Lactose intolerance refers to an inability to digest and absorb lactose in your food and drinks. This causes digestive symptoms such as bloating and diarrhoea. If you have lactose intolerance, you can still consume small amounts of lactose without any symptoms; however, how much varies from person to person.

In the UK, up to 1 in 20 people has lactose intolerance. However, globally, almost two-thirds of the population has lactose intolerance. This is because the main type of lactose intolerance is caused by your genes and its prevalence varies according to ethnic groups.

Here, we will explore the symptoms of lactose intolerance, the different types of lactose intolerance, getting a diagnosis and how to manage your symptoms.

What is lactose intolerance?

Lactose intolerance is caused by reduced levels of an enzyme in your gut called lactase. Lactase is found in your small intestine, specifically along its lining called the brush border. Its job is to break down lactose, which is found in dairy products such as milk, cheese and butter. Lactose is broken down into 2 sugars called glucose and galactose, which are then absorbed by your gut.

When lactase levels are too low, lactose remains in your gut and passes from your small intestine into your large intestine. Here, the presence of lactose causes more water to be drawn into your stools, which causes diarrhoea.

Also, bacteria in your large intestine feed off the lactose and produce gassy byproducts, namely hydrogen, methane and carbon dioxide. These gases cause bloating, abdominal pain and flatulence (wind).

Types of lactose intolerance

There are several types of lactose intolerance, but in this article, we will focus on the most common form: primary lactase deficiency, which is also known as lactase non-persistence. It does not occur in infants and only develops after age 2, most commonly after age 8.

It’s caused by your genes and results in lactase levels in your small intestine significantly decreasing after age 2. This leads to lifelong lactose intolerance.

The 3 other types of lactose intolerance are:

Congenital lactase deficiency — a rare genetic condition where hardly any or no lactase is produced from birth
Developmental lactase deficiency — temporary lactose intolerance in babies born prematurely, before they have developed enough lactase-producing cells
Secondary lactase deficiency — lactose intolerance due to damage to the small intestine by a food allergy, gut infection or gut condition, such as coeliac disease or Crohn's disease; treating the underlying cause often resolves this type of lactose intolerance

Lactose intolerance vs milk allergy: what’s the difference?

Lactose intolerance is due to low levels of the enzyme lactase, which breaks down lactose into products that your gut can absorb. This is different to a milk allergy, also known as a dairy allergy, which does not involve lactose.

A milk allergy is caused by the immune system reacting to proteins present in dairy products, specifically casein and whey. The immune system mistakenly identifies these dairy proteins as harmful and tries to neutralise them by producing antibodies called IgE against these proteins.

That means when you consume dairy products, IgE antibodies recognise and attack these proteins. This causes immune cells to release chemicals including histamine, which produces the symptoms of an allergic reaction, including:

Abdominal pain
Breathlessness and wheezing
Hives
Swelling of the lips, tongue and/or throat
Tingling around the mouth and lips
Vomiting and diarrhoea

Risk factors for lactose intolerance

The main risk factor for developing lactose intolerance caused by primary lactase deficiency is your genes. People of East Asian descent are most likely to have lactose intolerance, while people of Northern European descent are least likely to have lactose intolerance.

In East Asian populations, 70–100 people per 100 people have lactose intolerance, compared to just 5 in every 100 people of Northern European descent. Other ethnic groups with a high prevalence of lactose intolerance include people of South Asian, African, Afro-Caribbean, Middle Eastern, Native American, Hispanic or Jewish descent.

Your family history also affects your risk. The way in which lactose intolerance is inherited means that your risk is only higher if both of your parents have lactose intolerance. Your risk does not increase if only one parent has lactose intolerance.

Age has a role to play in primary lactase deficiency too. As lactase levels decrease with age, you are more likely to develop lactose intolerance as you get older.

Signs of lactose intolerance

If you have lactose intolerance, you can develop symptoms a few minutes or a few hours after consuming dairy products. Common symptoms include:

A rumbling or gurgling tummy
Abdominal pain and/or cramps
Bloating
Diarrhoea or constipation
Flatulence
Nausea and/or vomiting

You can also develop symptoms outside of your gut, including:

Headaches
Eczema
Fatigue
Joint pain
Mouth ulcers
Muscle pain

Your symptoms may persist until the food or drinks containing lactose passes out of your body. This can take a day or 2.

If you have these symptoms for 3 weeks or more, or you notice that you persistently develop these symptoms after consuming dairy products, see your GP.

Getting a diagnosis

Your GP will ask you about your symptoms and when they occur. It can help to keep a food diary so you can track what foods and drinks trigger your symptoms.

Based on your symptoms, their severity and your medical history, your GP may recommend the following tests:

Lactose elimination diet

You will be asked to avoid eating or drinking anything containing dairy products for 2–4 weeks to see if your symptoms improve. If your symptoms improve, you are likely to have lactose intolerance.

Lactose tolerance blood test

You will be asked to fast (ie to avoid eating and drinking) for a certain number of hours and will then have your blood sugar levels tested. Next, you will be asked to drink a lactose solution and have your blood sugar levels tested again within the next 2 hours.

If your blood sugar levels do not rise, the lactose was not broken down into simpler sugars that could be absorbed into your bloodstream. This means you may have lactose intolerance.

Hydrogen breath test

You will be asked to follow a lactose-free diet for the whole day before your test and to fast from the evening before your test. On the day of your test, you will be asked to provide a breath sample by blowing into a handheld hydrogen breath test device.

Then, you will drink a lactose solution and provide breath samples regularly for up to 4 hours to see if your hydrogen levels rise. If your hydrogen levels rise, this is a sign that bacteria in your gut are feeding off undigested lactose and producing hydrogen gas as a result. This means you may have lactose intolerance.

Confirming a diagnosis

The results of the lactose intolerance test and hydrogen breath test only tell you that your body was unable to break down and absorb lactose — this is called lactose malabsorption.

They do not tell you whether you have lifelong lactose intolerance (primary lactase deficiency) or temporary lactose intolerance caused by another condition affecting the gut (secondary lactase deficiency).

To reach a clear diagnosis, you must also develop the common symptoms of lactose intolerance after drinking the lactose solution and, in some cases, may need further tests to rule out other causes of these symptoms.

Conditions often mistaken for lactose intolerance

Although the most common type of lactose intolerance is primary lactase deficiency, which is a lifelong condition, there are also several gut conditions that can prevent the breakdown and absorption of lactose, which cause temporary lactose intolerance. In these latter cases, appropriate treatment can mean that you are able to tolerate consuming lactose again.

Gut conditions that can produce similar symptoms as primary lactase deficiency include:

Irritable bowel syndrome
Coeliac disease
Small intestinal bacterial overgrowth (SIBO)
Inflammatory bowel disease ie Crohn's disease and ulcerative colitis

Symptoms of a milk allergy can also overlap with lactose intolerance. It is, therefore, important to have any persistent bowel symptoms investigated to identify the underlying cause and receive appropriate treatment.

Living with lactose intolerance

If you are diagnosed with lactose intolerance, your doctor will advise you to reduce how much lactose you consume or avoid lactose-containing foods and drinks completely.

In addition to avoiding traditional dairy products ie milk, cheese and butter, you will also need to pay closer attention to the ingredients in processed foods, which often contain dairy. This includes:

Baked goods (eg biscuits, cakes, crackers and pastries)
Cereals
Protein shakes, meal replacement shakes and diet drinks
Salad dressings and sauces

If you would like to continue consuming lactose, taking a lactase supplement in the form of drops or a tablet before you eat or drink lactose may help reduce or prevent your symptoms. However, the effectiveness of lactase supplements varies from person to person, and you may, therefore, still need to reduce how much lactose you consume.

To compensate for having fewer or no dairy products in your diet, it’s important to take a calcium supplement and a vitamin D supplement. This is because dairy is a good source of calcium and, in the UK, dairy products are often fortified with vitamin D.

A diet with reduced amounts of dairy can, therefore, cause you to become deficient in both calcium and vitamin D. This increases your risk of reduced bone density (osteopenia), which, if left untreated, can lead to brittle bones (osteoporosis).

If following a lactose-free diet is causing you to lose weight or become tired, see your GP. They may refer you to a dietitian, who can help you follow a diet that provides all the vitamins and nutrients you need.