Fructan intolerance is a non-allergic sensitivity to a type of carbohydrate that causes gastrointestinal distress. Fructans are chains of fructose molecules classified as oligosaccharides, which fall under the larger umbrella of Fermentable Oligosaccharides, Disaccharides, Monosaccharides, and Polyols (FODMAPs). Common sources include wheat, barley, rye, onions, and garlic, as well as fruits and vegetables like artichokes and asparagus. The condition stems from a physiological inability to properly digest these compounds in the upper digestive tract.
Why Fructans Are Not Broken Down
The fundamental cause of fructan intolerance is a universal feature of human biology: the small intestine does not produce the specific enzyme needed for their hydrolysis. Unlike sucrose, which is easily broken down by the enzyme sucrase, fructans require a specialized enzyme called fructanase or inulinase. Since this enzyme is not naturally present in the human digestive system, fructans are considered largely indigestible.
When a person consumes fructan-rich foods, the carbohydrate chains pass through the small intestine mostly intact. Only about 5 to 15% of ingested fructans are broken down or absorbed before they exit the small intestine. This inability to break the specific bonds linking the fructose units is a normal characteristic of human digestive physiology, not a deficiency or a disease.
As this undigested matter moves past the small intestine, it enters the large intestine, or colon, where a massive population of gut microbes resides. These microbes are perfectly equipped to process the compounds that the human host cannot, setting the stage for the symptoms of intolerance.
How Undigested Fructans Cause Symptoms
When undigested fructans arrive in the large intestine, they become a rapid food source for resident gut bacteria. This process is known as fermentation, where microbes quickly consume the fructans and produce gaseous byproducts. The main gases generated are hydrogen, carbon dioxide, and sometimes methane, which accumulate rapidly within the colon.
The resulting increase in gas volume causes physical symptoms like bloating, abdominal distension, and flatulence. This rapid gas production can also lead to painful cramping and general digestive discomfort.
Osmotic Effects
Fructans are osmotically active molecules, meaning they draw water into the intestinal lumen. As the concentration of undigested fructans increases, they pull fluid from the body’s tissues into the bowel. This excess water accelerates the transit of bowel contents, often manifesting as diarrhea. These two mechanisms—fermentation and osmosis—work together to create the spectrum of gastrointestinal distress characteristic of fructan intolerance.
Fructan Intolerance vs. Allergies and Celiac Disease
Fructan intolerance is different from a food allergy or an autoimmune condition like Celiac Disease. A true food allergy involves an immediate immune system reaction to a food protein. Fructan intolerance, conversely, is a non-immune digestive process characterized by malabsorption.
Fructans are carbohydrates, not proteins, so they do not trigger the immune response or cause intestinal damage associated with Celiac Disease. Confusion often arises because many foods high in fructans, such as wheat, are also high in gluten.
Emerging research suggests that many people sensitive to gluten are actually reacting to the fructans in wheat. They experience relief on a gluten-free diet because it inadvertently eliminates their primary source of fructans. Fructan intolerance is a specific type of carbohydrate malabsorption that commonly affects people diagnosed with Irritable Bowel Syndrome (IBS).
Testing and Managing Fructan Intolerance
Diagnosis of fructan intolerance often begins with a physician suspecting carbohydrate malabsorption based on reported symptoms. One diagnostic tool is the hydrogen breath test, where a patient ingests a fructan solution, and the hydrogen gas produced by colonic fermentation is measured in the breath. An elevated hydrogen level, typically exceeding a rise of 20 parts per million above baseline, indicates malabsorption.
The most reliable method for identifying and managing fructan intolerance remains a structured dietary approach. This involves implementing a temporary low-FODMAP elimination diet, which strictly limits all high-FODMAP foods. After symptom relief, fructan-containing foods are systematically reintroduced, one at a time, to determine the individual’s specific tolerance threshold.
By identifying the exact dose and type of fructan a person can tolerate, a registered dietitian can help craft a sustainable diet. Targeted digestive enzyme supplements containing fructan hydrolase may also offer assistance by helping to break down the carbohydrate chains in the small intestine. Dietary modification focused on individual tolerance remains the cornerstone of effective management.