For a long time, cow's milk has been known to be a source of unpleasant symptoms such as gastric upsets and urticaria (formation of groups of acne buttons on several places on the body). The reactions are due to lactose intolerance or allergy to milk protein. In babies, both food reactions can co-exist. Milk is a baby's first foreign protein source, and is a very important source of nutrients for babies; thus, it cannot be removed easily from infant diets.

Lactose intolerance is the inability to digest significant amounts of lactose, the predominant sugar of milk. This inability results from a shortage of the enzyme lactase, which is normally produced by the cells that line the small intestine. Lactase breaks down milk sugar into simpler forms that can then be absorbed into the bloodstream. When there is not enough lactase to digest the amount of lactose consumed, the results, although not usually dangerous, may be very distressing. While not all persons deficient in lactase have symptoms, those who do are considered to be lactose intolerant.

Common symptoms include nausea, cramps, bloating, gas, and diarrhoea, which begin about 30 minutes to 2 hours after eating or drinking foods containing lactose. The severity of symptoms varies depending on the amount of lactose each individual can tolerate.

Mode of action of lactase (from www.indiana.edu/~ensiweb/ lessons/tp.milk3.html)

Some causes of lactose intolerance are well known. For instance, certain digestive diseases and injuries to the small intestine can reduce the amount of enzymes produced. In rare cases, children are born without the ability to produce lactase. For most people, though, lactase deficiency is a condition that develops naturally over time. After about the age of 2 years, the body begins to produce less lactase. However, many people may not experience symptoms until they are much older.

Although most people of northern European descent produce enough lactase throughout their lives, lactase deficiency is common among people from the Middle East, India and parts of Africa and their descendants in other parts of the world. Around 70 percent of adults have some degree of lactose intolerance. In Europe in most countries lactose deficiency is present in about five percent of white people and a much larger proportion in other ethnic groups.

The quantity of milk and dairy products that will lead to symptoms of intolerance varies widely between individuals. Many individuals who have low intestinal lactase activity can drink a glass of milk without experiencing discomfort. Hard cheeses, which are low in lactose, and fermented milk products such as yoghurt, are usually well tolerated. This could explain why cultured milk products such as yoghurts are widely consumed in areas of the world where lactase deficiency is common

Diagnosis

The most common tests used to measure the absorption of lactose in the digestive system are the lactose tolerance test, the hydrogen breath test, and the stool acidity test. These tests are performed on an outpatient basis at a hospital, clinic, or doctor's office.

The lactose tolerance test begins with the individual fasting (not eating) before the test and then drinking a liquid that contains lactose. Several blood samples are taken over a 2-hour period to measure the person's blood glucose (blood sugar) level, which indicates how well the body is able to digest lactose.

Normally, when lactose reaches the digestive system, the lactase enzyme breaks it down into glucose and galactose. The liver then changes the galactose into glucose, which enters the bloodstream and raises the person's blood glucose level. If lactose is incompletely broken down, the blood glucose level does not rise and a diagnosis of lactose intolerance is confirmed.

The hydrogen breath test measures the amount of hydrogen in a person's breath. Normally, very little hydrogen is detectable. However, undigested lactose in the colon is fermented by bacteria, and various gases, including hydrogen, are produced. The hydrogen is absorbed from the intestines, carried through the bloodstream to the lungs, and exhaled. In the test, the patient drinks a lactose-loaded beverage, and the breath is analyzed at regular intervals. Raised levels of hydrogen in the breath indicate improper digestion of lactose. Certain foods, medications, and cigarettes can affect the accuracy of the test and should be avoided before taking it. This test is available for children and adults.

Lactose content in milk

All milk of animals contains lactose. Soy-milk, which is actually a juice, does not contain lactose. However, it contains other sugars that sometimes can cause similar symptoms in lactose intolerant people.

Average lactose content of different animal milks (in gram per 100 ml, sorted from high to low lactose content)

Unlike people suffering from cow's milk allergy, lactose intolerant people should avoid all animal milks. The use of goat milk does not reduce the symptoms.

Hidden lactose

Although milk and foods made from milk are the only natural sources, lactose is often added to prepared foods. People with very low tolerance for lactose should know about the many food products that may contain even small amounts of lactose, such as

• bread and other baked goods
• processed breakfast cereals
• instant potatoes, soups, and breakfast drinks
• margarine
• lunch meats (other than kosher)
• salad dressings
• candies and other snacks
• mixes for pancakes, biscuits, and cookies
• powdered meal-replacement supplements

Some products labelled non-dairy, such as powdered coffee creamer and whipped toppings, may also include ingredients that are derived from milk and therefore contain lactose.

Smart shoppers learn to read food labels with care, looking not only for milk and lactose among the contents, but also for such words as whey, curds, milk by-products, dry milk solids, and non-fat dry milk powder . If any of these are listed on a label, the product contains lactose.

In addition, lactose is used as the base for more than 20 percent of prescription drugs and about 6 percent of over-the-counter medicines. Many types of birth control pills, for example, contain lactose, as do some tablets for stomach acid and gas. However, these products typically affect only people with severe lactose intolerance.