Lactose intolerance

Last updated:

Reviewed by:

Dr Bryony Henderson

, Lead GP at Livi

Medically reviewed

People with lactose intolerance don’t produce enough of the enzyme lactase. This means their body is unable to break down and absorb lactose, a sugar found in dairy-based food. Lactose remains in the digestive system, and is fermented by bacteria. This leads to a build up of gases, which can cause painful bloating and other symptoms.

What are the symptoms of lactose intolerance?

Lactose intolerance symptoms can happen after eating dairy-based products. These are the most common symptoms: 

  • Bloating 

  • Stomach pain and aches

  • Excessive gas

  • Loose or watery poo – this usually occurs a couple of hours after eating dairy

  • Itchy anus

Lactose intolerance is different to having an allergy to lactose. Being allergic to lactose causes your body’s immune system to react abnormally to lactose when you eat it. The symptoms will more closely resemble an allergic reaction like itching or wheezing. 

What causes lactose intolerance?

The main causes of lactose intolerance can be split into: 

1. Primary lactase deficiency 

This is the most common cause and happens when you don’t produce enough lactase. 

2. Secondary lactase deficiency

This usually happens after the lining of your intestines become damaged – by an illness or chronic condition like IBS or coeliac disease. If the disease goes away and the intestines heal, the deficiency also goes away. 

3. Developmental lactase deficiency

This is seen in premature babies, born at less than 34 week. It usually goes away as the baby gets older and the intestines develop.

How common is lactose intolerance?

In the UK, lactose intolerance is more common in people of Asian or African-Caribbean ethnicity. It can develop at any age, but is more common between the ages of 20 and 40.

How is lactose intolerance diagnosed?

If you have symptoms of lactose intolerance, speak to a doctor and they can help. They may ask you to keep a symptom diary, to see if your symptoms happen after particular meals. 

They may then ask you to exclude dairy-based products from your diet to assess whether your symptoms improve.

Sometimes, a doctor might request further tests. These help to confirm the diagnosis and to find the cause. 

When should I speak to a doctor?

If you’re worried that you might have an intolerance to lactose, speak to a doctor. Book an appointment if you have any of these symptoms, particularly after eating dairy products:

  • Persistent bloating, stomach pain or gas

  • Persistent loose or watery poo 

Last updated:
Reviewed by:
Dr Bryony Henderson, Lead GP at Livi