Cow milk allergy and lactose intolerance are common food hypersensitivities in infancy and young childhood.

Cow milk allergy is a food allergy, an adverse immune reaction to a food protein that is normally harmless to the non-allergic individual. Lactose intolerance is a non allergic food hypersensitivity: a reaction to food that does not involve the immune system.

Lactose is a disaccharide (two sugars joined together) that is naturally found in breastmilk and is broken down in the gut by the action of the enzyme lactase, to its constituent monosaccharides (glucose and galactose). This is essential, as lactose itself cannot be absorbed.

Lactose intolerance can be either primary (generated by a genetic predisposition) or secondary. Secondary lactose intolerance is caused by injury to the intestinal mucosa and is secondary to various gastrointestinal diseases and conditions such as gastroenteritis.

Lactose intolerance results in a number of symptoms such as bloating, abdominal pain, flatulence, diarrhoea or constipation.

Lactose intolerance is a popular but not-evidence based diagnosis in crying babies, but primary lactose intolerance at this age is almost non-existing, and secondary lactose intolerance is at this age very transient and related to infectious gastroesophagitis or caused by cow milk allergy as primary factor.