The Small Intestine and Lactose Intolerance

by Robert Tallitsch, PhD | January 29, 2024

Video explaining the Small Intestine and Lactose Intolerance with a patient example!

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Lactose intolerance occurs in individuals who lack a sufficient amount of the enzyme lactase, which is needed to break down lactose, the primary sugar in milk and other milk-containing food products. Lactose intolerance is not the same as having a food allergy to milk.

In this Brain Builder we will discuss

foods containing lactose,
the anatomy of the small intestine,
the source of the enzyme lactase,
the enzymatic breakdown and absorption of lactose,
the symptoms of lactose intolerance and lactose malabsorption, and
the diagnosis and treatment for lactose intolerance.

Lactose is the primary carbohydrate found in mammalian milk and several other food products, including

yogurt and cream,
bread and other baked goods, such as cookies, biscuits, pancakes, and cake,
processed meats, such as luncheon meats, hot dogs, bacon, and some sausages,
nondairy powdered and liquid coffee creamers, as well as nondairy whipped toppings, and
many processed foods, such as soups, margarine, salad dressings, flavored snack foods and chips.

Gross Anatomy of the Small Intestine

The function of the small intestine, which averages 20 feet (6 m) in length, is the absorption of nutrients from ingested materials. The small intestine is subdivided into three segments:

Duodenum
Jejunum
Ileum

The first and shortest segment of the small intestine is the duodenum. The duodenum is mostly located within the upper right quadrant of the abdomen. It is c-shaped, passing around the head of the pancreas from right to left, ending at approximately the level of the 2^nd lumbar vertebra.

The jejunum, which is mostly located within the left upper quadrant of the abdominal cavity, accounts for 2/5 of the remaining length of the small intestine.

In contrast, the ileum is mostly located within the right lower quadrant of the abdominal cavity and is the largest segment of the small intestine. Externally no clear line of demarcation marks the transition from the jejunum to the ileum. However, each segment does possess distinctive characteristics that are surgically important.

Microscopic Anatomy of the Small Intestine

The wall of the small intestine is composed of a series of layers. From superficial to deep these layers are termed the mucosa, lamina propria, muscularis mucosae, submucosa, muscularis externa, and serosa.

Examination of the interior of the small intestine (following a longitudinal incision) would yield the following:

The epithelium of the small intestine’s mucosa is composed of simple columnar epithelial cells possessing microvilli (also termed the brush border of the small intestine). These microvilli are microscopic folds to the apical surface of the epithelial surface cells.
A series of definite folds run parallel to each other and pass in circular or oblique directions on the inner surface of the intestine. These folds of the mucosa are termed plicae circulares.
The entire surface of the plicae and mucosa of the small intestine are covered with a series of finger-like projections termed villi.
The mucosa also contains numerous depressions termed the intestinal crypts (or crypts of Lieberkuhn), which open between the villi and extend as deep as the muscularis mucosae.
The muscularis mucosae is quite thin, and is composed of an inner layer of circular smooth muscle and an outer layer of longitudinal smooth muscle.
Deep to the muscularis mucosae is the submucosa, which is composed of irregularly arranged connective tissue.
The muscularis externa contains two layers of smooth muscle — an inner circularly arranged layer and an outer longitudinally arranged layer.
The deepest layer of the small intestine is a thin layer of connective tissue termed the serosa.

The microvilli, intestinal villi, and plicae circulares are modifications within small intestine that increase the surface area for secretion of enzymes and the absorption of nutrients.

The three regions of the small intestine (duodenum, jejunum, and ileum) exhibit subtle but important variations in their microscopic anatomy that aid in the identification of the various portions of the small intestine.

Source of Lactase, and the Enzymatic Breakdown and Absorption of Lactose

In order to digest lactose, the disaccharide must first be broken down into the monosaccharides glucose and galactose. This is accomplished by the enzyme lactase, which is produced by the microvilli of the mucosal epithelium of the small intestine. This enzymatic activity allows the uptake of glucose and galactose into the small intestine.

The Symptoms of Lactose Intolerance

Congenital lactose deficiency (also termed congenital alactasia, or congenital lactose intolerance) is an inherited condition in which infants are unable to break down the lactose in breast milk or infant formula. This results in severe diarrhea in the infant. If untreated the infant will become severely dehydrated and will not gain adequate weight.

Adult lactose intolerance (also termed lactase nonpersistence) develops after infancy, and is caused by an inadequate production of lactase by the microvilli of the mucosa of the small intestine. Symptoms, which includes diarrhea, flatulence, nausea, abdominal pain, and/or abdominal bloating, typically develop within 2 hours following the ingestion of lactose-containing dairy products. In severe cases symptoms can develop within as short as thirty minutes following the ingestion of lactose-containing food products.

Causes of Lactose Deficiency and Lactose Intolerance

Congenital Lactose Deficiency is a genetic deficiency in the gene required for the production of lactase. This genetic deficiency is believed to interfere with the production of and/or the function of the enzyme lactase.

Two factors contribute to the development of adult lactose intolerance. The most prominent is the gradual reduction in the production of the enzyme lactase by the microvilli of the intestinal surface epithelial cells. Only a small number of populations worldwide do not demonstrate this reduced production of lactase with increased age.

A second, and significantly less frequent cause, is an increased sensitivity of the mucosa of the small intestine to lactose. This increased sensitivity to lactose promotes flatulence and diarrhea, but to a lesser degree than that seen with adult lactose intolerance.

Diagnosis of Lactose Intolerance

Lactose intolerance is diagnosed by a complete physical and family medical history. Some of the tests that may be conducted in the diagnosis include the following:

Lactose tolerance test. In this test you will be asked to fast for eight hours. At the completion of the fast you will be asked to drink ten to twelve ounces of whole milk followed by blood tests over a two-hour period. The blood samples will be analyzed for blood glucose content. If blood glucose does not rise during that time period, you may be lactose intolerant.
Hydrogen breath test. Here you will be asked to fast for eight hours. At the completion of the fast you will be asked to drink ten to twelve ounces of whole milk followed by a breath analysis over a period of two hours. A high level of hydrogen in your breath may mean that you are lactose intolerant.
Stool acidity test. This test is typically conducted on infants and young children. With this test stool samples will be collected for twenty four or more hours following the ingestion of whole milk. If the individual is lactose intolerant the stools will contain higher than normal levels of lactic acid, glucose, and other fatty acids.

Treatment of Lactose Intolerance

The most common forms of treatment involve either the elimination of all lactose from one’s diet or the ingestion of lactase supplements during the ingestion of food. If all dairy products are eliminated from one’s diet, it is important to take vitamin D supplements to prevent vitamin D insufficiency.

Other individuals are able to manage their diet until they are able to determine how much lactose can be ingested without discomfort or other symptoms. This is typically accomplished by ingesting approximately twelve or fewer grams of lactose at one time, which is the equivalent of one cup of milk or one scoop of ice cream or less day.

Lactose intolerance is a natural variation in how your digestive tract digests lactose as you age. With careful monitoring and proper variations in one’s diet one can live a healthy life following the diagnosis of lactose intolerance.

Key Terms

Jejunum - The third, and largest segment of the small intestine.

Microvilli - Microscopic folds to the apical surface of the epithelial surface cells of the small intestine.

Congenital Lactose Deficiency - A genetic deficiency in the gene required for the production of lactase.

Lactase - The enzyme required to break down the disaccharide lactose into the monosaccharides glucose and galactose.

Stool acidity test - With this test (which is typically conducted on infants and small children) stool samples are collected for twenty four or more hours following the ingestion of whole milk. If the individual is lactose intolerant the stools will contain higher than normal levels of lactic acid, glucose, and other fatty acids.

Lactase nonpersistence - Another name for lactose intolerance. This develops after infancy, and is caused by an inadequate production of lactase by the microvilli of the mucosa of the small intestine.

Mucosa, lamina propria, muscularis mucosae, submucosa, muscularis, externa, serosa - The layers of the small intestine, as examined from superficial to deep

Adult lactose intolerance - This condition, which develops after infancy, is caused by an inadequate production of lactase by the microvilli of the mucosa of the small intestine.

Duodenum - The first and shortest segment of the small intestine is the duodenum. The duodenum is mostly located within the upper right quadrant of the abdomen. It is c-shaped, passing around the head of the pancreas from right to left, ending at approximately the level of the 2^nd lumbar vertebra.

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