Lactose, commonly known as "milk sugar", is a disaccharide or two sugars linked. It is a combination of the two sugars, glucose and galactose. If intestines lack or are deficient in lactase you will not adequately digest lactose and you will experience gas, bloating, abdominal cramps and diarrhea shortly after eating something containing lactose.

Lactose or milk sugar may not be digested well if you are either born with absent or low levels of the enzyme lactase or if your intestine has been injured resulting in absent or low levels of lactase.

Lactase is the enzyme that breaks down lactose to two simple sugars. It resides on tip of the villi the small intestine cells where it is very vulnerable to intestinal injury. Some people are born with absent or low levels of lactase, therefore have congenital alactasia or hypolactasia.

Lactose intolerance commonly causes symptoms of gas, bloating, abdominal pain or cramps and diarrhea that occur within minutes to a couple of hours of ingestion of dairy products. Since lactose can be in other foods not considered dairy, some people may not recognize a pattern that they associate with lactose. Others may be misdiagnosed as having irritable bowel syndrome.

The diagnosis is made formally by either a hydrogen breath test or lactose tolerance test. Measuring hydrogen in the breath after ingestion of lactose that has been radio labeled or marked with a detectable carbon fragment is known as the hydrogen breath test. Lactose tolerance tests measure several blood sugar levels after ingestion of lactose looking for absence of an expected normal rise. Though a biopsy of the intestine can test for presence of the enzyme, this is expensive, invasive and rarely available outside of research labs.

Non-specific tests such as stool acidity test (pH) or lactose challenge are commonly utilized along with a clinical history and a trial of a lactose free diet to make the diagnosis. The stool is normally neutral to alkaline in pH unless sugars are fermented in the intestine or excessively acidic foods or beverages are ingested. Maldigested lactose fermented by bacteria in the intestine produces acid and lowers the stool pH. Stool pH can be easily and inexpensively measured. However, a low or acidic stool pH is not specific for lactose intolerance since other maldigested sugars can be fermented resulting in increased stool acidity.

Between 80-100% of people of Asian, Native American, or African ancestry are lactose intolerant for this reason compared to only 15% of those of Northern European ancestry.

Lactose challenge is simply eating or drinking high lactose containing foods or drinks and noting the presence or absence of symptoms. The "poor man's" lactose challenge is ingesting a quart of skim milk. Fat slows emptying of the stomach and delays the delivery of food or drink to the small intestine. Skim milk's lack of significant fat permits rapid passage from the stomach to small intestine of a large load of lactose. If someone is lactase deficient they will have rapid onset of symptoms of bloating, gas, abdominal cramps and diarrhea. The absence of any of these symptoms after a quart of skim milk makes lactose intolerance very unlikely.

Lactose intolerance treatment involves avoiding lactose-containing foods, not just dairy, since many other foods have lactose added. Alternatively, lactase enzymes may be taken with or just before eating lactose containing foods but their effectiveness is somewhat limited. Lactose free milk may also be used.

Secondary or acquired lactose intolerance can develop after any injury of the small intestine. Common causes include infection (e.g. after severe "stomach flu" or gastroenteritis), medication (e.g. chemotherapy), radiation, malnutrition, food intolerance (e.g. Celiac disease) or overgrowth of bad bacteria. The intestine may be temporarily damaged and recover tolerance for lactose. Permanent or ongoing damage can result in prolonged intolerance similar to congenital lactose intolerance. The levels of lactase enzymes are highest in young children and diminish with time, often by teenage years when symptoms may become prominent.