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Why do some people develop lactose intolerance?

Michael Blann/(c) Michael Blann

We ask the experts to settle common questions we've all wondered about.

QUESTION: Most of us drink milk when we are very young without difficulty. But why do some people run into problems later on, developing lactose intolerance?

ANSWER: Naturally found in breast milk, lactose is an important energy source for infants and provides more than half their energy needs for the first year of life. Lactase, an enzyme found on the wall of the intestines, breaks down lactose (the sugar found in milk) into galactose and glucose. The activity of lactase naturally becomes reduced after weaning (stopping breastfeeding), at which point the body no longer needs as much lactase. This reduction in lactase activity after infancy is a genetically programmed event.

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Lactose intolerance can occur if the body does not produce enough lactase to break down lactose. Symptoms include watery diarrhea, bloating, abdominal pain and excess gassiness. (Lactose intolerance is often suspected as the cause of these symptoms; however, they may be due to other gastrointestinal ailments and should be investigated by a physician.) Undigested lactose in the small intestine acts like an osmotic agent, causing water and electrolytes to be pulled into the intestines, which results in diarrhea. Digestion of the lactose by intestinal bacteria causes the bloating and gassiness.

The ability to tolerate lactose varies from person to person and is dependent on genetic and cultural factors. In fact, about two thirds of people in the world are lactose intolerant, mainly in areas where milk products are not commonly found in diets. In parts of Asia, the majority of individuals are lactose intolerant, while in South America and parts of Africa, approximately half the population is intolerant of this milk sugar.

Lactase activity is naturally reduced over time and can also be reduced transiently, after intestinal illness, which damages or disturbs the cells on the intestinal wall that produce the lactase. In the resulting secondary lactase deficiency, the ability to digest lactose eventually returns after recovery from the illness.

Treating lactose intolerance includes avoiding or decreasing intake of the foods that contain lactose, such as milk and dairy products. Some people have severe symptoms that require complete avoidance of all foods that contain lactose, while others can manage with variable amounts of these foods. If individuals are avoiding dairy, then nutrients found in these foods (like calcium, phosphorous and Vitamin D) must be replaced by other foods or supplements to prevent nutrient deficiencies.

Dietary options for people with lactose intolerance include consuming milk products that have been treated with enzymes. They can also eat and drink fermented products like yogurt or buttermilk, as the bacteria used for fermentation in these products can improve the digestion of lactose. Some individuals also benefit from lactase preparations in capsule or tablet form.

Lactose intolerance is generally manageable, and individual responses to dietary treatments can vary. Some people can tolerate a cup of milk with meals, while others can only tolerate half a cup of milk once a day. Evidence suggests introducing increasing amounts of milk and dairy products may improve lactose tolerance over time. Individuals with lactose intolerance may need to experiment with the various available options to find a healthy balance of beverages and foods that they can enjoy.

Daina Kalnins is manager of clinical dietetics at the Hospital for Sick Children in Toronto. She is the co-author of SickKids' series of books on nutrition, including Better Food for Kids.

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