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What causes lactose intolerance?

Lactose malabsorption is the reason for lactose intolerance. If you have lactose malabsorption, your small intestine makes low levels of lactase—the enzyme that breaks down lactose—and can’t digest all the lactose you eat or drink.

Lactose intolerance can develop at any age. Many cases first develop in people aged 20 to 40, although babies and young children can also be affected.

As we grow older, our ability to digest lactose decreases. With age, our intestines make less of the lactase (enzyme) that digest lactose, a sugar that contains in milk and dairy products. Studies has found that only 35% of people worldwide can digest lactose beyond the age of about 7 or 8.

There is a chance that your genes are the reason for lactose intolerance. They play a role in the following conditions, and these conditions can lead to low levels of lactase in your small intestine and lactose malabsorption:

  • Lactase nonpersistence. Body of people with lactase nonpersistence makes less lactase after infancy. Lactase levels get lower with age. Symptoms may not begin until later childhood, the teen years, or early adulthood. Lactase nonpersistence, also called primary lactase deficiency, is the most common cause of low lactase levels.
  • Congenital lactase deficiency. In this rare condition, the small intestine makes little or no lactase, starting at birth so infants with this condition do not produce any lactase. Unable to digest lactose, the infants have diarrhea from birth. This condition was deadly before the development of lactose-free infant formulas

There are also other reasons for lactose intolerance, like the following:

  • Injury to the small intestine. Infections, diseases, celiac disease, Crohn’s disease or other conditions that injure your small intestine. Treatments for these conditions may also injure your small intestine. Lactose intolerance caused by injury to the small intestine is called secondary lactose intolerance. If the cause of the injury is treated, you may be able to tolerate lactose again.
  • Premature birth. In premature babies, or babies born too soon, the small intestine may not make enough lactase for a short time after birth. The small intestine usually makes more lactase as the baby gets older.

In most cases lactose intolerance in adults are inherited and tend to be lifelong, but there are a lot of cases in young children that are often caused by an infection in the digestive system and may only last for a few weeks. While most infants can digest lactose, many people begin to develop lactose malabsorption—a reduced ability to digest lactose—after infancy. Experts estimate that about 65 % of the population has lactose malabsorption

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