What Is Lactose Intolerance?

Only humans have the ability to digest lactose as adults. Lactose is a sugar found in milk. Lactose intolerance is the inability to fully digest lactose, resulting in gastrointestinal issues about 30 minutes to 2 hours after consuming dairy products.

Symptoms of lactose intolerance include:

• Abdominal bloating
• Stomach rumbling (borborygmus)
• Abdominal pain or cramps
• Diarrhea
• Gas, or flatulence
• Nausea, sometimes with vomiting

Importantly, lactose intolerance is not the same thing as a milk allergy, which occurs when your immune system overreacts to the proteins in milk. If you have a milk allergy, you will likely immediately experience typical allergy symptoms after consuming milk, including hives, wheezing, and vomiting.

How Lactose Intolerance Develops

When you consume food or drink containing lactose, an enzyme in the small intestine called lactase helps you digest the milk sugar via hydrolysis, or the chemical splitting of molecules with water. In effect, lactase catalyzes the hydrolysis of lactose, producing two simpler sugars: glucose and galactose, which are then absorbed into the bloodstream.

Many people have a lactase deficiency, or hypolactasia, in which their small intestine produces low levels of lactase. This deficiency may lead to lactose malabsorption, wherein the undigested lactose makes its way into the large intestine and colon. Bacteria in the colon break down the lactose, resulting in increased gas and fluid in colon. You are lactose intolerant if your lactase deficiency or lactose malabsorption causes digestive symptoms (though this doesn’t always happen). The amount of lactose required to cause symptoms varies between people, and most people are able to consume small amounts of lactose without issues, according to the National Institute of Diabetes and Digestive and Kidney Diseases.

Types of Lactose Intolerance

There are several types of lactase deficiencies that lead to lactose intolerance. Congenital lactase deficiency is an extremely rare inherited disorder, in which the small intestine produces little to no lactase from birth. It's caused by a mutation of the LCT gene, which provides instructions for making lactase. You can only be born with this disorder if both of your parents pass the mutated gene on to you. 

The most common type of lactase deficiency is primary lactase deficiency, or lactase nonpersistence, and it occurs when lactase production slowly declines as you age, generally beginning after the age of 2. Lactase nonpersistence develops when the activity, or expression, of the LCT gene decreases over time (its expression is controlled by a regulatory DNA sequence in MCM6, a nearby gene). You can digest lactose into adulthood if you have a mutation of the MCM6 gene that, in a sense, keeps the LCT gene permanently switched on. Lactose intolerance isn't always related to genetics, however. Secondary or acquired lactase deficiency arises when an infection or disease — including celiac disease, infectious enteritis, or Crohn's disease (an inflammatory bowel disease, or IBD) — damage the small intestine. Treating the underlying issue usually reverses the lactose intolerance.

Prevalence and Demographics of Lactose Intolerance

Lactase persistence — the ability to digest lactose throughout adulthood — is unique to humans, and only evolved within the last 10,000 years.However, about 65 percent of the world's population has some degree of lactose intolerance in adulthood, according to a 2012 article in the International Dairy Journal. The prevalence of lactose intolerance also varies heavily by race.The mutation that causes lactase persistence is believed to have originally sprung up through natural selection along with the rise of dairy farming. As such, lactase persistence is very common in people of northern European decent, with only a few percent of some populations unable to digest lactose. But lactose intolerance is common among people of numerous descents, including:

• West African
• Arab
• Jewish
• Greek
• Italian

Lactose intolerance also affects almost all Asian and Native American people, according to a 2006 report in the journal Pediatrics. In the United States, you are most likely to be lactose intolerant if you are African American, Native American, Asian American, or Hispanic or Latino.

culled from everyday health.com