The most common enzymatic reaction is intolerance to lactose, generally inherited and widespread in Asia and in some American regions. In Europe it is more frequent in the Mediterranean countries, in Italy for example and less frequent in Northern Europe.

Lactose is the sugar present in milk. Before being absorbed and metabolised by the body, lactose should be divided into its main components, glucose and galactose, thus requiring an enzyme called lactase. If the lactose quantity is not enough, part of the lactose can not be digested.

A low production of lactase does not necessarily leads to an intolerance to lactose. Therefore such an intolerance can be reduced through the gradual reintroduction of food holding lactose in the diet.

Symptoms are dose-dependent: the higher the ingested lactose quantity, the more evident are the symptoms, leading to flatulence, diarrhoea, swelling and abdominal pains.

In case of a diagnosed intolerance to lactose, not always it is necessary to entirely remove any lactose-holding food from the diet. Sometimes it is simply enough to identify the tolerated lactose quantity without triggering any symptom.
If the intolerance is light, symptoms can be controlled, drinking milk during meals, or replacing fresh dairy products with fermented products, drinking products poor in lactose.

Some cheeses hold a few lactose, as Parmesan cheese, Emmental, Cheddar and Edam. If the intolerance is severe, pay attention to accurately read any product label: in fact lactose is used in many ready-to-cook food.