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Symptoms & Causes of Lactose Intolerance

by Shilpi Singh
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Lactose intolerance refers to the inability of the body to fully digest or break down lactose- a sugar present in milk and milk products. This medical condition is also known as lactose malabsorption. It is usually harmless but its symptoms can be uncomfortable. It must not be confused with having an allergy to milk as milk allergies are caused by the response of the immune system to one or more proteins in the milk and milk products. Lactose intolerance typically appears in later life while milk allergy often shows up in newborns in the first year. Lactose intolerance can’t be treated as there is no such way to make the body produce more lactase. However, one can manage the symptoms of lactose intolerance by lifestyle modifications. This is possible by looking for those dairy products that produce fewer symptoms of lactose intolerance and also provide enough calcium and other essential nutrients. [1, 3]

What causes Lactose Intolerance?

Lactose intolerance is the result of the small intestine not producing sufficient lactase enzyme to digest all the lactose that one’s eats or drinks. The lactase enzyme functions in turning the milk sugar (lactose) into two simpler sugars (glucose and galactose) which further get mixed with the bloodstream after passing through the intestinal lining.

When the lactase enzyme is deficient, the undigested lactose does not go for processing and absorption instead it passes into the colon. Inside the colon, normal bacteria reside which interact with the undigested lactose and break it down to create gas and fluid. This results in symptoms and signs of lactose intolerance.

There are different kinds of lactose intolerance and each type has its causative factor that leads to lactase deficiency. These include:

  • Primary Lactose Intolerance- Initially, infants who need lactase to get nutrition from milk start life by producing a sufficient amount of lactase enzyme. But as they grow up, they start replacing their milk feed with other foods which normally drops the production of the lactase enzyme in their body. However, the lactase production is enough to digest the lactose present in dairy consumed in a typical adult diet. The lactase non-persistence by adulthood makes it difficult for people to digest milk and they are known to be having primary lactose intolerance. It is the most common type.
  • Secondary Lactose Intolerance- This happens when the small intestine is not able to produce enough lactase as it gets injured, has undergone surgery, or radiation therapy has been subjected to medications; or is affected by some illness. Lactose levels can be restored by treating the underlying condition of the small intestine.
  • Congenital or Developmental Lactose Intolerance- It can be possible but rarely seen that babies have lactose intolerance as their birth defect. This could be because the disorder where the person has lactase deficiency is inherited as an autosomal recessive trait. It means both the parents have the same gene variant for lactase deficiency which is passed onto their child. Premature infants are also found to suffer from lactose intolerance because of an insufficient lactase level in them. [1, 2]

Risk Factors of Lactose Intolerance

The factors that make an individual susceptible to lactose intolerance include:

  • Ethnicity- Most cases of lactose intolerance are reported in African Americans, Asian Americans, Native Americans, and Mexican Americans.
  • Advancing Age- Lactose intolerance is uncommon in infants and young children and is mostly seen in adulthood.
  • Diseases or Illnesses Associated with the Small Intestine- This includes celiac disease, bacterial overgrowth, intestinal infection, and Crohn’s disease.
  • Premature Birth- The babies born prematurely might have lower lactose levels because the small intestine develops the lactase-producing cells by the end of the third trimester and not before that.
  • Certain Cancer Treatments- For those who have stomach cancer, chemotherapy can increase their risk of lactose intolerance. The same goes for people who have intestinal complications from radiation therapies. [2]

Symptoms of Lactose Intolerance

People who have lactose intolerance, usually exhibit symptoms within a few hours of having milk, any dairy products, or any food or drinking item that contains lactose. These symptoms can be mild or severe depending upon how much lactose the body has taken up. The common signs and symptoms include:

  • Diarrhea
  • Stomach cramps
  • Bloating
  • Gas
  • Nausea
  • Vomiting
  • Pain in the abdomen
  • Stomach making rumbling or growling sounds.

If a person frequently experiences symptoms of lactose intolerance after having milk products and is worried about calcium deficiency, then he or shemust take an appointment with a general physician to get relief. [1, 2]

How do Doctors Check for Lactose Intolerance?

Doctors first discuss with their patients their medical records and family history. They then physically examine the patient. The patient may be asked to not consume milk or milk products for some time to check if the symptoms are getting better. The healthcare provider may also run some tests to diagnose lactose intolerance. These include-

  • Lactose tolerance test- It checks for how the digestive system absorbs lactose. After 8 hours of fasting, the patients are asked to drink milk. Their blood samples are taken after every 2 hours to check for blood glucose levels. If the levels don’t rise, the patients are reported as lactose intolerant.
  • Stool acidity test- This is done for paediatric patients to look for the presence of lactic acid, glucose, and other fatty acids in their stool sample. If these are found, this means they have undigested lactose meaning lactose intolerance.
  • Hydrogen breath test- The patients are asked to consume milk and then checked several times for hydrogen levels in their breath. High levels indicate lactose intolerance. [3]

Tips to Manage Lactose in The Diet

  • Slowly Start the Intake of Lactose-Containing Foods and Drinks. It’s advised to add small amounts of milk and milk products to the diet and check for the response of the body.
  • Consume Products that Naturally have Low Levels of Lactose. These include yogurt and hard cheeses.
  • Do not Take Milk and Milk Products Alone. It has been found that lactose intolerant people will experience fewer symptoms if they have milk and milk products with other foods. So, it is better to have milk with cereals instead of drinking only a glass of milk.
  • Buy Products that are Marked as Lactose-Free or Lactose-Reduced. Many food stores have milk and milk products available that are just like regular products but with added lactase enzyme.
  • Ask Healthcare Providers about the Intake of Lactase Products. Talk to the doctor about to whether have any lactase pills or lactase drops whenever eating or drinking milk products. [3]


  1. https://www.niddk.nih.gov/health-information/digestive-diseases/lactose-intolerance/symptoms-causes
  2. https://www.mayoclinic.org/diseases-conditions/lactose-intolerance/symptoms-causes/syc-20374232#:~:text=People%20with%20lactose%20intolerance
  3. %20are,its%20symptoms%20can%20be%20uncomfortable.
  4. https://www.hopkinsmedicine.org/health/conditions-and-diseases/lactose-intolerance

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