Carbohydrate intolerance

Carbohydrates are usually divided based on the degree of polymerization (how many simple sugar units are linked together): monosaccharides (single sugar units), disaccharides (two sugar units), oligosaccharides (3-10 sugar units) and polysaccharides (long chains of many sugar units).

Small intestine, where the carbohydrate malabsorption/maldigestion occurs.

Carbohydrate intolerance refers to adverse food reactions, specifically intolerance, to dietary carbohydrates, often caused by the lack or dysfunction of specific digestive enzymes. This, in turn, can be caused by acquired conditions or a genetic predisposition.

Common gastrointestinal symptoms include abdominal pain, cramping, bloating, flatulence, and diarrhea; other symptoms may include nausea, fatigue, and heartburn. These symptoms occur because the body is unable to digest certain carbohydrates, which then attract fluid into the intestines via osmosis, and produce gas via fermentation.

Terminology

Some clinical sources, including the Merck Manuals, limit the definition of "carbohydrate intolerance" specifically to malabsorption of disaccharides caused by enzyme deficiencies, sometimes also termed "maldigestion". Others use the term more broadly, referring to all adverse food reactions to dietary carbohydrates.

Types

Common examples include lactose intolerance (lack of enzyme lactase), fructose malabsorption, and sorbitol intolerance (poor intestinal absorption of sorbitol). Other types include rare genetic disorders, such as sucrase-isomaltase deficiency leading to sucrose intolerance, and glucose-galactose malabsorption.

Pathophysiology

Villi & microvilli of small intestine.

Villus of small intestine in more detail.

Carbohydrates are normally split into disaccharides, which are further broken down in the small intestine into monosaccharides by enzymes called disaccharidases (eg, lactase, maltase, isomaltase, sucrase [invertase]). These enzymes are located at the brush border of small intestine enterocytes.

In carbohydrate intolerance this normal function is impaired, leading to carbohydrate malabsorption/maldigestion and subsequent fermentation by gut microbiota in the large intestine. This fermentation produces gas (hydrogen, CO₂, methane) and short-chain fatty acids, producing symptoms such as bloating, gas and diarrhea. Undigested carbohydrates also produce an osmotic load that attracts water and electrolytes into the intestine, causing watery diarrhea.

Diagnosis and management

Diagnosis includes hydrogen breath tests, tolerance tests, biopsies, and sometimes genetic testing, as well as a careful clinical history and symptom assessment. The main aim is to find the specific carbohydrate that causes malabsorption or maldigestion. The treatment focuses on the limiting of this carbohydrate, usually by using specific diet that avoids or reduces certain foods. Enzyme supplements (like lactase for lactose intolerance) can also help. For conditions such as prediabetes, type 2 diabetes, and obesity that are associated with carbohydrate intolerance, lifestyle changes like a low-carb diet, exercise, and stress management can result in significant improvements.