An individual can develop lactose intolerance at any age. In rare cases, it can be present from birth, but the symptoms generally emerge as a person ages, presenting in childhood, adolescence, or adulthood.

Lactose intolerance refers to difficulty digesting foods that contain lactose, such as dairy products. This can result in symptoms such as gas, bloating, and diarrhea. There are four different forms of lactose intolerance, all of which have different causes. Some types occur more frequently than others, and they also vary in the average age of onset.

In this article, we discuss the different types of lactose intolerance and when they may occur.

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Lactose intolerance is the term for a food intolerance to the sugar lactose, which occurs naturally in dairy products such as milk, cheese, and yogurt. This intolerance is typically due to lactose malabsorption, which is the inability to digest or absorb lactose properly.

The insufficient production of an enzyme known as lactase, which breaks down lactose, leads to this malabsorption. Undigested lactose passes into the colon, where bacteria break it down, creating the fluid and gas that are responsible for the digestive symptoms that people experience.

Evidence suggests that although the prevalence of lactose intolerance varies among different regions, about two-thirds of the world’s population experiences lactose malabsorption. However, not everyone with lactose malabsorption will develop symptoms of lactose intolerance.

Lactose intolerance can develop at any age. However, it tends to be uncommon in children under the age of 5 years. In most cases, the onset of symptoms occurs gradually due to lactase levels getting lower as a person ages. As lactase production decreases, a person may begin to notice worsening symptoms. Many cases of lactose intolerance first develop when a person is aged 20–40 years.

There are four main types of lactose intolerance that can occur at different ages. Some forms may develop due to a person’s genes, but not all causes are genetic.

Primary lactase deficiency

Also known as lactase nonpersistence, this is the most common type of lactose intolerance. It occurs due to decreasing activity, or expression, of the LCT gene that causes a gradual decrease in lactase. The decrease in lactase typically occurs after the age of 2 years.

Secondary lactase deficiency

This cause of lactose intolerance does not occur due to genetics. Instead, it results from a disease or injury that affects the small intestine and lactase production. It can develop at any age.

According to the American College of Gastroenterology, the most common cause of secondary lactase deficiency in infants and children is an infection of the intestinal tract that damages the lining. Other possible causes include trauma to the small intestine, health conditions such as Crohn’s disease or celiac disease, or medical interventions, such as surgery, radiation therapy, or certain medications.

Congenital lactase deficiency

People with this genetic form of lactose intolerance are born without the ability to produce lactase. This condition is very rare and can cause severe symptoms, such as failure to thrive and watery diarrhea.

Developmental lactase deficiency

This type of lactose intolerance may occur in infants born prematurely, usually before 34 weeks of gestation. Usually, it only lasts a short period. The symptoms typically resolve as the intestinal mucosa develops further and matures.

Lactose intolerance results from a deficiency in lactase. Without adequate levels of this enzyme, the body cannot digest food and drinks containing lactose. The causes of lactase deficiency include:

  • an inherited gene leading to a decrease in lactase or an inability to produce it
  • injury to the small intestine that affects lactase production
  • premature birth

Although anyone can develop lactose intolerance, it appears to be more prevalent in certain populations. The risk factors may include:

  • Family history: As primary lactase deficiency occurs due to an inherited gene, family history plays a role.
  • Race: According to the American College of Gastroenterology, about 85% of African American adults in the United States may have lactose intolerance. However, it is possible to have lactase malabsorption and not display symptoms of lactose intolerance.
  • Ethnic background: Lactose intolerance also appears to be more common in people of Hispanic, Latino, Asian, or Native American descent.

The symptoms of lactose intolerance may vary in severity from person to person. Additionally, how much lactose a person consumes can influence the symptoms. Some common symptoms include:

  • excess gas
  • bloating
  • diarrhea
  • abdominal cramping or pain
  • sudden urge to have a bowel movement

Doctors typically diagnose lactose intolerance based on a physical examination, during which they will check for abdominal tenderness, pain, and bloating. They will also take into account the person’s symptoms and medical history and the results of a few diagnostic tests. Tests to diagnose lactose intolerance may include:

  • Dietary elimination: This involves eliminating all foods and drinks containing lactose for a set period to see whether the symptoms resolve.
  • Hydrogen breath test: A healthcare professional will take breath samples from the person after they have consumed foods or beverages containing lactose. People who have lactose intolerance will exhale breath with higher levels of hydrogen. When the value of hydrogen increases above a certain value, doctors may confirm a diagnosis of lactose intolerance.
  • Milk tolerance test: This test involves measuring blood glucose levels before and after drinking 500 milliliters of milk. If blood sugar levels do not rise, this suggests that a person is not digesting lactose, which indicates lactose intolerance.

The primary way to manage lactose intolerance is to make dietary modifications. In the past, doctors recommended avoiding all products containing lactose. However, research suggests that most people can tolerate up to 12–15 grams of lactose per day without experiencing significant symptoms. Many lactose-free products, such as plant milks, are available to help people limit their consumption.

Other treatment options may include addressing the underlying condition causing intolerance, if applicable, or supplementing with lactase tablets, which contain lactase and help break down lactose.

Other digestive and bowel conditions can cause symptoms similar to those that occur with lactose intolerance. When symptoms of lactose intolerance develop, doctors may first rule out other conditions, such as:

  • Irritable bowel syndrome (IBS): IBS refers to changes in bowel function that can cause symptoms, such as stomach cramps, diarrhea, and constipation, without any visible signs of intestinal damage.
  • Celiac disease: This autoimmune condition occurs in some people when their immune system overreacts following the consumption of foods containing gluten. The symptoms may include diarrhea, nausea, and fatigue.
  • Inflammatory bowel disease (IBD): IBD typically refers to either Crohn’s disease or ulcerative colitis. These conditions may cause long-term intestinal inflammation. Either condition can lead to symptoms, including stomach pain, diarrhea, and nausea.

Lactose intolerance can develop at any age. It occurs due to low levels of the enzyme lactase, which result in difficulty digesting lactose, a common sugar in dairy products.

Lactose intolerance can be present from birth, but most cases develop gradually, typically presenting in adulthood. The most common management strategy involves limiting or avoiding lactose-containing foods and drinks.