Lactose is the primary carbohydrate of breast milk, including those belonging to animals; cow's, goat's and sheep's milk contain high levels of lactose.¹ The carbohydrate is known as a disaccharide and is comprised of simple sugars, glucose and galactose¹.

Before lactose can be absorbed by the body, it must be hydrolysed to its two components, glucose and galactose, by the enzyme lactase.¹ A deficiency of this enzyme can result in lactose malabsorption.⁴ Lactose that is not absorbed is fermented by the gut microbiota which results in the production of gases such as hydrogen, carbon dioxide and methane. Lactose intolerance is clinically defined as substantial lactose malabsorption associated with gastrointestinal symptoms such as bloating, abdominal pain, diarrhoea and vomiting.⁴ Normal, healthy infants are born with the ability to tolerate lactose as it is the primary carbohydrate of breastmilk. In most infants, intestinal lactase activity reaches its maximum at birth.¹ Some children experience a physiological gradual decline of lactase activity (hypolactasia) after weaning, however, significant gastrointestinal symptoms generally do not occur before 5 years of age.³ The peak onset of hypolactasia related gastrointestinal symptoms occurs in adolescence and adulthood.³

Lactose intolerance results from lactose malabsorption and is primarily due to alactasia (total absence of lactase activity) or hypolactasia (low lactase activity).¹ Lactose intolerance is diagnosed when there is a symptomatic response produced in response to substantial lactose malabsorption e.g. diarrhea, abdominal discomfort, bloating.⁵ It is important to distinguish between primary lactase deficiency related to genetics and secondary causes of lactase deficiency, including coeliac disease, infectious enteritis, or Crohn’s disease, which have distinct pathogenic and therapeutic implications.⁵ Lactose intolerance can be further subdivided into four classifications depending on its origin.¹ Congenital Lactase Deficiency Congenital lactase deficiency is a very rare genetic disorder and is characterised by a total absence or significant reduction of the lactase enzyme.¹ This condition is present at birth and persists throughout the individual’s life.¹ There have only been a few dozen cases reported, mainly in Finland.¹ Developmental (Neonatal) Lactase Deficiency Developmental lactase deficiency is defined as the low level of lactase activity observed in preterm infants (28-32 weeks).¹ Lactase and other disaccharide enzymes are deficient until at least 34 weeks’ gestation and reach maximum activity at birth.¹ Primary lactase deficiency or adult hypolactasia Primary lactase deficiency results from a progressive and permanent decrease in lactase activity.¹ Approximately 70% of the world’s population have primary lactase deficiency.² The prevalence of primary lactase deficiency varies according to geographical region, ethnicity and is also related to the consumption of dairy products in the diet, resulting in genetic selection of individuals with varied capacities to digest lactose.² Although primary lactase deficiency may present with a relatively acute onset of milk intolerance, its onset typically is subtle and progressive over many years. Most lactase deficient individuals experience maximum onset of symptoms in late adolescence and adulthood.³ Secondary lactase deficiency Secondary lactase deficiency is a temporary condition that occurs when the production of the enzyme lactase is interrupted due to different mechanisms such as chronic enteropathy, atrophy of villi or other gastrointestinal diseases that can damage the brush border e.g. infections.¹ Lactase is the first disaccharidase enzyme to be compromised and the last to regenerate following damage to the gut.¹ This condition is usually reversible once the underlying pathology has been resolved resulting in restoration of normal lactase activity.⁶

Lactose intolerance is a physiological response to poorly digested and/or absorbed lactose in the gut due to lactase deficiency or inactivity. This does not involve the immune system, sticking to causing issues in a child's digestive system.⁶ In contrast, cow's milk protein allergy is an adverse immune system reaction to cows’ milk protein that can be either immunoglobulin E (IgE) or non-IgE mediated.⁶ Although their symptoms may be similar in some cases, intolerance is not the same as an allergy. An allergy can cause a severe allergic reaction and anaphylactic shock, whereas intolerances usually do not cause such a reaction.⁷ Lactose intolerance and cow's milk protein allergy are often incorrectly used interchangeably due to similarities in the gastrointestinal clinical symptoms.³ Misdiagnosis or delayed diagnosis may result in inappropriate treatment options and/or unnecessary dietary restriction.³

Similar to other food intolerances, the onset and severity of lactose intolerance symptoms is related to dose, frequency of lactose ingested and the individual’s capacity to digest it.¹ Clinical symptoms may appear between 1-3 hours after consumption, however, this has been reported to be highly variable among individuals.⁵ The most common symptoms of lactose intolerance include:

• Stomach pain and cramps
• Flatulence
• Bloating
• Diarrhoea
• Nausea
• Vomiting¹

A referral to a Paediatric Gastroenterologist may be required for cases of persistent symptoms of lactose intolerance in infants and young children. The following tests may be used to diagnose of lactose intolerance: A trial of a lactose-free diet When lactose intolerance is suspected, a lactose-free diet can be tried. During a diagnostic lactose-free diet, it is important that all sources of lactose be eliminated, requiring the reading of food labels to identify “hidden” sources of lactose. Generally, a 2-week trial of a strict lactose-free diet with resolution of symptoms and subsequent reintroduction of dairy foods with recurrence of symptoms can be diagnostic.¹ Many lactose containing foods are rich in calcium and Vitamin D, important nutrients for infants, thus an elimination diet should always be performed for a specified period, under strict medical supervision and preferably in consultation with a Paediatric Dietitian. Stool acidity test This test measures the amount of acid in the stool.  Undigested lactose fermented by colon bacteria creates lactic acid and other short-chain fatty acids that can be detected in a stool sample.¹ Fecal pH will normally be lower (5.0–5.5) in infants compared with older children and adolescents due to relative enzyme inadequacy in relation to the high amount of lactose in breastmilk.¹ Hydrogen Breath Test Measures the presence of hydrogen in the expired air. Hydrogen is produced when lactose is fermented in the colon and diffuses into the blood stream and is expired through the lungs. This test is simple, inexpensive and non-invasive but is not specific to lactose since any unabsorbed carbohydrates can cause hydrogen production.¹ Antibiotic use within one month of the test may modify the gut flora and produce false negative results.¹

No, food allergy or more widely known as ‘food hypersensitivity’ is an immune response whereas food intolerance does not involve the immune system.^3,5 Allergies are an overreaction of the body’s immune system to a normally harmless substance as if it were toxic. Such substances, known as allergens, may include certain foods, pollens, house dust, animal hair or moulds. Food intolerances describe non-immunological effects including direct pharmacologic effects of chemicals in food or metabolic defects such as lactase deficiency. It can be hard to tell the difference between symptoms of food allergy and intolerance. Generally, in sufferers with what is known as IgE mediated allergy, symptoms usually appear fairly rapidly within first consumption of the food (from immediate to less than 1 hour) and usually involve the skin, respiratory and/or gastrointestinal systems.³  Signs or symptoms include:

• Swelling of the face, eyes, tongue or throat
• Itching
• Acute hives
• Vomiting Diarrhoea
• Breathing difficulties or, in severe cases, anaphylaxis.³

Removal or reduction of lactose containing foods is advised for the management of lactose intolerance, while ensuring an adequate nutritional intake.¹ Unless there is evidence of secondary lactase deficiency, most infants with cows’ milk protein allergy can tolerate lactose.⁵ Fully breastfed infants Breastmilk is best for babies and infants with lactose intolerance, breastfeeding should be continued if possible.² In infants with congenital lactase deficiency, breastmilk or lactose-containing formula may cause persistent diarrhea and failure to thrive.² Consultation with a healthcare professional is recommended to determine if a lactose-free formula is an appropriate alternative for infants with congenital lactase deficiency.² Partially breastfed infants Lactose intolerant infants who are partially breastfed and partially bottle fed may benefit from smaller more frequent breastfeeds during the period of lactose intolerance in order to manage the lactose load. Partially formula fed infants may benefit from the use of a lactose free cows’ milk based formula. Infants who are partially breast and partially formula fed should be breastfed before the formula feed to protect the supply of breastmilk. Formula fed infants Formula fed infants with confirmed lactose intolerance may benefit from the use of a lactose free cows’ milk based formula. Feeding solids to infants and young children During periods of lactose intolerance, the provision of lactose containing foods should be avoided or minimised. Many lactose containing foods are rich in calcium and Vitamin D, important nutrients for infants, thus an elimination diet should always be performed for a specified period, under strict medical supervision and preferably in consultation with a Paediatric Dietitian.