Lactose is a naturally occurring sugar in milk that consists of two units: glucose and galactose. In normal digestion, lactose is broken down into these simple sugars and absorbed by the body. To do this effectively, we need enough of the enzyme lactase.

Lactase enzyme production is high during the first year of life to help digest breast milk, which contains even higher lactose levels than cow’s milk. For most people, lactase production starts to reduce in early childhood after weaning from breast milk. This can affect their ability to tolerate milk and dairy products.(1) Without enough lactase enzyme, the lactose sugar is not broken into its constituents and will not be digested or absorbed. This is called lactose maldigestion. For some individuals, lactose maldigestion results in uncomfortable digestive symptoms such as bloating, gas, abdominal pain and diarrhoea. This is known as lactose intolerance.(2, 3)

Primary and secondary lactose intolerance

There are two main types of lactose intolerance: primary and secondary. People with primary lactose intolerance produce insufficient amounts of the lactase enzyme to fully digest lactose in the diet. This type is ongoing and can worsen with age for some people. Secondary lactose intolerance is most often temporary and can affect people of all ages. It occurs when a person’s ability to digest lactose becomes limited by other illnesses, such as gastrointestinal injury or inflammation. In these cases, digestion of other food components can also be affected. Secondary lactose intolerance generally resolves after recovering from the original illness.(1)

Confirming a diagnosis in adults

Many adults suspect they are lactose intolerant and self-diagnose. It is important for health professionals to also consider and rule out more serious causes of digestive discomfort. Other conditions such as cow’s milk protein allergy, coeliac disease, inflammatory bowel disease or irritable bowel syndrome may present similarly and should not be left untreated. Lactose intolerance can be diagnosed by elimination trials, a hydrogen breath test, or measuring lactase levels in a biopsy taken via gastroscopy.(3) Health professionals should consider the symptom types and severities, individual medical history and lifestyle factors when choosing a diagnostic method.

Management of lactose intolerance in adults

It is important to note that lactose intolerance is not an allergy. Most people who are lactose intolerant can still enjoy small amounts of dairy. In contrast, those with a cow’s milk protein allergy should fully eliminate dairy from their diet. These conditions should always be distinguished to prevent potential harm and avoid unnecessary removal of dairy from the diet.

People with lactose intolerance can usually tolerate small amounts of lactose at a time, equivalent to around 1 cup of milk (250 mL) per day.(4-6) This is because most lactose intolerant individuals still have some, although small amounts of, lactase enzyme. If only a small portion of lactose is consumed at a time, it is less likely to exceed the amount of lactase enzyme available.

Another way for lactose intolerant people to enjoy dairy is by selecting low-lactose or lactose-free products. Lactose-free milk is produced by adding the lactase enzyme to regular milk, which breaks down lactose before consumption. The resulting glucose and galactose molecules give lactose-free products their characteristic sweet taste.

Other dairy products, including cheese and yoghurt, contain lower levels of lactose than regular cow’s milk. When milk is fermented to produce these products, beneficial bacteria feed on lactose, which can significantly reduce its content.(7)

Other dairy products, including cheese and yoghurt, contain lower levels of lactose than regular cow’s milk. When milk is fermented to produce these products, beneficial bacteria feed on lactose, which can significantly reduce its content.(7)

It is possible for people with lactose intolerance to enjoy a range of dairy products while managing their digestive symptoms. Dairy is a rich source of nutrients and makes up one of the five core food groups in the Australian Dietary Guidelines. Removing dairy when unnecessary can make it difficult to meet all nutrient requirements.(3, 6, 7)

Lactose intolerance in infants

It is very rare for infants to have lactose intolerance caused by insufficient lactase production.(1) But true lactose intolerance during infancy is important to identify correctly, as it can severely impact growth and development. These infants may need a lactose-free cows’ milk-based formula or soy-based formula for adequate nutrition.(1, 9, 10) That being said, many parents incorrectly assume their baby is lactose intolerant when they show signs of digestive discomfort. These symptoms can have a range of causes, including cows’ milk protein allergy for formula-fed infants, that cannot be treated by eliminating lactose from the diet.

Secondary lactose intolerance is not uncommon in infancy. However, it is more important in these cases to address the underlying cause rather than the lactose intolerance itself. Infants can temporarily become lactose intolerant secondary to illnesses such as gastroenteritis, food allergies and infections. The lactose intolerance usually resolves when the underlying condition is treated.(1)

Infants showing signs of gastrointestinal discomfort, whether sudden or ongoing, should be seen by their GP or paediatrician. It can be dangerous to attempt treating these symptoms without professional advice.

Lactose overload in breastfed infants

Infants can develop lactose overload when they consume large volumes of breast milk. This often occurs when an infant’s mother produces more milk than required, also known as ‘breast milk oversupply’. Lactose overload can sometimes be confused with lactose intolerance as they both present with digestive discomfort. Infants with lactose overload often present with unsettled behaviour, excessive wind, frequent wet nappies (>10/day) and stools that are green, frothy and loose.(11, 12) But an infant with lactose overload is not necessarily intolerant.

When breast milk is produced in large volumes it has a lower fat content than usual. It passes through the digestive tract more quickly, which can lead to incomplete lactose digestion. The undigested lactose draws water into the bowel and acts as a substrate for bacteria to ferment. Ultimately, this results in gas production and acidic, loose stools. An infant experiencing these symptoms may appear ‘unsettled’. This can be misinterpreted as hunger and encourage a mother to offer even more breast milk. The infant may find some temporary relief with feeding, as it can stimulate movement of gas and stool through the bowel. But the additional breast milk ultimately continues the cycle of lactose overload.(11, 12)

Mothers with breast milk oversupply can seek advice from an Australian Breastfeeding Association counsellor, International Board Certified Lactation Consultant, paediatric dietitian or other specialised health professional.(12) These experts can discuss strategies to reduce milk volume for healthy digestion in the infant. The lactose content in breast milk is completely independent of the mother’s dietary lactose intake, so adjusting a mother’s dairy intake will not change the lactose content of her breast milk. Some mothers may be advised to cease breastfeeding and switch to a lactose-free formula. This is rarely necessary, and there are far more benefits for both mother and baby in continuing to breastfeed with professional guidance. Lactose overload can be fully managed without switching to formula.