Lactose intolerance is the inability to digest the main sugar found in milk and other dairy products. This is caused by a deficiency of lactase, the enzyme responsible for metabolizing lactose in the small intestines, according to the National Institutes of Health (NIH).

The prevalence of lactose intolerance in adults varies from less than 5 percent to almost 100 percent among different populations, according to research published in the Scandinavian Journal of Gastroenterology. The lowest prevalence is in northwestern Europe, around the North Sea, and the highest prevalence is in Asians and American Indians. About 30 million American adults are somewhat lactose intolerant by the age of 20, according to the NIH.

People can acquire lactose intolerance at any point in their life, and some people develop it over time, said Dr. Sophie Balzora, a gastroenterologist at NYU Langone Medical Center in New York City.

Lactose intolerance is different from having a milk allergy, since the latter is a reaction to the proteins in milk rather than lactose. It's also not like celiac disease, an autoimmune disease caused by gluten, which can have detrimental effects if ingested.

Symptoms and causes

People can be genetically predisposed not to produce the lactase enzyme, or the condition can result from illness or injury to the small intestine, including surgery or infections, according to the NIH.

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In lactose-intolerant adults, the lactose is fermented and metabolized by bacteria in the colon to produce gas and short chain fatty acids. This results in abdominal cramps, bloating, diarrhea, flatulence and nausea, Balzora said. The severity of symptoms largely depends on how quickly the lactase available in the digestive system is used up.

Although reduced levels of lactase could result in improper absorption of lactose, only people with low lactase levels who exhibit the common symptoms would be properly considered lactose intolerant. According to the Mayo Clinic, most people with lactase deficiencies do not display any signs or symptoms.

Premature babies can also be intolerant to breast milk, but full-term babies don't show signs of the problem before the age of 2, according to the NIH. The intolerance can develop earlier in African American children than in Caucasian ones.

Diagnosis & tests

Lactose intolerance should be suspected in people with abdominal symptoms — such as cramps and bloating — after consuming milk and other dairy products. The symptoms usually appear 30 minutes to two hours after ingesting a milk product.

The initial diagnosis of lactose intolerance can be very simple.

"The quick and dirty way is to have a patient avoid lactose products for a certain amount of time," usually about two weeks, Balzora said. Then, these foods can be gradually reintroduced into the diet again, and if the symptoms return, the person is likely somewhat lactose intolerant, she said.

Most patients do not need a referral to a specialist, or diagnostic laboratory tests. However, the symptoms of lactose intolerance can overlap with other gastrointestinal problems, such as irritable bowel syndrome and Crohn’s disease. A hydrogen breath test is an objective, non-invasive, inexpensive and easy-to-perform test that can be used to confirm an initial diagnosis of lactose intolerance. A properly administered breath test can help patients determine whether they need to cut back on milk and dairy products, according to research published in the World Journal of Gastroenterology.

Treatment & medication

Cutting lactose out of the diet is an option, but patients should make sure they aren't depriving themselves of calcium and vitamin D, Balzora said.

A study published in the June 2017 issue of the Journal of Nutrition found that those with lactose intolerance that cut milk out of their diet have lower levels of vitamin D in their blood. This study of 1,495 Canadian men and women also found that those who cut out milk were also shorter.

Over-the-counter pills or drops that contain lactase can be taken before meals to help alleviate or eliminate symptoms. For example, Lactaid pills or Lactaid milk allow many people to process dairy products without any difficulty, Balzora said. Some people find that taking probiotics can help them digest lactase better, but Lactaid is really the standard, Balzora said.

However, according to the Mayo Clinic, these products do not help all patients. Adults who are lactose intolerant can ultimately recondition their digestive system to tolerate up to 8.5 fluid oz. (250 milliliters) of milk — about a glass — if they drink the milk in gradually increasing amounts. According to a 21-day intervention study conducted in 2000, most people who do this will experience minimal or no discomfort.

A 2017 study by scientists at the North Carolina School of Medicine and North Carolina State University found that probiotics may also be a useful treatment. The study, published in the journal Proceedings of the National Academy of Sciences, found that 70 percent of those that took prebiotics for 35 days had reduced abdominal pain when they resumed drinking milk. Ninety percent of the subjects showed a significant increase in lactose fermenting bacteria, as well.

Coping strategies

Lactose intolerance can be treated with simple dietary changes. The most straightforward way is for a person to reduce the amount of milk or daily products in his or her diet. Also, it may help to divide daily milk and dairy products into several small portions and consume them with other foods. Processed dairy such as yogurt and cheeses are usually better tolerated, because the lactose has been partially metabolized by bacteria during their preparation.

Foods high in lactose, according to The Cleveland Clinic, are:

Milk, milkshakes and other milk-based beverages

Foods made with milk

Whipping cream and coffee creamer

Cheese

Ice cream, ice milk, sherbet

Puddings, custards

Butter

Cream soups, cream sauces

There are many products on the market that are lactose-free. This is a good option for those that don't want to give up their favorite milk products. There are more options on the way. The lactose-free food market is predicted to grow 11.10 percent between 2017 and 2021.

There are also other options, such as rice, soy and almond milk, that can be used as an alternative to cow's milk. Additionally, there are some milk products that can be easier to digest. According to the NIH, they include:

Buttermilk and cheeses

Fermented milk products, like yogurt

Goat's milk

Ice cream

Milkshakes

Aged or hard cheeses

Additional reporting by Alina Bradford, Live Science Contributor.

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