Lactose Intolerance: Why Milk Can Upset Your Stomach

Many people enjoy milk, ice cream, cheese, or creamy foods without a second thought. But for others, dairy can lead to stomach pain, bloating, gas, or diarrhea. If this sounds familiar, lactose intolerance may be the reason.

Lactose intolerance is common, and the good news is that it can often be managed with simple diet changes and lactase supplements. Understanding what is happening in your body can make it much easier to handle.

What Is Lactose Intolerance?

Lactose is a natural sugar found in milk and many dairy products.

To digest lactose, your small intestine needs an enzyme called lactase. An enzyme is a substance that helps break down food. Lactase breaks lactose into smaller sugars that the body can absorb.

When the body does not make enough lactase, lactose is not fully digested. It then moves into the large intestine, where bacteria break it down. This can lead to uncomfortable symptoms.

This problem is called lactose malabsorption. When it causes symptoms such as bloating, diarrhea, or stomach cramps, it is called lactose intolerance.

Why Does Lactose Intolerance Happen?

Lactose intolerance happens because the body has low levels of lactase.

There are a few reasons this can occur:

1. Primary Lactase Deficiency

This is the most common cause. It happens because of a genetic change that causes the body to make less lactase over time.

Symptoms often begin after lactase levels start to fall during childhood.

2. Secondary Lactase Deficiency

This happens when the lining of the small intestine is damaged by another condition or infection.

Examples include:

• Certain autoimmune conditions, such as gluten-sensitive enteropathy

• Viral stomach infections, such as rotavirus

This type can happen at any age, but it most often affects infants.

3. Congenital Lactase Deficiency

This is extremely rare. Babies with this condition develop symptoms in the first few days of life.

4. Developmental Lactase Deficiency

This can happen in premature babies because lactase activity develops late in pregnancy. It is usually temporary.

How Common Is It?

Lactase deficiency is very common worldwide. Around 70% of people have some degree of lactase deficiency, although not everyone develops symptoms.

It is more common in people of African, Asian, or Native American ancestry. Lactose intolerance also becomes more common with age.

What Symptoms Can Lactose Intolerance Cause?

Symptoms usually start about one hour to several hours after eating or drinking foods that contain lactose.

Common symptoms include:

• Watery diarrhea

• Cramping stomach pain

• Pain around the belly button or lower belly

• Bloating

• Gas

• Nausea

Symptoms can vary a lot from person to person. Some people can tolerate small amounts of lactose, while others feel symptoms after larger amounts.

In general, symptoms tend to be stronger when a person consumes more lactose.

Why Does Lactose Cause These Symptoms?

When lactose is not digested in the small intestine, it passes into the large intestine.

There, it can:

• Pull water into the bowel, which can cause diarrhea

• Be broken down by bacteria, which produces gas

• Increase movement in the intestines, which can cause cramping pain

• Lead to bloating and flatulence

  
  

This is why a glass of milk may cause diarrhea, cramps, and gas in someone with lactose intolerance.

How Is Lactose Intolerance Diagnosed?

A common first step is a strict lactose-free diet for 2 weeks.

During this time, lactose should be avoided carefully, including hidden sources in foods or even some medicines.

If symptoms go away and then return when lactose is added back into the diet, lactose intolerance is likely.

If symptoms continue despite avoiding lactose, further testing may be needed. A healthcare professional may recommend tests such as:

Hydrogen Breath Test

This is one of the least invasive tests.

After fasting, a person drinks lactose mixed with water. Breath samples are then checked over several hours. If undigested lactose reaches the large intestine, bacteria produce hydrogen gas, which can be measured in the breath.

Lactose Tolerance Test

This test measures blood sugar levels after drinking lactose. If lactose is not digested and absorbed properly, blood sugar does not rise as expected.

Endoscopy With Small Intestine Biopsy

In some cases, a doctor may look at the small intestine and take a small tissue sample. This can help check for low lactase levels or damage to the intestinal lining.

Could It Be Something Else?

Yes. Several conditions can cause symptoms similar to lactose intolerance.

These include:

• Cow’s milk allergy

• Other food allergies

• Fructose intolerance

• Intolerance to certain fermentable carbohydrates

• Celiac disease

• Irritable bowel syndrome

• Inflammatory bowel disease

• Stomach or bowel infections

• Small bowel bacterial overgrowth

• Cystic fibrosis

• Bowel malignancy

Because many conditions can cause bloating, diarrhea, and abdominal pain, it is important to seek medical advice if symptoms are severe, persistent, or unclear.

How Is Lactose Intolerance Managed?

The main treatment is reducing lactose intake.

This does not always mean removing all dairy forever. Many people can tolerate some lactose, depending on the amount and the food.

Helpful steps include:

• Choose lactose-free or low-lactose products

• Reduce the amount of milk and dairy you consume

• Read food labels carefully

• Watch for hidden lactose in foods such as bread, salad dressing, and some medications

• Take oral lactase supplements when eating foods that contain lactose

• Eat lactose-containing foods with other solid foods to slow digestion

• Try yogurt with live cultures, which is often better tolerated

Some people can tolerate about 8–12 fluid ounces of cow’s milk per day, especially when taken with food. Tolerance varies, so it helps to pay attention to your own symptoms.

Living With Lactose Intolerance

Lactose intolerance can be frustrating, especially when symptoms interrupt your day. But once you understand your triggers, it often becomes much easier to manage.

You may not need to avoid every dairy product. Small changes, such as choosing lactose-free milk, checking labels, or using lactase tablets, can make a big difference.

Key Takeaways

• Lactose intolerance happens when the body does not have enough lactase to digest lactose, the sugar found in milk and dairy products.

• Common symptoms include bloating, gas, stomach cramps, nausea, and diarrhea after consuming lactose.

• A 2-week lactose-free diet can help identify whether lactose is causing symptoms. If symptoms do not improve, testing may be needed.

• Treatment usually involves a lactose-restricted diet, lactose-free products, and oral lactase supplements.

With the right approach, most people can manage lactose intolerance and still enjoy a comfortable, flexible diet.