Autoimmune reaction to gluten damaging small intestine	Chronic diarrhea, bloating, weight loss, nutrient malabsorption
Starch Malabsorption	Sucrose and/or Starch	Genetic enzyme deficiency (CSID)	Diarrhea, gas, bloating upon ingesting starch or sucrose

Diagnosis and Management of Carb-Related Diarrhea

If you suspect carbohydrates are causing your diarrhea, a healthcare provider can help determine the root cause. A common diagnostic tool is the hydrogen breath test, which measures hydrogen levels in your breath after consuming a specific sugar (e.g., lactose or fructose). Elevated hydrogen levels indicate that bacteria are fermenting undigested sugars in your colon.

Dietary Management

For many, dietary modification is the primary way to manage symptoms. This often involves an elimination diet to identify trigger foods. The low-FODMAP diet is a clinically proven approach for managing IBS and other functional gut disorders. It involves an elimination phase, systematic reintroduction of FODMAP groups, and personalization to create a long-term diet.

Other Strategies

Beyond elimination diets, other strategies can help manage carb-related diarrhea:

Enzyme Supplements: For conditions like lactose intolerance, taking a lactase enzyme supplement can help digest dairy products.
Probiotics: Specific probiotic strains may help improve digestion in some cases, particularly in those with IBS and carbohydrate intolerances.
Fiber Adjustment: Soluble fiber can help with some diarrhea by absorbing water, while insoluble fiber may worsen it. Adjusting fiber intake and drinking plenty of water is important.
Treating Underlying Conditions: Addressing conditions like celiac disease or diabetic enteropathy is necessary for symptom resolution.

Conclusion

It is clear that carbohydrates can cause diarrhea, but the connection is complex and depends on an individual's unique digestive makeup and any existing health conditions. From common intolerances like lactose and fructose malabsorption to conditions like IBS and celiac disease, the root cause is often the fermentation of poorly digested carbs in the large intestine. By consulting a healthcare professional, using diagnostic tools like the hydrogen breath test, and employing strategies like elimination diets, enzyme supplements, and gut health support, you can identify your triggers and find an effective path to relief.

For more information on the low-FODMAP diet and its management, consult authoritative sources like the Monash University Low FODMAP Diet website.

Frequently Asked Questions

Not all carbs are digested the same way. The ones that cause diarrhea are often poorly absorbed in the small intestine, like FODMAPs, and instead travel to the large intestine where they are fermented by bacteria, causing gas and water retention.

For some people, especially those with IBS-D, reducing the intake of fermentable carbohydrates (FODMAPs) can significantly improve symptoms like diarrhea. However, a very low-carb diet is not always necessary and should be managed under medical guidance to avoid nutritional deficiencies.

A carbohydrate intolerance is a digestive issue caused by an inability to properly digest certain carbohydrates, while a food allergy is an immune system response that can be more severe. Intolerances, like lactose intolerance, cause digestive symptoms, whereas allergies, like wheat allergy, can trigger hives, swelling, and breathing issues.

It depends on the type of fiber. Soluble fiber can be helpful as it absorbs water and helps firm up stools. Insoluble fiber, on the other hand, can speed up bowel movements and may worsen diarrhea. People with diarrhea-predominant IBS should focus on soluble fiber.

A hydrogen breath test is the most accurate way to diagnose these specific intolerances. A healthcare provider will have you consume a liquid containing the sugar, and if hydrogen is detected in your breath, it indicates malabsorption.

Yes, diabetes can cause nerve damage (neuropathy) that affects the digestive system, leading to altered gut motility and diarrhea. Additionally, some diabetes medications like metformin and artificial sweeteners can also trigger diarrhea.

The best approach is a personalized one, guided by a healthcare professional. A low-FODMAP diet is often recommended for initial management, followed by a systematic reintroduction of foods to identify specific triggers. For specific intolerances, like lactose, avoidance or enzyme supplements may be sufficient.