A lactose intolerance is caused by a permanent or temporary lack of the lactase enzyme which is needed in the digestion of the natural sugar in milk, known as Lactose.
This results in large amounts of undigested lactose accumulating in the colon causing gas and diarrhoea. A lactase deficiency is more common in adults, but some babies are born lacking the enzyme.
Babies are generally born with high levels of lactase enzymes in order to digest milk, which is their exclusive diet for approximately the first 6 months of their life. These levels usually stay high for the first 1-2 years of life and then the levels start to decline, nearly disappearing by adulthood in some populations. (Watson et al, 2003)
Other conditions such as Crohn’s disease and Celiac disease, both of which affect the small intestine, cause damage to the cells which produce the enzymes. Once the cells are damaged they often can no longer produce enough lactase in order to digest the lactose in the milk, resulting in a lactose intolerance. Thus a person with Celiac disease will often also have a lactose intolerance; this may recover once the lining of the gut has recovered. Damage to the lactase-producing areas in the intestine, for example parasites, can have the same effect. (Watson et al, 2003)
Note that a temporary lactose intolerance can also occur after a bout of severe diarrhoea, which will mimic a cow’s milk allergy. In this situation however the child is not allergic to the protein, but sensitive to the carbohydrate (lactose) as the enzyme (lactase) used to break it down in the body, has been diminished as a consequence of the diarrhoea. In this case stools will be watery, acidic and frothy. (Petropulos, 2011). Use of a lactose free milk for a few weeks will help while the lining of the gut recovers.
A test for lactose intolerance should be performed and, if evident, then an appropriate formula should be used under the supervision of a medical practitioner. (Petropulos, 2011) Breastmilk also contains lactose as its main sugar, so in the case of lactose intolerance in a baby, 1-2 feeds a day may have to be substituted with a lactose free formula; or in some cases lactose in the breastmilk may be reduced by giving the baby drops of lactase enzyme or by feeding in such a way as to reduce the foremilk (which is lactose rich). Always consult your medical practitioner for the best approach.
Note that a lactose intolerance is not the same as a milk allergy. In a milk allergy the body launches an immune response to one or more of the components of milk, normally a protein. A person allergic to milk is not necessarily lactose intolerant, as an intolerance results from poor enzyme activity and not an allergic response. (Watson et al, 2003)