Only a small number of human beings live like this today.
But from the time our species evolved some 200,000 years ago until the not too distant past, all of us lived as hunter gatherers.
Then, around 10,000 years ago, people started domesticating animals for food, living in settlements and cultivating crops. These cultural changes have profound biological impacts on our species. And you're about to encounter one surprising example. It has to do with the familiar food.
I'm talking about milk, the main ingredients, some of our favorite things. Almost all of us can digest it as babies. But the story of how many adults can use it as a food is a fascinating case study. A study of the coevolution of human culture and biology.
All infant mammals can digest milk. In fact, producing milk for babies is a key trait that distinguishes mammals from all other types of animals.
The main sugar in milk, lactose can't easily pass through the intestinal wall, so cells here make an enzyme called lactase, which breaks lactose into glucose and galactose.
These two simpler sugars can then enter the bloodstream where they can be used for energy.
Around the time young mammals stop drinking milk. Almost all of them stop making lactase so they lose their ability to digest milk. They become lactose intolerant. What typically happens when an adult mammal drinks milk? It's not pretty. The lactose goes undigested straight through the small intestine to the large intestine. Here, bacteria eat the sugar and can cause cramps, gas, and diarrhea.
It's a bad idea to offer a bowl of milk to an adult cat.
We only know of one mammal species in which some adults can drink milk without getting sick. Yes, it's us. Not all of us, but worldwide, about a third of adults can digest lactose. This minority is called lactase persistent because their ability to produce the enzyme that breaks down lactose persists beyond childhood and in fact, throughout their lives. How did lactase persistence come about? Why does it occur only in some people?
I've come to University College London to start my quest to find out.
Geneticist Dallas Swallow will show me how to figure out whether someone can digest the sugar in milk. You're going to do a, uh, lactose tolerance test. I am. The idea is to look to see what the level of glucose is in the blood of the volunteer before the lactose load has been taken of the blood of. After measuring my baseline glucose level. Okay. I now have to chug a liter of milk. You're allowed to breathe in between. It's all right. If my body is still making lactase, my blood glucose will shoot up. After I drank the milk. Here's what happened.
No doubt about it. My lactase enzyme is still working. Where do your family come from? Britain. On my father's side. Um, Denmark. Holland on my mother's side, but kind of. Northern Europe. Northern Europe. Okay. You can see, first of all, that most people in Europe are lactase persistent. My family background makes sense.
In only a few regions is a large majority of people lactase persistent?
In other parts of the world, few adults easily digest lactose.
What exactly is different about people who are lactase persistent?
To get a clue. Researchers looked at DNA. They first compared the part of the lactase gene that encodes the enzyme across persistent and non persistent people. They didn't find a change in the DNA to distinguish the two traits. So what could explain the difference? We know that genes, including lactase, are regulated, turned on or off, dialled up or down by other pieces of DNA that act like switches. In search of a possible mutation in a lactase switch. A research team identified Finnish families that had members who were lactase persistent, as well as those who weren't.
Statistical geneticist Joe Terwilliger was part of the team. We then looked to see if they shared DNA around the region where the gene was that we knew was affecting the metabolism of lactose. On chromosome two. In and around the lactase gene. A number of shared markers in the DNA allowed Terwilliger and his colleagues to hone in on a segment of DNA likely to contain the lactase persistence mutation.
By comparing this segment based by base across lactase persistent and non persistent individuals, they discovered the critical one base difference a T instead of a C at one non-coding position. The researchers had made an important discovery. They found a mutation that causes lactase persistence in Finns and other Europeans. Do all lactase persistent people carry this mutation? I thought that would be one mutation and that would be it. So we went off, uh, to study, uh, samples from Africa. And to our surprise, we found that the mutation barely existed. Was a different mutation at work on this continent. Then a young professor, geneticist Sarah Skov, traveled to a number of African countries to find out. We've now looked at Tanzania, Kenya and Sudan and Ethiopia. And so we've really looked at a broad range of
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Activity Student Handout
Blood Glucose Data Analysis
INTRODUCTION
Although we can easily digest milk as babies, most of us lose this ability as adults. This is because we usually stop producing lactase, an enzyme that breaks down the main sugar in milk. However, about one-third of people worldwide still produce lactase as adults. In this activity, you will learn about a test for whether someone is likely to produce lactase. This test is based on the amount of a sugar called glucose in a person’s blood.
MATERIALS
• ruler • colored pencils
BACKGROUND
Milk is packed with proteins, fats, and carbohydrates that support the growth, development, and survival of baby mammals. The main carbohydrate in milk is a sugar called lactose. To digest milk, lactose must be cleaved, or broken down, by lactase, an enzyme produced in the small intestine. Lactase cleaves lactose into two smaller sugars, glucose and galactose, which are easily absorbed through the walls of the small intestine. Once these sugars are absorbed into the bloodstream, they can be delivered to the cells of the body and used for energy.
Figure 1. A diagram showing how the lactase enzyme cleaves the sugar lactose. As baby mammals grow up and stop drinking their mother’s milk, their bodies usually stop producing the lactase enzyme (presumably because it is no longer needed). Individuals that do not produce lactase as adults are called lactase nonpersistent. Most mammals are lactase nonpersistent and do not drink milk as adults. Humans are unusual in that some adults continue to drink milk from other mammals, such as cows.
When an individual who is lactase nonpersistent drinks milk, they cannot easily break down the lactose in the milk. The lactose passes from their small intestine to their large intestine, where it is fermented by bacteria. Fermentation produces various gases in the large intestine, which can cause abdominal pain, bloating, flatulence, and diarrhea — all symptoms of lactose intolerance, the inability to digest lactose. Most adults are lactase nonpersistent and thus typically lactose intolerant (although some may not know it because their symptoms are mild). However, about 35% of the global human population continues to produce lactase into adulthood. These individuals are called lactase persistent and are typically lactose tolerant, meaning that they can digest lactose easily and drink milk without problems.
Activity Student Handout Blood Glucose Data Analysis
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PROCEDURE
There are several ways to test whether someone is lactase persistent or nonpersistent. One method, the blood glucose test, is shown in the short film Got Lactase? The Co-evolution of Genes and Culture. Table 1 shows the blood glucose levels of the film’s narrator, Spencer Wells, and six other individuals over time. The glucose levels were measured using glucose strips and a glucose reader similar to the one in the film. After baseline levels (i.e., the ones at “0 minutes”) were measured, each person drank a liter of milk. Their blood glucose levels were measured again at 15, 30, 45, and 60 minutes after drinking the milk. Table 1. Blood glucose test results for seven adults tested in Sarah Tishkoff’s laboratory.
Blood Glucose (mg/dL)
Individual 0 minutes 15 minutes 30 minutes 45 minutes 60 minutes
Spencer Wells 117 128 146 160 152
Peter 97 111 135 154 143
Rachel 96 99 105 101 98
Katherine 95 97 99 101 102
Sarah 108 116 129 141 139
Michael 94 109 128 143 140
Arthur 97 96 94 83 88
1. Plot the data in Table 1 on the graph below, which already includes the results for Spencer Wells. Add more entries to the legend as needed.
80
90
100
110
120
130
140
150
160
0 10 20 30 40 50 60
Bl oo
d G
lu co
se (m
g/ dL
)
Time (minutes)
Blood Glucose Test Results
Spencer Wells
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After graphing the data, answer the following questions.
2. Why might someone’s blood glucose levels after drinking milk indicate their lactase activity?
3. Divide the individuals in Table 1 into two groups (A and B) based on their blood glucose test results. Write the names of the individuals in each group, including Spencer Wells, below.
Group A:
Group B:
4. Explain your rationale for dividing the individuals into these two groups. Use data from your graph to support your answer.
5. Based on these data, do you think the individuals in Group A are lactase persistent or nonpersistent? Describe the evidence that supports your answer.
6. Based on these data, do you think the individuals in Group B are lactase persistent or nonpersistent? Describe the evidence that supports your answer.
7. If the blood glucose test was performed on people from the Maasai population in Kenya, would their results be more like those of the individuals in Group A or Group B? Explain your prediction. (Hint: As discussed in the film Got Lactase? The Co-evolution of Genes and Culture, the Maasai traditionally raise cows for food.)
8. A person taking a blood glucose test is usually told to fast (i.e., to not eat or drink anything but water) before the test. Why do you think that might be necessary?
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EXTENSION: The Hydrogen Breath Test
Another common way to test whether a person is lactase persistent or nonpersistent is the hydrogen breath test. This test uses the amount of hydrogen in a person’s breath to check for lactose fermentation. As described in the “Background” section, undigested lactose is fermented by bacteria in the large intestine. Fermentation produces several gases, including hydrogen, that can exit the body through the anus. These gases can also be absorbed into the blood, circulated to the lungs, and eliminated through the breath.
Figure 2. A diagram comparing what happens to lactose in the intestines of individuals with and without the lactase enzyme. Table 2 shows the levels of hydrogen in the breath of four adults tested for lactase persistence. As in Table 1, the measurements at “0 minutes” represent baseline levels before drinking milk. The other measurements were taken at various times after drinking milk.
Table 2. Hydrogen test results for four adults.
Hydrogen Breath Levels (ppm)
Individual 0 minutes 30 minutes 60 minutes 90 minutes 120 minutes
Lisa 5 6 9 8 5
Dan 4 9 8 29 35
Cindy 6 8 10 31 32
Brian 4 7 6 9 6
Activity Student Handout Blood Glucose Data Analysis
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1. Create your own graph of the data in Table 2. Your graph should include a title, labels for the x- and y-axes, and a legend.
After graphing the data, answer the following questions.
2. Which individuals in Table 2 are likely to be lactase persistent? Use data from your graph to support your answer.
3. Which individuals in Table 2 are likely to be lactase nonpersistent? Use data from your graph to support your answer.
4. Think of another type of test to determine whether a person is lactase persistent or nonpersistent. Describe your idea in one or two sentences.
- introduction
- Materials
- Background
- Procedure
- Extension: The Hydrogen Breath Test
- Extension question 4:
- Extension question 3:
- Extension question 1:
- Extension question 2:
- question 3 group A:
- question 3 group B:
- question 2:
- question 4:
- question 5:
- question 6:
- question 7:
- question 8:
