Lactose intolerance is a condition resulting from an inability to digest lactose (which is a carbohydrate present in milk and milk products).
This results in gastric symptoms including abdominal pain, bloating, diarrhea, nausea, etc.
Pathophysiology
Lactose intolerance occurs due to decreased levels or absence of enzyme lactase in the body.
Lactose (a disaccharide) can’t be directly absorbed through the wall of the small intestine into the bloodstream. It needs to be broken down into simple sugars (monosaccharides) – glucose and galactose (by the enzyme lactase) which are then absorbed within the duodenum (part of small intestine).
In case of deficiency of lactase, lactose passes intact into the colon (large intestine). This lactose gets fermented in the colon by normal bacterial flora producing large amounts of gas (a mixture of hydrogen, carbon dioxide, and methane) which is responsible for the various abdominal symptoms. Unprocessed sugars and fermentation products cause the osmotic pressure to rise within the colon resulting in an increased flow of water into the bowels causing diarrhea.
Types of Lactose Intolerance
There are three types of lactose intolerance.
Primary lactose intolerance
This is the most common type of lactose intolerance. Levels of enzyme lactase are normal in infancy. As the child grows, milk is replaced by other foods. With growing age, lactase production decreases making it difficult to digest milk and milk products in adulthood.
It is a genetically determined disorder, occurring in a large proportion of people of African, Asian or Hispanic origin. It is also commonly seen in people of Mediterranean or Southern European descent.
Secondary/Acquired lactose intolerance
It occurs when the production of lactase by small intestine decreases after an illness, injury or surgery involving the small intestine. It is associated with acute gastrointestinal parasites (such as giardia), celiac disease, bacterial overgrowth, Crohn’s disease, ulcerative colitis, chemotherapy, etc. It is usually reversible. Treatment of the underlying disorder is able to restore lactase levels bringing about improvement in signs and symptoms.
Congenital or developmental lactose intolerance
It is an extremely rare condition. It is a genetic disorder having an autosomal recessive inheritance. There is a complete absence of lactase (alactasia) right from birth. Since these babies can’t digest milk, they are not able to survive. Introduction of soybean-derived infant formulas and commercially available lactose-free dairy products in recent times have increased the chances of survival in such infants.
Premature infants may also have lactose intolerance because of an insufficient lactase level.
Epidemiology
Approximately 65% of the world’s population is estimated to have a reduced ability to digest lactose after infancy.
The incidence varies from region to region. It is most prevalent in people of East Asian descent, affecting more than 90 percent of adults in some of these communities. It is also prevalent in people belonging to West Africa, Arabs, Jewish, Greek, and Italian regions. It is relatively infrequently seen in North Americans and Northern Europeans.
Congenital lactase deficiency is an extremely rare condition. It is most commonly seen in Finland, where it affects an estimated 1 in 60,000 newborns.
Difference between Lactose intolerance and Milk allergy
The two conditions are entirely different.
Lactose intolerance is due to deficiency or absence of enzyme lactase resulting in an inability to digest milk and milk products which contain lactose.
Milk allergy is a type of allergy in which the immune system of the body over-reacts to ingestion of milk proteins. Milk allergy usually refers to cow’s milk, although allergy can be against other types of milk, including soy. It is most common in infants and children, however, it can develop at any age. It is the most common food allergy in children. With increasing age, most of the children outgrow milk allergy.
Signs and Symptoms
The signs and symptoms of lactose intolerance usually begin 30 minutes to two hours after consumption of foods that contain lactose. Common signs and symptoms include:
- Diarrhea
- Nausea, and vomiting
- Abdominal cramps and pain
- Bloating
- Flatulence (wind)
The severity of symptoms and duration after which they appear depend on the amount of lactose consumed and the level of lactase enzyme present in the body.
Diagnosis
Lactose intolerance is usually diagnosed on the basis of clinical signs and symptoms. Secondary causes leading to lactose intolerance should be investigated and ruled out.
The following tests may be carried out to confirm the diagnosis.
Lactose tolerance test
After overnight fasting, a sample of blood is withdrawn to test for the amount of glucose present in the fasting state. The patient is then asked to drink a solution containing a high level of lactose. After two hours, the level of glucose in the blood is again determined. If the glucose level doesn’t rise, it indicates that the body is unable to digest lactose.
Hydrogen breath test
After overnight fasting, the baseline level of hydrogen in the breath is measured. The patient is then asked to drink a solution of lactose. The amount of hydrogen in the breath is measured at regular intervals thereafter by clinical gas chromatography or compact solid-state detector. If the patient is unable to digest lactose, the bacteria present in the colon will ferment it producing hydrogen and other gases. These gases are absorbed by the intestine and exhaled out through the lungs. Abnormally large levels of hydrogen gas measured during a breath test indicate that the patient is not able to digest and absorb lactose.
Stool acidity test
It is used to diagnose lactose intolerance in infants, for whom other forms of testing cannot be carried out. The infant is given lactose to drink. If the child is unable to digest lactose, its fermentation will create lactic acid and other acids that can be detected in the stool sample. In patients of lactose intolerance pH of stool is less than 5.5.
Intestinal biopsy
It is not routinely performed to diagnose lactose intolerance. However, it can help to rule out secondary causes like celiac disease, Crohn’s disease, ulcerative colitis, etc.
Genetic testing
Genetic tests can be carried out to detect primary lactose intolerance. Molecular assays can be performed on DNA sample extracted and amplified from patient’s blood or saliva to look for polymorphisms associated with lactose intolerance.
Treatment
Secondary lactose intolerance can be cured by treating the underlying cause. In celiac disease, consumption of gluten-free diet brings about an improvement in symptoms of lactose intolerance after some time.
There is no definite cure for primary lactose intolerance. However dietary modifications can reduce the symptoms and discomfort associated with the disease.
Reduction of intake of dairy products
Patients suffering from lactose intolerance vary in the amount of lactose that their bodies can tolerate. Also, this tolerance varies over time. Most of the patients are able to consume a small amount of lactose in their diet without producing any significant symptoms. Moreover, regular consumption of a small amount of lactose in diet enables the body to adapt to it (colonic bacteria adaptation).
So complete avoidance of dairy products is not required in most of the patients.
Various dairy products differ in the amount of lactose present in them. The following milk products contain reduced levels of lactose and can be consumed safely by lactose intolerant patients.
- Clarified butter and ghee contain a negligible amount of lactose.
- Curd or yogurt: lactobacilli used in their production remove a varying amount of lactose. Besides, the bacteria present in these also produce lactase which helps to digest the ingested lactose.
- Cheese: fermentation and processing reduce the amount of lactose present in cheese. Different types of cheese have different amounts of lactose but it is usually less than the amount present in an equivalent quantity of milk.
Milk Substitutes / Non-dairy sources of calcium
In patients who are not able to tolerate even a small amount of milk or milk products, other substitutes can be consumed. These include soy milk, rice milk, almond milk, coconut milk, etc. which are completely lactose-free. Also, such patients are encouraged to have calcium-rich foods like broccoli, soybean, figs, tofu, and foods fortified with calcium.
Enzyme supplements
Enzyme lactase in the form of tablets may be taken along with a lactose-rich diet. It is also available in liquid form; a few drops of which may be added to the food containing lactose. These enzyme supplements have proved to be beneficial in a significant percentage of people though they are ineffective if a higher amount of lactose is ingested. They are produced commercially from fungi of the genus Aspergillus.
Probiotics
Probiotics in the form of yogurts or curd or commercially available supplements may aid in the digestion of lactose.
