Carbohydrate intolerance describes the body’s impaired ability to properly break down or absorb certain carbohydrates. This condition can lead to various uncomfortable digestive symptoms such as bloating, gas, abdominal pain, and diarrhea after consuming carbohydrate-rich foods. It is not a food allergy, but rather a digestive issue stemming from problems within the digestive process itself.
How Carbohydrates Are Digested
Carbohydrate digestion begins in the mouth, where chewing mechanically breaks down food and salivary amylase starts chemically breaking down starches. This process pauses in the stomach, where the acidic environment inactivates salivary amylase. The food mixture, now called chyme, then moves into the small intestine.
In the small intestine, pancreatic amylase resumes starch breakdown. The small intestine lining then produces specialized enzymes called disaccharidases, including lactase, sucrase, and maltase. These enzymes further break down two-sugar units (disaccharides) into simpler single sugar units (monosaccharides).
For example, lactase breaks down lactose into glucose and galactose, while sucrase breaks down sucrose into glucose and fructose. Once carbohydrates are reduced to monosaccharides like glucose, fructose, and galactose, they are ready for absorption. These single sugars are absorbed across the small intestine’s wall into the bloodstream. Glucose and galactose are transported into intestinal cells and then move into the blood, while fructose is absorbed primarily through facilitated diffusion.
When Digestive Enzymes Are Insufficient
One common cause of carbohydrate intolerance involves an insufficient supply or reduced activity of specific digestive enzymes. When these enzymes are lacking, carbohydrates cannot be properly broken down. Undigested carbohydrates travel to the large intestine, where gut bacteria ferment them, producing gas and other byproducts that cause symptoms like bloating, gas, and diarrhea.
Lactase deficiency, which causes lactose intolerance, is a prevalent example, affecting up to 70% of the world’s adult population. Lactase is the enzyme responsible for breaking down lactose, the sugar found in milk and dairy products. Primary lactase deficiency, the most common type, involves a natural decline in lactase production after childhood. This decline is often genetically determined and varies among ethnic groups, being more common in people of African, Asian, Hispanic, and American Indian descent.
Secondary lactase deficiency results from damage to the small intestine’s lining, which temporarily reduces lactase production. Conditions such as acute gastroenteritis, celiac disease, inflammatory bowel disease (Crohn’s disease and ulcerative colitis), and certain medications can cause this damage. Developmental lactase deficiency can occur in premature infants, and congenital lactase deficiency is a rare genetic disorder where babies are born with a severe lack of lactase.
Another enzyme insufficiency is congenital sucrase-isomaltase deficiency (CSID), a rare inherited disorder where the enzymes sucrase and isomaltase are deficient or absent. This deficiency impairs the breakdown of sucrose (table sugar) and certain starches. Symptoms, including watery diarrhea, abdominal pain, and excessive gas, typically appear when infants begin consuming foods containing sucrose or starch, such as fruit juices and grains. Some individuals with CSID may see an improvement in symptoms with age.
When Carbohydrate Absorption Goes Wrong
Beyond enzyme deficiencies, carbohydrate intolerance can also arise when the body struggles to absorb carbohydrates, even if they have been properly broken down into monosaccharides. Problems with these transport mechanisms can lead to malabsorption, meaning the sugars remain in the intestine instead of being used by the body.
Fructose malabsorption is a common example of an absorption issue, where the small intestine has difficulty absorbing fructose. Fructose is a monosaccharide found naturally in fruits, some vegetables, and honey, and is also added to many processed foods as a sweetener. While healthy individuals can absorb a certain amount of fructose per sitting, typically between 25-50 grams, those with malabsorption absorb less.
Impaired absorption is linked to deficient fructose carriers in the small intestine’s lining. When unabsorbed fructose reaches the large intestine, gut bacteria ferment it, producing gases and other byproducts. This fermentation process, combined with the osmotic effect of unabsorbed fructose drawing water into the bowel, leads to symptoms such as abdominal pain, bloating, and diarrhea. The severity of symptoms can vary widely depending on the amount of fructose consumed and an individual’s tolerance.
Impact of Gut Health and Other Conditions
Various underlying health conditions and overall gut health significantly influence carbohydrate digestion and absorption, contributing to intolerance. Inflammatory bowel diseases (IBD), such as Crohn’s disease and ulcerative colitis, can damage the intestinal lining through inflammation. This damage impairs the small intestine’s ability to produce digestive enzymes and absorb nutrients, including carbohydrates. The resulting malabsorption can lead to symptoms similar to those of carbohydrate intolerance.
Celiac disease, an autoimmune disorder triggered by gluten, causes damage to the small intestine’s villi, compromising nutrient absorption. Even after adopting a gluten-free diet, some individuals with celiac disease may initially experience sensitivity to carbohydrates until the intestinal lining fully heals.
Small intestinal bacterial overgrowth (SIBO) occurs when there is an excessive amount of bacteria in the small intestine. These overgrown bacteria ferment carbohydrates prematurely, producing gases that cause bloating, pain, and altered bowel habits like diarrhea or constipation. SIBO can also damage the brush border enzymes in the small intestine, further contributing to carbohydrate maldigestion.
Acute gastroenteritis, an infection that inflames the lining of the stomach and intestines, can temporarily reduce digestive enzymes and impair nutrient absorption. While symptoms are usually temporary, a persistent infection can lead to malabsorption syndrome. These diverse conditions highlight the interconnectedness of gut health with the body’s ability to process carbohydrates effectively.