Lactose is the sugar making up around 2-8% of the solids in milk. The name comes from the Latin for milk, plus the -ose ending used to name sugars. Lactose is a disaccharide consisting of two subunits, a galactose and a glucose linked together. Its empirical formula is C12H22O11 and its molecular weight is 342.3. In the young of mammals, an enzyme called lactase is secreted by the intestinal villi, and this enzyme cleaves the molecule into its two subunits for absorption.
Normally, as the young grow up, production of lactase gradually ceases, and they are then unable to metabolise lactose. This is perhaps an evolutionary mechanism to enforce weaning of the young. This loss of lactase on maturation is also the default pattern in most of the human race. However, in some humans (mostly those with ancestry in the approximate geographic region of Europe, the Middle East and India), the enzyme is retained in adulthood, and dairy products form a substantial part of the adult diet. It would appear that millennia of animal husbandry in those regions, with the milking of sheep, cattle, goats and water buffalo, has caused an evolutionary adaptation to an adult diet containing milk. (This process of retaining infant characteristics into adulthood is one of the simplest routes of evolutionary adaptation, and is known as neoteny) The fact that at least some humans have made adaptations to lactose in the adult diet would, incidentally, appear to cast doubt on some arguments by proponents of the so-called 'Stone-age diet', who argue that human metabolic needs have not changed since the last ice age.
What is lactose intolerance?
Lactose intolerance is the inability to digest significant amounts of lactose, the major sugar found in milk. Lactose intolerance is caused by a shortage of the enzyme lactase, which is produced by the cells that line the small intestine. Lactase breaks down milk sugar into two simpler forms of sugar called glucose and galactose, which are then absorbed into the bloodstream. Not all people deficient in lactase have the symptoms commonly associated with lactose intolerance, but those who do are said to have lactose intolerance.
People sometimes confuse lactose intolerance with cow’s milk intolerance because the symptoms are often the same. However, lactose intolerance and cow’s milk intolerance are not related. Being intolerant to cow’s milk is an allergic reaction triggered by the immune system. Lactose intolerance is a problem caused by the digestive system.
What are the symptoms of lactose intolerance?
People who do not have enough lactase to digest the amount of lactose they consume may feel very uncomfortable when they digest milk products. Common symptoms, which range from mild to severe, include nausea, cramps, bloating, gas, and diarrhea. Symptoms begin about 30 minutes to 2 hours after eating or drinking foods containing lactose. The severity of symptoms depends on many factors, including the amount of lactose a person can tolerate and a person’s age, ethnicity, and digestion rate.
How is lactose intolerance diagnosed?
Lactose intolerance can be hard to diagnose based on symptoms alone. People sometimes think they suffer from lactose intolerance because they have the symptoms associated with the disorder, not knowing other conditions such as irritable bowel syndrome can cause similar symptoms. A doctor can use tests to diagnose lactose intolerance but may first recommend eliminating cow’s milk from the diet to see if the symptoms go away.
The most common tests used to measure the absorption of lactose in the digestive system are the lactose tolerance, hydrogen breath, and stool acidity tests.
The lactose tolerance and hydrogen breath tests are not given to infants younger than 6 months of age. A large lactose load can be dangerous prior to this age, as infants are more likely to become dehydrated from diarrhea that can be caused by lactose intolerance.
What causes lactose intolerance?
Some causes of lactose intolerance are well known. Primary lactase deficiency is a condition that develops over time. After about age 2 the body begins to produce less lactase, though most people will not notice symptoms until they are much older.
Secondary lactase deficiency occurs 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.
Researchers have identified a genetic link for lactose intolerance. Some people are born with a likelihood of developing primary lactase deficiency because it has been passed to them genetically (inherited from their parents). This discovery may be useful in developing a diagnostic test to identify people with the condition.
Who is at risk for lactose intolerance?
Between 30 and 50 million Americans are lactose intolerant and certain ethnic and racial populations are more affected than others. Up to 80 percent of African Americans, 80 to 100 percent of American Indians, and 90 to 100 percent of Asian Americans are lactose intolerant. The condition is least common among people of northern European descent.
Babies that are born prematurely are also more likely to be lactose intolerant, because lactase levels do not increase until the third trimester of a woman’s pregnancy.
How is lactose intolerance treated?
Lactose intolerance is easy to treat. No treatment can improve the body’s ability to produce lactase, but symptoms can be controlled through diet.
Young children and infants with lactase deficiency should not consume lactose-containing formulas or foods until they are able to tolerate lactose digestion. Most older children and adults do not have to avoid lactose completely, but people differ in the amounts and types of foods they can handle. For example, one person may have symptoms after drinking a small glass of milk, while another can drink one glass but not two. Others may be able to manage ice cream and aged cheeses, such as cheddar and Swiss, but not other dairy products. People can also tolerate more lactose by having smaller amounts of it at one time. The level of dietary control needed with lactose intolerance depends on how much lactose a person’s body can handle.
For those who react to very small amounts of lactose or have trouble limiting their intake of foods that contain it, the lactase enzyme is available without a prescription to help people digest foods that contain lactose. The tablets are taken with the first bite of dairy food. Lactase enzyme is also available as a liquid. Adding a few drops of the enzyme makes lactose more digestible for people with lactose intolerance.
Lactose-reduced milk and other products are available at most supermarkets. The milk contains all of the nutrients found in regular milk and remains fresh for about the same length of time, or longer if it is super-pasteurized.
How is nutrition balanced?
Milk and other dairy products are a major source of nutrients in the American diet. One of the most important of these nutrients is calcium. Calcium is essential for the growth and repair of bones throughout life. In the middle and later years, a shortage of calcium may lead to thin, fragile bones that break easily, a condition called osteoporosis. A concern for both children and adults with lactose intolerance is getting enough calcium in a diet that includes little or no dairy products.
The Institute of Medicine released a report listing the requirements for daily calcium intake. How much calcium a person needs to maintain good health varies by age group. Recommendations from the report are shown in the following table.
In addition, pregnant and nursing women need between 1,200 and 1,500 mg of calcium daily.
In planning meals, people with lactose intolerance should make sure that each day’s diet includes enough calcium, even if dairy products are not included. Many non-dairy foods are high in calcium, including dark green vegetables such as broccoli, or fish with soft, edible bones, such as salmon and sardines. To help in planning a high-calcium, low-lactose diet, the table that follows lists some common foods that are good sources of dietary calcium and shows how much lactose they contain.
Recent research shows that yogurt with active cultures may be a good source of calcium for many people with lactose intolerance. Even though yogurt is fairly high in lactose, the bacterial cultures used to make it produce some of the lactase enzyme required for proper digestion.
Clearly, many foods can provide the calcium and other nutrients the body needs, even when intake of milk and dairy products is limited. However, factors other than calcium and lactose content should be kept in mind when planning a diet. Some vegetables that are high in calcium (Swiss chard, spinach, and rhubarb, for example) are not listed in the chart because the body cannot use the calcium they contain because these foods also contain substances called oxalates, which stop calcium absorption.