Sugar intolerance is a common digestive issue occurring when the body cannot properly break down specific carbohydrates, resulting in malabsorption. Unlike a food allergy, which is an immune system reaction, intolerance involves the digestive tract’s inability to process sugar molecules for bloodstream absorption. Symptoms like bloating, gas, and diarrhea arise when undigested sugars travel to the large intestine, where gut bacteria ferment them.
The Essential Role of Digestive Enzymes
Before the body can use the energy from sugars, complex carbohydrates must be broken down into simpler forms. Disaccharides, made of two linked sugar units, must be split into single-unit monosaccharides by specialized digestive enzymes.
These enzymes, including lactase, sucrase, and maltase, are primarily located on the brush border of the small intestine. The brush border is a layer of microvilli that increases the surface area for nutrient absorption. For instance, lactase splits lactose, the disaccharide found in milk, into the monosaccharides glucose and galactose.
Once cleaved into single-unit components, these sugars are absorbed through the intestinal wall into the circulation. When a specific enzyme is deficient or non-functional, the target sugar remains intact and passes through the small intestine undigested. This failure to complete the digestive step directly causes sugar intolerance symptoms.
Primary Causes: Genetic and Developmental Deficiencies
Primary sugar intolerances stem from the body’s genetic programming, where the ability to produce a necessary enzyme either naturally declines or was never fully present. The most common example worldwide is lactase non-persistence, or adult-onset lactose intolerance. In this condition, lactase enzyme production naturally declines after infancy, affecting approximately two-thirds of the global population.
This decline is controlled by genetic factors, specifically changes near the LCT gene. This genetic change results in low levels of functional lactase in adulthood. The ability to maintain high lactase levels throughout life, known as lactase persistence, is a dominant trait that arose as an evolutionary adaptation in certain populations.
Other primary causes include rare genetic conditions, such as Congenital Sucrase-Isomaltase Deficiency (CSID). This inherited disorder is present from birth and involves mutations in the SI gene, which instructs the making of the sucrase-isomaltase enzyme. Individuals with CSID cannot effectively break down sucrose (table sugar) and maltose, causing severe gastrointestinal symptoms.
Secondary Causes: Intestinal Damage and Underlying Disease
Secondary sugar intolerance occurs when the enzyme-producing machinery in the small intestine is damaged by an external factor or underlying medical condition. This damage strips away the brush border where the enzymes reside, causing a temporary or long-term enzyme deficiency. Unlike primary causes, secondary intolerances are acquired.
Acute infections, such as viral or bacterial gastroenteritis, frequently cause secondary sugar intolerance. The inflammation and damage to the intestinal lining temporarily reduce the production of brush border enzymes, most commonly lactase. In these cases, the intolerance is often transient, with enzyme levels returning to normal once the infection resolves and the intestinal lining heals.
Chronic diseases causing persistent inflammation or structural changes also lead to secondary intolerance. Celiac disease, for instance, causes villous atrophy—a flattening of the intestinal lining—which directly destroys the enzyme-bearing brush border. Inflammatory Bowel Disease (IBD), including Crohn’s disease, and certain intestinal surgeries can similarly damage the lining. Treating the underlying disease is the primary method for restoring digestive enzyme function.
Pinpointing the Cause: Diagnostic Methods
Determining the specific cause of sugar intolerance requires diagnostic methods that identify the offending sugar and assess the health of the intestinal lining. A common, non-invasive tool is the hydrogen breath test, which measures hydrogen gas concentration in the patient’s exhaled breath. When undigested sugar reaches the large intestine, gut bacteria ferment it, producing hydrogen gas that is absorbed into the bloodstream and expelled through the lungs.
The patient consumes a measured dose of the suspect sugar (e.g., lactose, fructose, or sucrose) after fasting, and breath samples are collected regularly. A significant rise in breath hydrogen levels indicates the sugar was not properly digested in the small intestine. Physicians may also use elimination diets to pinpoint the carbohydrate triggering symptoms.
To identify secondary causes, a doctor may perform an endoscopy followed by a small intestine biopsy. This allows for a direct examination of the intestinal tissue to check for damage, such as the villous atrophy seen in Celiac disease. For suspected genetic deficiencies, specific genetic testing can check for mutations in genes like LCT or SI.