7 Signs of Lactose Intolerance, Meaning and Why It Occurs

Lactose intolerance is the most common food intolerance, affecting an estimated 75% of the world’s population to some degree. Contrary to popular belief, lactose intolerance is not always evident from childhood. In fact it may only become apparent in adulthood although lactose intolerance can subside temporarily, like during periods of pregnancy.

Meaning of Lactose Intolerance

Lactose is a type of sugar found in milk. It is a disaccharide, made up of two monosaccharides (simple sugars) known as galactose and glucose. In order for the body to digest lactose, it has to break it down through the action of an enzyme known as lactase. In lactose intolerance, there is a deficiency of the digestive enzyme lactase. As a result lactose cannot be broken down and remains in the gut where it causes a host of disturbances.

It is important to understand that lactose intolerance is not a serious condition although it can cause severe symptoms in some people. The discomfort and debility associated with lactose intolerance can be avoided by not consuming milk and other dairy products. While milk has several important nutrients, there is no risk of vitamin or mineral deficiencies with proper and balanced nutrition.

In addition, digestive enzyme supplements may also help ease the symptoms of lactose intolerance. It works by digesting the lactose so that the simpler sugars can be absorbed within the digestive tract, thereby leaving no residual lactose. These supplements may only be useful when consuming small amounts of dairy. It also does not provide relief for every person with lactose intolerance.

Read more on what is lactose intolerance.

Lactose Intolerance vs IBS

Irritable bowel syndrome (IBS) is a seperate condition from lactose intolerance. IBS is a functional bowel disorder while lactose intolerance is a food-related digestive disorder. In IBS, movement through the gastrointestinal tract may be faster (IBS with diarrhea) or slower (IBS with constipation) than normal. However, the exact mechanism behind IBS is not completely understood.

The symptoms of both IBS and lactose intolerance are largely similar. Therefore these two conditions may be confused at times. Some people with irritable bowel syndrome (IBS) may also have lactose intolerance. Therefore a lactose-free diet may ease the symptoms of intolerance but not completely resolve those symptoms due to the IBS. The conditions can also be distinguished by conducting lactose intolerance tests.

How to Spot Lactose Intolerance

Lactose intolerance causes a host of digestive disturbances. To understand the reason for these disturbances and associated symptoms, it is important to look at the effects of undigested lactose in the gut. Normally lactose is digested into monosaccharides and absorbed in the gut because lactose in its undigested form cannot be absorbed by the body. However, when it stays in the gut then it changes the osmotic concentration within the bowel.

This upset in the osmotic concentration means that more water is drawn out of the body and into the bowels. It causes the small intestine to dilate (enlarge) and this speeds up movement through the bowels. The faster movement further upsets digestion of other nutrients. In addition, the undigested nutrients are consumed by the bacteria in the colon which releases larger amounts of hydrogen gas than normal.

All of these changes then gives rise to the following signs and symptoms. These symptoms usually arise within 30 minutes to two hours after consuming milk and dairy products. It is important to note that many people with lactose intolerance can tolerate small amounts of lactose. In these cases there may be no signs or symptoms until large amounts of dairy are consumed.

Read more on foods to avoid in lactose intolerance.

Nausea and Vomiting

Any irritation of the upper gut can lead to nausea. The disturbances of lactose intolerance start in the small intestine where lactose digestion should occur. Signals from the swollen small intestine are relayed to the brain. These signals stimulate the vomit centers as the body tries to expel the irritant substance through vomiting. However, vomiting does not always occur with the nausea in lactose intolerance. Only some people experience vomiting due to lactose intolerance.


Diarrhea is a common symptom of lactose intolerance. The severity of the diarrhea often correlates to the quantity of dairy that is consumed and the amount of lactose that is undigested. Loose and watery stool is passed out frequently, with some people experiencing explosive diarrhea. Usually, diarrhea only lasts until the lactose is expelled and the osmotic concentration within the bowels returns to normal. Sometimes diarrhea is the only symptom of lactose intolerance.

Abdominal Cramps

Abdominal cramping is another common symptom. These cramps are a result of the intestinal wall muscles contracting forcefully. It occurs with widening of the intestines with water and gas. The stretching causes the tiny intestinal wall muscles to respond thereby increasing movement through the bowels. Passing stool can sometimes ease the abdominal cramps temporarily, until the water and gas content in the bowels return to normal.


Bloating is a sensation of abdominal fullness. The feeling arises with the influx of water and electrolytes into the bowel which causes the small intestine to distend. Together with the other symptoms, it is often contributes to a loss of appetite. Sometimes there is also excessive belching in an attempt to relieve the bloating. As with other symptoms, bloating may ease to some degree with passing stool.


Increased gas production in the colon results in excessive flatulence. This is due to the extra hydrogen gas that is produced by the colonic bacteria that have additional nutrients to consume. Although this is initially the lactose, the faster than normal movement through the bowels also hampers digestion of absorption of other nutrients. Collectively this provides a larger store of nutrients for the colonic bacteria to consume thereby producing additioanl gas as a metabolic byproduct.


Dehydration is not a symptom of lactose intolerance. It is a complication that may arise with excessive fluid and electrolyte loss due to diarrhea and vomiting. However, in most instances the diarrhea due to lactose intolerance is usually not severe or prolonged to lead to dehydration. With proper hydration, it can be averted. Fatigue, increased thirst, dryness of the mouth, eyes and skin, dizziness, headaches and low urine output are some of the signs and symptoms of dehydration.

Please note that any information or feedback on this website is not intended to replace a consultation with a health care professional and will not constitute a medical diagnosis. By using this website and the comment service you agree to abide by the comment terms and conditions as outlined on this page