Lactose intolerance happens when a person’s body cannot break down lactose properly. Symptoms include bloating, flatulence, and diarrhea.

Lactose is a sugar found only in milk. It is also present in dairy products and products made from milk, including cheese and ice cream.

If a person has lactose intolerance, their digestive system produces too little of an enzyme known as lactase. Lactase is needed to break down lactose.

Lactose intolerance is different from a milk allergy. In a milk allergy, the body reacts to milk proteins, not milk sugar. A milk allergy can result in severe symptoms, including anaphylaxis.

Worldwide, an estimated 68% of people have trouble digesting lactose. The figure is lower in the U.S., affecting 36% of individuals.

This article looks at the symptoms, diagnosis, causes, and treatments for lactose intolerance. It also discusses foods to avoid and alternatives to dairy products.

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People with lactose intolerance experience symptoms after eating or drinking milk or dairy products that contain lactose.

Symptoms can range from mild discomfort to a severe reaction. This depends on how much lactase a person’s body produces and how much lactose they consumed.

Most people with lactose intolerance can eat some amount of lactose without experiencing symptoms. Each person has a different tolerance level.

Symptoms of lactose intolerance can include:

The person may have a sudden urge to use the bathroom 30 minutes to 2 hours after consuming lactose.

Chronic diarrhea can lead to dehydration, so it is important for a person to drink plenty of water if they have diarrhea.

If a person suspects that they are lactose intolerant, they can keep a food diary that lists the foods they eat and any symptoms that arise. This can help them and their physician work out which foods are causing the symptoms.

Most healthcare providers recommend that people try a lactose free diet for a period to see if symptoms improve.

Some tests can detect a lactose intolerance. They are:

  • Hydrogen breath test: The person fasts overnight and then takes a lactose solution the following morning. A physician then measures the levels of hydrogen in exhaled air. High levels of hydrogen indicate lactose intolerance.
  • Lactose tolerance test: The person consumes a lactose solution, and a physician takes blood samples to measure their glucose levels. If blood glucose levels remain the same, the body has not broken down the lactose properly.
  • Stool sample test: Lactose tolerance tests and hydrogen breath tests are not suitable for infants, so a physician may perform a stool test. High levels of acetate and other fatty acids in the stool can be a sign of lactose intolerance.

Physicians will also want to rule out other conditions that can cause similar symptoms. These include inflammatory bowel disease, irritable bowel syndrome, and celiac disease.

To avoid symptoms, a person with a severe lactose intolerance may need to avoid milk, milk products such as ice cream, and other processed foods that contain milk powder and whey.

Avoiding lactose may require some trial and error, but food labeling can help, as a product that contains lactose must have a label stating that it contains “milk.”

Many people with lactose intolerance can tolerate up to 10 grams of lactose, or around a glass of milk, without significant symptoms. Spreading consumption throughout the day, and consuming lactose-containing products with meals, can increase tolerance.

People can use lactase enzyme supplements, which contain lactase, to break down the lactose in milk and milk products. This lowers the chances of lactose intolerance symptoms.

Lactase is an enzyme produced in the small intestine. The body uses lactase to break down lactose into components called galactose and glucose. The glucose is then absorbed into the bloodstream.

If a person’s lactase levels are low, the lactose does not break down and it does not absorb into the bloodstream. Instead, it moves into the large intestine, or colon. Bacteria in the colon react to any product that contains lactose by creating more gas. That can lead to discomfort and other unpleasant symptoms for the individual.

In some regions, more people carry a gene that allows them to digest lactose. Lactose intolerance is less common in people who are from Europe, or whose families are from Europe.

In the U.S., the following ethnic and racial groups are more likely to have trouble digesting lactose:

  • African Americans
  • American Indians
  • Asian Americans
  • Hispanics
  • Latinos

Some people believe that it is not natural for people to drink milk, as humans are the only mammals that continue to consume milk products after weaning. Lactose intolerance, they argue, is not a medical condition but a natural occurrence.

Lactose intolerance is often inherited. The types include:

  • Primary lactase deficiency, which is the most common type of lactose intolerance. It is genetic and symptoms often appear when an infant is weaned from milk to solids. Lactase production drops, and symptoms of intolerance appear.
  • Congenital lactase deficiency, which is when a person is born with a genetic mutation results in them producing very little or no lactase.
  • Familial lactase deficiency, which is when a person produces enough lactase, but the person does not break down the lactose for absorption into the bloodstream.

In some people, lactase levels drop from an early age, but the symptoms of lactose intolerance may not appear until they are an older child or adult.

There is also a condition called secondary lactase deficiency, in which a problem with the small intestine results in too little lactase production. Possible causes are:

If the underlying condition is chronic, such as Crohn’s disease, the resultant intolerance is often long-term too. An intolerance that starts with a short-term condition, such as gastroenteritis, normally passes within a few days or weeks.

Most milk and milk products have lactose in them, and many processed foods have milk and dairy products added to them.

Lactose is a sugar found in the milk of most mammals in similar amounts, except for some sea mammals. Different dairy products can, however, contain different amounts of lactose depending on how they are processed.

Any product with milk, lactose, whey, curds, milk byproducts, dry milk solids or non-fat dry milk powder listed in its ingredients will have lactose in it.

Foods that commonly contain lactose include:

  • cakes and biscuits
  • cheese sauce
  • cream soups
  • custard
  • milk chocolate
  • pancakes
  • scrambled eggs
  • quiche

To avoid symptoms, a person with a lactose intolerance should check food labels carefully, as some foods may contain hidden lactose.

Examples include:

  • muesli bars
  • breads
  • breakfast cereals
  • margarine
  • some instant soups
  • boiled candies
  • chocolate candies and bars
  • some processed meats, such as sliced ham
  • salad dressing and mayonnaise

Around 20% of drugs contain lactose as a filler. These might include birth control pills, over-the-counter drugs, and treatments for stomach acid.

In many societies, dairy products are an important source of nutrients, including calcium, protein, and vitamins A, B12, and D. When eliminating dairy, it is important to get these nutrients from elsewhere.

Many alternative dairy products, including soy milk, almond milk, and alternative cheeses, are reinforced with vitamins.

Sources for these vitamins other than dairy include:

  • Calcium: Seaweeds, nuts and seeds, blackstrap molasses, beans, oranges, figs, quinoa, amaranth, collard greens, okra, rutabaga, broccoli, dandelion leaves, kale, and fortified products such as orange juice and plant milks. Read more here.
  • Vitamin A: Carrots, broccoli, sweet potatoes, cod liver oil, liver, spinach, pumpkin, cantaloupe melon, egg, apricot, papaya, mango, and peas.
  • Vitamin D: Levels can be enhanced by exposure to natural sunlight, consuming fatty fish, egg yolk, fish liver oils, and some fortified plant milks, as well as other fortified products, including many breakfast cereals.
  • Lactose-free milk: A person with severe symptoms should check the label to ensure that lactose levels are zero, and not just reduced. Vegetable-based milks also contain less protein than cow’s milk.

It is best to talk to a doctor or dietician before making significant dietary changes.

Read about alternatives to milk, cheese, cream, and more here.

Lactose intolerance happens when a person’s body is unable to break down proteins in milk. It is very common, and can cause mild to severe symptoms.

While there is no cure for lactose intolerance, people can prevent symptoms by avoiding milk and products that contain milk, or by taking lactase enzymes.

Alternatives to milk and other dairy products are becoming increasingly popular in the West, and more of those products are becoming available all the time.