Even if symptoms are often similar, lactose intolerance is completely different from a milk allergy: people with lactose intolerance, also called milk intolerance, can't digest a complex sugar that is present in milk and in other dairy products, the lactose. Lactose intolerant people, indeed, lack the required enzyme, the lactase, in their intestine and, consequently, they are not able to process lactose into its elementary parts, the two simple sugars galactose and glucose, that are adsorbed from the intestine into the bloodstream.

Thus, in lactose intolerance there is not an allergic reaction to lactose but a problem caused by the digestive system, while in cow milk allergy there is an allergic reaction to cow milk proteins, triggered by the immune system.

Causes of lactose intolerance can be either genetic or environmental.

Genetic, or congenital, disorder prevents lactase from being produced by the organism. Children with this type of intolerance are never able to digest lactose, symptoms appear early in infancy and are permanent.

Environmental lactose intolerance is often related to the lactase enzyme destruction by intestinal parasites and viruses. This lactose intolerance may be permanent, when the ability to produce lactase gets lost, or temporary, if it is destroyed just the amount of intestinal lactose.

Other lactose intolerance causes occur when injury to the small intestine or certain digestive diseases reduce the amount of lactase a person produces. These diseases include celiac disease, inflammatory bowel disease, and Crohn's disease.

Symptoms characterizing lactose intolerance begin about 30 minutes to 2 hours after eating or drinking foods containing lactose and include gas, cramps, bloating, and flatulence. These manifestations are derived from activities of bacteria that are present in the lower gastrointestinal tract, that start using lactose for their metabolism.

Watery diarrhoea is another symptom of lactose intolerance: sugar accumulation in the colon tract keeps water from being adsorbed as normally, thereby causing watery diarrhoea.

Due to symptoms variety, the definitive diagnosis of lactose intolerance is difficult because there are a lot of gastrointestinal disorders and inflammations with similar manifestations. Moreover, due to the important cow's milk nutrient content, it is important to be careful in the total elimination of cow's milk from the diet, before diagnosis has been ascertained. A definitive diagnosis can be performed through measuring the absorption of lactose in the digestive system.

In infants and young children, diagnosis is performed by the Stool Acidity Test, measuring the amount of acids and glucose in their stool. In fact, gastrointestinal bacteria brake lactose in lactic acid and other fatty acids, that are eliminated with stools.
Other tests, not suitable for infants that can't drink too much lactose, are the Lactose Tolerance Test and the Hydrogen Breath Test.

As for Cow Milk Allergy, the main treatment of lactose intolerance is a voiding lactose-containing products in the diet, being careful to keep a balanced nutrition, especially because milk is normally a major source of calcium, an essential element for bone growth: the American Dietetic Association recommended calcium requirements are 400 mg/day for 0–6 months infants and 600 mg/day for a 6–12 months old child. Milk is important also for potassium intake.

In some cases, when lactose avoidance is not possible, you can ask your doctor about enzymatic lactase supplements, that are lactase enzymes similar to those produced in the small intestines of humans but produced industrially by fungi or yeasts.

Source: the National Digestive Diseases Information Clearinghouse