Some people are lactose intolerant. Milk has long been thought to cause unpleasant symptoms, such as stomach discomfort and urticaria (clusters of acne in multiple parts of the body), caused by lactose intolerance or allergies to milk protein. In infants, the two reactions can coexist. Milk is the first foreign protein source for infants and is a very important source of nutrition. Therefore, dairy products can not be easily excluded from infant diet.
Lactose intolerance is due to the human body does not have enough capacity to digest a considerable amount of lactose (the most important sugar in milk). The main cause of lactose intolerance is the lack of lactase secreted by small intestinal cells; lactase breaks down the sugar in milk into simple forms and can be absorbed by blood. When there is not enough lactase to digest lactose, there is usually no real danger, but it can be very uncomfortable. In people who are considered lactose intolerant, not all people who lack lactase show symptoms. Common symptoms include nausea, cramps, swelling, bloating, and diarrhea, often occurring 30 minutes to 2 hours after taking foods containing lactose. The severity of the symptoms varies with individual tolerance to lactose.
The lactose + water = galactose + glucose lactase reaction model ((Figure)) some causes of lactose intolerance have been known. For example, some digestive diseases and small intestine injuries can reduce the production of lactase. It is rare that a child is born without the ability to produce lactase. For most people, lactase deficiency develops over time. After about two years old, the body can produce a small amount of lactase. However, many people do not show symptoms of lactase deficiency until they are very old.
Although the majority of Nordic people can produce enough lactase, there is a widespread lack of lactase in the Middle East, India and some African people and their descendants around the world. About 70% of adults have varying degrees of lactose intolerance. In most European countries, 5% of white people and a larger proportion of other ethnic groups have lactase deficiency. For different people, the amount of milk and dairy products that cause allergies varies. Many people with low intestinal lactase activity don't feel uncomfortable drinking a glass of milk. Cheese with low lactose content and fermented yoghurt generally do not cause intolerance. This is why some dairy products, such as yoghurt, are widely consumed in areas of the world where lactase deficiency is common.