Carbohydrate intolerance


Carbohydrate intolerance is the inability of the small intestine to completely process the nutrient carbohydrate (a classification that includes sugars and starches) into a source of energy for the body. This is usually due to deficiency of an enzyme needed for digestion. Lactose intolerance is the inability to digest the sugar found in milk.


Carbohydrates are the primary source of energy and, along with fats and proteins, one of the three major nutrients in the human diet. Carbohydrates are classified according to their structure, based on the number of basic sugar, or saccharide, units they contain.

A monosaccharide, called a simple sugar, is the simplest carbohydrate. Simple sugars include glucose (the form in which sugar circulates in the blood), fructose (found in fruit and honey), and galactose (produced by the digestion of milk). These simple sugars are important because they can be absorbed by the small intestine.

Two simple sugars linked together form a disaccharide. Disaccharide sugars present in the diet are maltose (a product of the digestion of starch), sucrose (table sugar), and lactose (the sugar in milk). These disaccharides must be broken down by enzymes into two simple sugars so that they can be absorbed by the intestine.

Polysaccharides are much more complex carbohydrates made up of many simple sugars. The most important polysaccharides are glycogen, which is stored in the liver, and cellulose (starch).

Digestion of sugars

Digestion of food begins in the mouth, moves on to the stomach, and then into the small intestine. Along the way, specific enzymes are needed to process different types of sugars. An enzyme is a substance that acts as a catalyst to produce chemical changes without being changed itself. The enzymes lactase, maltase, and isomaltase (or sucrase) are needed to break down the disaccharides; when one or more is inadequate, the result is carbohydrate intolerance.

Types of intolerance

Carbohydrate intolerance can be congenital, primary, or secondary. Congenital deficiency is caused by an enzyme defect present at birth. Alactasia is a very rare congenital condition and the result of a genetic defect that causes the complete absence of lactase, the enzyme needed to digest milk sugar. Primary deficiency is caused by an enzyme defect developed over time. The most common is lactose intolerance. Secondary deficiencies, often caused by a disease or disorder of the intestinal tract, disappear when the underlying cause is treated. Secondary deficiencies include protein deficiency, pancreatitis, celiac disease , short-bowel syndrome, and some intestinal infections. Chronic renal failure and certain medications also can cause secondary deficiencies.


Lactose intolerance is widespread, affecting about 20 percent of American children and up to 70 percent of the world's adult population. Lactose intolerance is the most common of all enzyme deficiencies, and an estimated 30–50 million Americans have this condition. Some racial and ethnic populations are affected more than others. Lactose intolerance is found in as many as 75 percent of African Americans, Jewish Americans, Mexican Americans, and Native Americans, and in 90 percent or more of Asian Americans and some Mediterranean peoples. Descendants of Northern Europe usually do not develop the condition (incidence is less than 20 percent in these populations). Deficiencies in enzymes other than lactase are extremely rare.

Causes and symptoms


Enzymes play an important role in breaking down carbohydrates into forms that can pass through the intestine and be used by the body. Usually they are named by adding ase to the name of the substance they act on (for example, lactase is the enzyme needed to process lactose). In the mouth, cooked starch is broken down to a disaccharide by amylase, an enzyme in the saliva. The disaccharides maltose, sucrose, and lactose cannot be absorbed until they have been separated into simple sugar molecules by their corresponding enzymes present in the cells lining the intestinal tract. If this process is not completed, digestion is interrupted.

Although not common, a deficiency in the enzymes needed to digest lactose, maltose, and sucrose is sometimes present at birth. Intestinal lactase enzymes usually decrease naturally with age, but this occurs at varying degrees. Because of the uneven distribution of enzyme deficiency based on race and ethnic heritage, especially in lactose intolerance, genetics are believed to play a role in the cause of primary carbohydrate intolerance.

Digestive diseases such as celiac disease and tropical sprue (which affect absorption in the intestine), as well as intestinal infections and injuries, can reduce the amount of enzymes produced. In cancer patients, treatment with radiation therapy or chemotherapy may affect the cells in the intestine that normally secrete lactase, leading to intolerance.


The severity of the symptoms depends upon the extent of the enzyme deficiency, and ranges from a feeling of mild bloating to severe diarrhea . In the case of a lactase deficiency, undigested milk sugar remains in the intestine, which is then fermented by the normal intestinal bacteria. These bacteria produce gas, cramping, bloating, a "gurgly" feeling in the abdomen, and flatulence. In a growing child, the main symptoms are diarrhea and a failure to gain weight. Lactase deficiency causes gastrointestinal distress to begin about 30 minutes to two hours after eating or drinking foods containing lactose.

Food intolerances can be confused with food allergies , since the symptoms of nausea , cramps, bloating, and diarrhea are similar. Food intolerances involve an exaggerated or abnormal physical reaction to a food or food additive, and are not associated with an immune reaction. Food allergies involve an immune reaction.

Sugars that are not broken down into one of the simplest forms cause the body to push fluid into the intestines, which results in watery diarrhea (osmotic diarrhea). The diarrhea may sweep other nutrients out of the intestine before they can be absorbed, causing malnutrition .

When to call the doctor

If a child has the following symptoms, the parent should contact the child's pediatrician or gastroenterologist:

  • abdominal pain, vomiting , or diarrhea that awakens the child during the night
  • persistent or severe abdominal pain or diarrhea
  • unexplained weight loss
  • rectal bleeding
  • blood or mucus in stools
  • fever


The doctor may recommend a lactose-free diet for two or three weeks to determine if lactose intolerance is causing the symptoms. During the lactose-free period, the child should avoid all products containing lactose. The parent and child are asked to record the intake of all foods and beverages and note when symptoms occur after eating or drinking.

To identify other problem-causing foods or beverages, it is helpful for the parent and child to keep a diary of symptoms for two or three weeks. The doctor can then review the diary with the parent and child to identify possible problem foods.

The diagnosis of carbohydrate or lactose intolerance is supported by the presence of symptoms related to the condition. In addition, the primary pediatrician or gastroenterologist may confirm the diagnosis after questioning the child (if old enough to provide an accurate history of symptoms) or parent about his or her physical health, performing a physical examination, and ordering laboratory tests to rule out other conditions that resemble carbohydrate intolerance.

When carbohydrate intolerance is suspected, the diagnosis can be confirmed using oral tolerance tests. The carbohydrate being investigated is given by mouth in liquid form. Several blood levels are measured and compared to normal values. This helps evaluate the individual's ability to digest the sugar.

To identify lactose intolerance in children and adults, the hydrogen breath test is used to measure the amount of hydrogen in the breath. The patient drinks a beverage containing lactose and the breath is analyzed at regular intervals. If undigested lactose in the large intestine (colon) is fermented by bacteria, various gases are produced. Hydrogen is absorbed from the intestines and carried by the bloodstream into the lungs, where it is exhaled. Normally, there is very little hydrogen detectable in the breath; therefore, its presence indicates faulty digestion of lactose.

When lactose intolerance is suspected in infants and young children, many pediatricians recommend simply changing from cow's milk to soy formula and watching for improvement. If needed, a stool sample can be tested for acidity. The inadequate digestion of lactose will result in an increase of acid in the waste matter excreted by the bowels and the presence of glucose.


Carbohydrate intolerance caused by temporary intestinal diseases disappears when the condition is successfully treated. In primary conditions, no treatment exists to improve the body's ability to produce the enzymes, but symptoms can be controlled by diet.

An over-the-counter product marketed by the brand name Beano contains the enzyme alpha-galactosidase that works with the body's digestive system to break down complex carbohydrates into simple sugars that are easily digested. Beano is taken just before consuming gas-producing foods.

Nutritional concerns

Because there is wide variance in the degree of lactose intolerance, treatment should be tailored for the individual. Milk products should be avoided in young children who have signs of lactose intolerance. The child's doctor or a registered dietitian can help in making dietary adjustments and can advise when to start gradually reintroducing milk products, if applicable.

In infants, switching to soy-based formula may help. Special formulas, such as a glucose polymer-based formula, or a casein-based formula, may be recommended in infants with severe carbohydrate intolerance or when symptoms are severe.

Older children can adjust their intake of lactose, depending on how much and what they can tolerate. For some, a small glass of milk will not cause problems, while others may be able to handle ice cream or aged cheeses such as cheddar or Swiss, but not other dairy products. Generally, small amounts of lactose-containing foods eaten throughout the day are better tolerated than a large amount consumed all at once.

For those who are sensitive to even very small amounts of lactose, the lactase enzyme supplement is available without a prescription. The supplement is available in liquid form for use with milk. The addition of a few drops to a quart of milk will reduce the lactose content by 70 percent after 24 hours in the refrigerator. Heating the milk speeds the process, and doubling the amount of lactase liquid will result in milk that is 90 percent lactose free. Chewable lactase enzyme tablets are also available. Three to six tablets taken before a meal or snack will aid in the digestion of solid foods. Lactose-reduced milk and other products are also available in stores. Lactose-reduced milk contains the same nutrients as regular milk.

Because dairy products are an important source of calcium, people who reduce or severely limit their intake of these foods and beverages may need to consider other ways to consume an adequate amount of calcium. Taking calcium supplements or choosing other foods high in calcium may be needed to meet the recommended daily requirement of calcium. In addition, foods high in vitamin A, riboflavin, and vitamin B 12 should be included in the daily diet to compensate for the nutrients normally found in cow's milk.

Alternative treatment

Alternative and complementary therapies include approaches that are considered to be outside the mainstream of traditional health care. The list of alternative treatments for carbohydrate intolerance includes aromatherapy, homeopathy, hydrotherapy, juice therapy, acupuncture, chiropractic, osteopathy, naturopathic medicine, and Chinese traditional herbal medicine.

Before learning or practicing any particular technique, it is important for the parent or caregiver and child to learn about the therapy, its safety and effectiveness, potential side effects, and the expertise and qualifications of the practitioner. Although some practices are beneficial, others may be harmful to certain patients.

Relaxation techniques and dietary supplements should not be used as a substitute for medical therapies prescribed by a doctor. Parents should discuss the alternative treatments with the child's doctor to determine the techniques and remedies that may be beneficial for the child.


Carbohydrate intolerance has a very low mortality rate. Newborns and infants have a higher risk of chronic diarrhea and malnutrition from carbohydrate intolerance. With good dietary management, children with carbohydrate intolerance can lead normal lives.


Since the cause of the enzyme deficiency leading to carbohydrate intolerance is unknown, there is no way to prevent this condition.

Nutritional concerns

To help prevent or decrease the child's symptoms, parents can:


Alactasia —A rare inherited condition causing the lack of the enzyme needed to digest milk sugar.

Celiac disease —A disease, occurring in both children and adults, which is caused by a sensitivity to gluten, a protein found in grains. It results in chronic inflammation and shrinkage of the lining of the small intestine. Also called gluten enteropathy or nontropical sprue.

Cellulose —The primary substance composing the cell walls or fibers of all plant tissues.

Constipation —Difficult bowel movements caused by the infrequent production of hard stools.

Defecation —The act of having a bowel movement or the passage of feces through the anus.

Diarrhea —A loose, watery stool.

Digestion —The mechanical, chemical, and enzymatic process in which food is converted into the substances suitable for use by the body.

Enzyme —A protein that catalyzes a biochemical reaction without changing its own structure or function.

Feces —The solid waste, also called stool, that is left after food is digested. Feces form in the intestines and pass out of the body through the anus.

Gastroenterologist —A physician who specializes in diseases of the digestive system.

Hydrogen breath test —A test used to determine if a person is lactose intolerant or if abnormal bacteria are present in the colon.

Lactose —A sugar found in milk and milk products.

Metabolism —The sum of all chemical reactions that occur in the body resulting in growth, transformation of foodstuffs into energy, waste elimination, and other bodily functions. These include processes that break down substances to yield energy and processes that build up other substances necessary for life.

Nutrient —Substances in food that supply the body with the elements needed for metabolism. Examples of nutrients are vitamins, minerals, carbohydrates, fats, and proteins.

Peristalsis —Slow, rhythmic contractions of the muscles in a tubular organ, such as the intestines, that move the contents along.

Sugars —Those carbohydrates having the general composition of one part carbon, two parts hydrogen, and one part oxygen.

  • help the child identify and avoid problematic foods
  • work with a registered dietitian to facilitate specific dietary changes
  • incorporate changes in the child's diet gradually, giving his or her body time to adjust
  • establish set times for meals, and not permit the child to skip a meal
  • encourage the child to drink at least eight glasses of water per day
  • encourage the child to eat more slowly
  • offer smaller, more frequent meals

Parental concerns

Parents should reinforce with the child that carbohydrate intolerance is not a life-threatening condition and that dietary changes can help reduce symptoms. They should remind the child that a few months may be needed before he or she notices substantial improvement in symptoms.

See also Lactose intolerance .



Macdonald, Ian. "Carbohydrates." In Modern Nutrition in Health and Disease, 9th ed. Maurice E. Shils, ed., et al. Philadelphia: Lippincott Williams & Wilkins, 1998.

Williams, Sue Rodwellm, and Eleanor Schlenker. Essentials of Nutrition and Diet Therapy, 8th ed. Philadelphia: C.V. Mosby, 2002.


American College of Gastroenterology (ACG). P.O. Box 3099, Alexandria, VA 22302. (703) 820-7400. Web site:

American Gastroenterological Association. 4930 Del Ray Ave., Bethesda, MD 20814. (301) 654-2055. Web site:

International Foundation for Functional Gastrointestinal Disorders (IFFGD). P.O. Box 170864, Milwaukee, WI 53217-8076. (888) 964-2001. Web site:

National Digestive Diseases Information Clearinghouse (NDDIC). 2 Information Way, Bethesda, MD 20892-3570. (800) 891-5389. Web site:

Karen Ericson, R.N.

Angela M. Costello

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