Chapter 6.5, Problem 1ET

Health Watch Lack of an Enzyme Leads to Lactose Intolerance

If you enjoy ice cream and pizza, it might be hard for you to imagine life without these treats. However, such dairy containing treats cannot be enjoyed by much of the world’s population. Although al young children normally produce lactase (the enzyme that breaks down lactose, or “milk sugar”), about 65% of people worldwide, including 30 to 50 million people In the United States, produce less of this enzyme as they progress through childhood, a condition called lactose intolerance. In the worst cases, people may experience abdominal pain, flatulence, nausea, and diarrhea after consuming milk products (FIG. E6-2).

Why do people stop synthesizing the enzyme for this nutritious food? From an evolutionary perspective, it makes sense not to continue expending energy to produce an enzyme that has no function. In our early ancestors (who had not yet domesticated livestock), lactase lost its function in very early childhood because, after weaning, these people no longer had access to milk—the main source of lactose. As a result, many modern adults cannot digest lactose because the gene that encodes lactase is regulated by being turned off after weaning. Lactose intolerance is particularly prevalent In people of East Asian, West African, and Native American descent. Genetic studies have revealed that between 10.000 and 6.000 years ago. some people in northern Europe and the Middle East acquired mutations that allowed them to digest lactose throughout their lives. These mutations were advantageous and gradually spread because they provided better nutrition for members of agricultural societies, who could obtain milk as well as meat from their livestock. Their descendants today continue to enjoy milk, ice cream, and extra-cheese pizzas.

FIGURE E6-2 Risky behaviour? For the majority of the world’s adults, drinking milk invites unpleasant consequences.

EVALUATE THIS A family brings their 8-year-old adopted child to a pediatric clinic because she has begun to suffer from diarrhea and stomach cramps after drinking milk. What would the pediatrician suspect was the cause? If tests confirm his suspicions, what approaches would he recommend to deal with the issue? Are there dairy products that would not cause the reaction? How might they work?