Lactose is a complex carbohydrate (sugar) that is present in milk. During digestion, lactose is broken down into two simpler sugars, glucose and galactose. An enzyme called lactase that is made in the small bowel is necessary for this process. Some people do not have enough lactase (lactase deficiency) and have difficulty digesting milk and milk products. This problem is known as lactose intolerance.
Symptoms of lactose intolerance
The main symptoms of lactose intolerance are abdominal discomfort or pain, a bloated feeling, wind and watery diarrhoea.
The symptoms usually develop 30 minutes to 2 hours after eating or drinking products containing lactose.
Lactose intolerance is different from a milk allergy. A genuine allergic reaction usually causes symptoms such as an itchy skin rash, swelling of the lips or difficulty breathing.
Tests for lactose intolerance
A lactose challenge test can help determine whether lactose intolerance is likely. This test involves drinking half a glass of milk and seeing if symptoms develop. If lactose intolerance is suspected, you doctor may suggest you avoid foods that contain lactose for a few weeks to see if your symptoms improve. If the symptoms return when you re-introduce lactose to your diet, you most likely have lactose intolerance.
Special tests, including a formal lactose tolerance test and measuring the lactase concentration in the small bowel are available.
Dietary modification is the main treatment for lactose intolerance. Most lactose-intolerant people don’t have to avoid lactose entirely, they just need to be sensible and not overdo it.
Choose products that are low in lactose or lactose-free when possible. Milk and milk products should generally still be included in the diet because they are valuable sources of calcium, but don’t eat or drink large quantities of lactose-containing products at one time — spread it out over the course of the day. Eating dairy products along with a meal can also help.
Small amounts of lactose (e.g. 10 grams a day) can usually be tolerated. Foods with a high lactose content (more than 4 grams per serve) include:
- dairy desserts; and
- yoghurt (although yoghurt is often tolerated because the lactose is partially digested by bacterial cultures in the yoghurt).
There is a moderate amount of lactose (0.1 to 4 grams per serve) in ice cream, cream and soft cheeses (such as cream cheese, cottage cheese and ricotta cheese).
Hard cheeses such as cheddar cheese have only very small amounts of lactose (less than 0.1 grams per serve) and generally cause no symptoms.
Lactose in medicines
Lactose is an ingredient in some medicines, including certain brands of the oral contraceptive pill. Usually, lactose in medicines only causes symptoms in people with severe lactose intolerance. It is generally not necessary to avoid taking medicines that contain small amounts of lactose. But you might want to check your medicines to find out if they contain lactose because there may be lactose-free alternatives available.
Lactase enzyme is available over-the-counter in pharmacies, and can be used by people who still experience symptoms despite dietary changes. It can be taken as drops or tablets before drinking milk or eating dairy products.
Secondary lactase deficiency
Lactase deficiency can occur on a temporary basis after a bout of gastroenteritis, or in people with coeliac disease or Crohn’s disease (a type of inflammatory bowel disease).