Parents often underestimate the seriousness of child's cows' milk allergy, thinking that CMA is just a more serious case of lactose intolerance. This is false: milk allergy and lactose intolerance are two different conditions, caused by different mechanisms and with different evolutions.

A food allergy, such as cows' milk allergy, involves a reaction, immediate or delayed, of the immune system to the protein in a particular food. Food intolerance differs in that the reaction does not involve an immune response.

Lactose intolerance is the inability to absorb lactose, the main sugar found in milk.
Lactose is a disaccharide (two sugars joined together) and is broken down in the gut by the action of the enzyme lactase - found in the upper small intestine - to its constituent monosaccharides (glucose and galactose), which can then be absorbed in the normal course of digestion.
However, some people do not have sufficient lactase to perform this digestive function. The lactose goes unabsorbed in the intestine, where the resident bacteria act upon it, causing it to ferment and produce gases, with symptoms such as pain, bloating, feeling of fullness and diarrhoea.

Lactose intolerance is quite common (approximately 70% of the world's population suffers from a lactase deficiency) and, even if typically worsens with age, it's less serious than milk allergy: some sufferers may be completely intolerant, but others may be able to tolerate small amounts of dairy products.

In a report published on September 2006⁽¹⁾, the Committee on Nutrition for the American Academy of Pediatrics (AAP) recommended that lactose intolerant kids should consume as much dairy as they can. A contradictory news, based on the fact non-dairy kids risk insufficient calcium in their diets: the AAP suggests slowly introducing dairy foods into a sensitive child's diet to determine their level of tolerance.

Children with cow's milk allergy, instead, have to completely avoid cows' milk protein, using formulas like Amino Acid-based ones to enable optimal growth and development.

Source: "Got a little milk? AAP report recommends some dairy products for lactose intolerant patients." AAP News, Sep 2006;27:12.