If you are lactose intolerant, you typically experience digestive symptoms, including bloating, diarrhea, and other symptoms, after consuming lactose.

Lactose is a type of sugar found naturally in the milk of most mammals.

Lactose intolerance is a condition characterized by symptoms such as stomach pain, bloating, gas and diarrhea, which are caused by lactose malabsorption (1).

In humans, an enzyme known as lactase is responsible for breaking down lactose for digestion. This is particularly important in infants, who need lactase to digest breast milk.

However, as children grow older, they generally produce less and less lactase.

By adulthood, nearly 70% of people across the globe no longer produce enough lactase to properly digest the lactose in milk, leading to symptoms when they consume dairy. Lactose intolerance is less common among people of European and northwestern Indian descent (1, 2).

Some people may also develop lactose intolerance after surgery or due to gastrointestinal conditions such as viral or bacterial infections (3).

Here are the 5 most common signs and symptoms of lactose intolerance.

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Stomach pain and bloating are common symptoms of lactose intolerance in both children and adults.

When the body is unable to break down lactose, it passes through the gut until it reaches the colon (4).

Carbohydrates such as lactose cannot be absorbed by the cells lining the colon, but they can be fermented and broken down by the naturally occurring bacteria that live there, known as the microflora (5).

This fermentation causes the release of short-chain fatty acids, as well as the gases hydrogen, methane and carbon dioxide (4).

The resulting increase in acids and gases can lead to stomach pain and cramps. The pain is usually located around the navel and in the lower half of the tummy.

The sensation of bloating is caused by an increase of water and gas in the colon, which causes the gut wall to stretch, also known as distention (5).

Interestingly, the amount of bloating and pain is not related to the amount of lactose ingested, but to the sensitivity of the individual to feelings of distention. Therefore, the frequency and severity of symptoms can vary significantly between individuals (5, 6).

It’s important to note that stomach pain and bloating are common symptoms that could result from other causes, such as overeating, other kinds of malabsorption, infections, medications and other illnesses.


Stomach pain and bloating are common with lactose intolerance. They are caused when bacteria in the colon ferment lactose that the body has left undigested, resulting in excess gas and water. Pain is most often situated around the navel and lower tummy.

2. Diarrhea

Officially, diarrhea is diagnosed when someone has 3 or more watery or loose stools in a 24-hour period (7).

Lactose intolerance causes diarrhea by increasing the volume of water in the colon, which increases the volume and liquid content of the stool (4).

In the colon, microflora ferment lactose to short-chain fatty acids and gases. Most, but not all, of these acids are absorbed back into the colon. The leftover acids and lactose increase the amount of water that the body releases into the colon (4, 5).

For some people with lactose intolerance, consuming as little as 3 grams of lactose can be enough to cause symptoms. However, many people can tolerate 12 grams of lactose, the amount found in 1 cup (240 ml) of milk (3, 5).

If you’re experiencing diarrhea, it doesn’t always mean you have lactose intolerance. There are many other causes of diarrhea including diet, other kinds of malabsorption, medications, infections and inflammatory bowel diseases (8).


Lactose intolerance can cause diarrhea, or frequent, watery stools. It occurs when undigested lactose ferments in the colon, producing short-chain fatty acids that increase the amount of water in the gut.

The fermentation of lactose in the colon increases the production of the gases hydrogen, methane and carbon dioxide (4, 9).

In fact, in people with lactose intolerance, the colon microflora become very good at fermenting lactose into acids and gases. This results in more lactose being fermented in the colon, which further increases flatulence (5).

The amount of gas produced can differ enormously from person to person due to differences in the efficiency of the microflora, as well as the rate of gas reabsorption by the colon (5).

Interestingly, gases produced from lactose fermentation have no odor. In fact, the odor of flatulence comes from the breakdown of proteins in the gut, not carbohydrates (5).


The fermentation of lactose in the colon can lead to increased flatulence, and the extent to which this occurs can vary significantly from person to person. The gas produced from the fermentation of lactose is odorless.

Lactose intolerance may result in nausea or even vomiting in some people, including in children. In one study, nausea was the second most common symptom associated with lactose intolerance in kids, after abdominal pain (1, 10).

When lactose ferments in the gut, the resulting gases and volatile fatty acids can leave you feeling sick to your stomach. Symptoms tend to appear about 30 minutes to 2 hours after consuming dairy (11).

However, nausea and vomiting can have many different causes. These include gastroenteritis (stomach flu), medications, motion sickness, pregnancy, and health conditions affecting your gut, nervous system, or hormones (12).


Nausea and vomiting can be symptoms of lactose intolerance in adults and children. Like other symptoms, they tend to develop about 30 minutes or up to 2 hours after consuming dairy.

While there’s less scientific support for these symptoms, some case studies have reported other symptoms, including (13, 14, 15, 16):

  • constipation
  • headaches
  • fatigue
  • loss of concentration
  • muscle and joint pain
  • mouth ulcers
  • problems urinating
  • eczema

However, these symptoms have not been established as true symptoms of lactose intolerance and may have other causes (9, 17).

Additionally, some people with a milk allergy may mistakenly attribute their symptoms to lactose intolerance.

Cow’s milk allergy is common in infants, affecting as many as 2% to 3% of young babies. In about 90% of cases, this allergy resolves by the time a child is 6 years old (18).

While cow’s milk allergy is more common in young children than in adults, the opposite is true for lactose intolerance. Lactose intolerance is considered rare in children under 5 years old (1).

A milk allergy and lactose intolerance aren’t related, although they can share symptoms such as abdominal pain, diarrhea, and vomiting (18, 19).

Other symptoms of a milk allergy can include (18):

  • hives
  • itching
  • wheezing
  • shortness of breath
  • blood in stools
  • colic
  • swelling in or around the mouth or throat
  • anaphylaxis, which is a medical emergency

Unlike lactose intolerance, a milk allergy can be life threatening, so it is important to get an accurate diagnosis of symptoms, particularly in children.


Other reported symptoms include headaches, fatigue, eczema, and muscle and joint pain, but these have not been confirmed as true symptoms. It is important not to confuse lactose intolerance with a milk allergy, which can be fatal.

Because the symptoms of lactose intolerance are rather general, it is important to get an accurate diagnosis before limiting dairy in your diet (20).

In fact, many people who think they have lactose intolerance because they’ve experienced the symptoms have been shown to absorb lactose normally.

To diagnose lactose intolerance, your doctor will ask you about your symptoms and medical history. Also, they will typically perform a physical exam and request diagnostic testing (1).

Doctors often use the hydrogen breath test to help diagnose lactose intolerance. This test involves ingesting a standard dose of lactose and testing for elevated levels of hydrogen in the breath, which are caused by bacteria fermenting lactose in the colon (4, 20).

Because the hydrogen breath test is sensitive to other factors in addition to lactose, it’s possible to have a false positive result or a false negative result. This means the test can show a positive result when you do not have lactose intolerance, or a negative result when you do (21).

Some estimates suggest that false negatives happen in up to 20% of cases. Even so, the hydrogen breath test is still considered a useful tool for diagnosing lactose intolerance. To help ensure an accurate result, it’s important to follow your doctor’s instructions carefully when preparing for the test (4, 21).

Your doctor will consider your medical history and symptoms when interpreting the results of the test, and may request other tests if needed to confirm your diagnosis or rule out other conditions (1).

Lactose intolerance is defined by the presence of reported symptoms, and that depends on how sensitive a person is to the effects of malabsorption, as well as the amount of lactose in their diet (5).

Treatment of lactose intolerance usually involves restriction or avoidance of high-lactose foods such as milk, soft and processed cheeses, cream, and ice cream (1).

However, people with lactose intolerance can often tolerate up to 1 cup (240 ml) of milk, especially when it’s spread throughout the day. This is equivalent to 0.4–0.5 ounces (12–15 grams) of lactose (4, 22).

Additionally, people often tolerate fermented milk products such as yogurt and certain cheeses better, so these foods may help people meet their calcium needs without causing symptoms (4, 5).


If you have symptoms of lactose intolerance, your doctor may determine your diagnosis by having you perform a hydrogen breath test. Treatment usually involves avoiding high-lactose foods like milk, though you may still tolerate small amounts.

Lactose intolerance is very common, affecting up to 70% of people worldwide.

The most common symptoms include stomach pain, bloating, diarrhea, gas, nausea and vomiting.

There have been reports of other symptoms, such as headaches, fatigue and eczema, but these are rarer and not well established. Sometimes people mistakenly attribute symptoms of a milk allergy, such as itching and rashes, to lactose intolerance.

If you have symptoms of lactose intolerance, a hydrogen breath test may help determine whether you have lactose malabsorption or your symptoms are caused by something else.

Treatment involves reducing or removing sources of lactose from your diet, including milk, cream and ice cream. However, many people with lactose intolerance can drink up to 1 cup (240 ml) of milk without experiencing symptoms.

The severity of symptoms differs from person to person, so it’s important to find out what amount of dairy works for you.