What Causes Lactose Intolerance?


Lactose intolerance is the inability to break down a type of natural sugar called lactose. Lactose is commonly found in dairy products, such as milk and yogurt. A person becomes lactose intolerant when his or her small intestine stops making enough of the enzyme lactase to digest and break down the lactose. When this happens, the undigested lactose moves into the large intestine. The bacteria that are normally present in the large intestine interact with the undigested lactose and cause symptoms such as bloating, gas, and diarrhea. The condition may also be called lactase deficiency.

Lactose intolerance is very common in adults, particularly those with Asian, African, Native American, or Mediterranean ancestry. According to Mayo Clinic, nearly 30 million American people over the age of 20 are lactose intolerant. The condition isn’t serious but may be unpleasant.

Lactose intolerance usually causes gastrointestinal symptoms, such as gas, bloating, and diarrhea, about 30 minutes to two hours after ingesting milk or other dairy products containing lactose. People who are lactose intolerant may need to avoid eating these products or take medicines containing the lactase enzyme before doing so.

Types of Lactose Intolerance

There are three main types of lactose intolerance, each with different causes:

Primary Lactose Intolerance (Normal Result of Aging)

This is the most common type of lactose intolerance.

Most people are born with enough lactase. Babies need the enzyme in order to digest their mother’s milk. The amount of lactase a person makes may decrease over time. This is because as people age, they eat a more diverse diet and rely less on milk.

The decline in lactase is gradual. This type of lactose intolerance is more common in people with Asian, African, Native American, or Mediterranean ancestry.

Secondary Lactose Intolerance (Due to Illness or Injury)


Intestinal diseases such as celiac disease and inflammatory bowel disease or a surgery or injury to your small intestine can also cause lactose intolerance. Lactase levels may be restored if the underlying disorder is treated.

Congenital or Developmental Lactose Intolerance (Being Born with the Condition)

In very rare cases, lactose intolerance is inherited. A defective gene can be passed from the parents to a child, resulting in the complete absence of lactase in the child. This is referred to as congenital lactose intolerance.

In this case, your baby will be intolerant of breast milk. They will have diarrhea as soon as human milk or a formula containing lactose is introduced. If it’s not recognized and treated early on, the condition can be life-threatening. The diarrhea can cause dehydration and electrolyte loss. The condition can be treated easily by giving the baby a lactose-free infant formula instead of milk.

Developmental Lactose Intolerance

Occasionally, a type of lactose intolerance called developmental lactose intolerance occurs when a baby is born prematurely. This is because lactase production in the baby begins later in the pregnancy, after at least 34 weeks.

What Are the Symptoms of Lactose Intolerance?

The symptoms of lactose intolerance typically occur between 30 minutes and two hours after eating or drinking a milk or dairy product, and may include:

  • abdominal cramps
  • bloating
  • gas
  • diarrhea
  • nausea

The symptoms can range from mild to severe. The severity depends on how much lactose was consumed and how much lactase the person has actually made.


How Is Lactose Intolerance Diagnosed?

If you’re experiencing cramps, bloating, and diarrhea after drinking milk or eating and drinking milk products, your doctor may want to test you for lactose intolerance. Confirmatory tests measure lactase activity in the body. These tests include:

Lactose Intolerance Test

This blood test measures your body’s reaction to a liquid that contains high lactose levels.

Hydrogen Breath Test

This test measures the amount of hydrogen in your breath after consuming a drink high in lactose. If your body is unable to digest the lactose, the bacteria in your intestine will break it down instead. The process by which bacteria break down sugars like lactose is called fermentation. Fermentation releases hydrogen and other gases. These gases are absorbed and eventually exhaled. If you aren’t fully digesting lactose, the hydrogen breath test will show a higher than normal amount of hydrogen in your breath.

Stool Acidity Test

This test is more often done in infants and children. It measures the amount of lactic acid in a stool sample. Lactic acid accumulates when bacteria in the intestine ferment the undigested lactose.

How Is Lactose Intolerance Treated?

There’s currently no way to make your body produce more lactose. Treatment for lactose intolerance involves decreasing or completely removing milk products from the diet.

Many people who are lactose intolerant can still have up to 1/2 cup of milk without experiencing any symptoms. Lactose-free milk products can also be found at most supermarkets. And not all dairy products contain a lot of lactose. You may still be able to eat some hard cheeses, such as cheddar, Swiss, and Parmesan, or cultured milk products like yogurt. Low-fat or nonfat milk products typically have less lactose as well.

An over-the-counter lactase enzyme is available in capsule, pill, drops, or chewable form to take before consuming dairy products. The drops can also be added to a carton of milk.

People who are lactose intolerant and not consuming milk or dairy products may become deficient in calcium, vitamin D, riboflavin, and protein. Taking calcium supplements or eating foods that are either naturally high in calcium or are calcium-fortified is recommended.

The Long Term: Adjusting to a Lactose-Free Diet and Lifestyle

Symptoms will go away if milk and milk products are removed from the diet. Learn to read food labels carefully to detect ingredients that may contain lactose. Aside from milk and cream, look out for ingredients derived from milk, such as:

  • whey or whey protein concentrate
  • casein or caseinates
  • curds
  • cheese
  • butter
  • yogurt
  • margarine
  • dry milk solids or powder
  • nougat

Many foods that you would not expect to contain milk may actually contain milk and lactose. Examples include:

  • salad dressings
  • frozen waffles
  • non-kosher lunch meats
  • sauces
  • dry breakfast cereals
  • baking mixes
  • many instant soups
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