Milk Makes Me Sick

1. Test your glucose test strips with positive and negative controls. Follow the directions for your brand of strip, and dip one strip into a glucose solution (your positive control) and another into water (the negative control). Wait as instructed on the box, note any color changes, and compare them to the key provided on the bottle to determine the glucose concentration of each fluid. If the strips indicate that the glucose solution has no glucose in it or registers far less than 2%, the strips are defective—throw them out.
2. Pour about 1/2 inch each of regular milk (about 3 mL) and lactose-free milk into separate test tubes. Have a friend label the tubes A and B to conceal the identity of the fluids from you. Make sure they write down which one is which!

Follow the directions on the bottle to determine the glucose concentration of tube A using a glucose test strip: Dip a test strip and then compare the color of the strip with the color-coded key on the side of the bottle.

Use a fresh test strip to determine the glucose concentration in tube B. Follow the directions on the bottle and compare the color of this test strip with the color-coded key on the side of the bottle.

Record the results on a piece of paper. Add one lactase drop to the liquid in tube A. Warm the tube by rolling it between your hands for two minutes. Repeat the glucose test for tube A with a fresh test strip. Is the glucose concentration in tube A the same or different after adding the lactase?

Add one lactase drop to tube B. Warm the tube as well by rolling it between your hands for two minutes. Repeat the glucose test for tube B with a fresh test strip. Is the glucose concentration the same or different after adding the lactase?

Explain your results. Which of the tubes had glucose before the drops? Which of the tubes had glucose after the lactase?

What if you introduced glucose to the milk as part of this experiment? You need one more test to rule this out! Use a glucose strip to test for the presence of glucose in the lactase drops. This will provide another piece of evidence that the glucose was or wasn't created in the tube.

Lactose is milk sugar. It is composed of two molecules of simple sugars—glucose and galactose—that are chemically bonded together. The enzyme lactase breaks down lactose into glucose and galactose, which are easily digested by humans.

Regular milk contains a high concentration of lactose but no glucose, so whichever tube had regular milk should have had a negative reaction with the glucose test strip. Lactaid—lactose-free milk—has been treated with lactase, which breaks down the lactose into glucose and galactose. The tube that the test strip indicated had a high concentration of glucose must therefore be the Lactaid. After adding the lactase drop, the tube of regular milk should show that there is glucose, since the lactase broke down the lactose into glucose and galactose while you were warming the tube.

Most human infants produce ample quantities of lactase for milk digestion. However, in the vast majority of adults, the gene which specifies production of lactase is turned off and these individuals cannot digest lactose—they are lactose intolerant.

The vast majority of the world’s people are lactose intolerant, including American Indians, Inuits, most sub-Saharan African peoples, Mediterranean and Near Eastern groups, people of Indian, Southeastern and East Asian descent, and Pacific Islanders. In general, only Northern Europeans, who drink a lot of milk, seem to have a high degree of lactose tolerance in the adult population. Lactose tolerance is strongly correlated with those whose ancestry included dairying as a means of subsistence. Many Americans are lactose tolerant, due to mixing of ethnicities.

This activity works well for the classroom. Give each pair or group of students three test tubes and a minimum of six glucose test strips (it’s a good idea to have additional test tubes and glucose test strips available for further experimentation).

Cover the label of the lactase drops with paper or colored tape to hide the identity of these “mystery” drops from your students. Follow the instructions above but don’t reveal what the mystery drops are until the experiment is completed.

Tabulate all the results on the board so everyone can see them. Acknowledge that small variations between groups may exist (for example, most strips require reading at an exact time after dipping; if students do not follow the instructions exactly, results may vary slightly). Small variations are okay, as long as your initial controls were tested correctly.

HHMI BioInteractive. The Making of the Fittest: Got lactase? The Co-Evolution of Genes and Culture. Background information, videos, additional classroom explorations and more.

Gerbault, Pascale et al. Evolution of lactase persistence: an example of human niche construction. Philos Trans R Soc Lond B Biol Sci. 2011 Mar 27; 366(1566): 863–877.

Durham, William H. Coevolution; Genes, Culture and Human Diversity. Stanford University Press, Stanford, CA. 1991. Of particular interest is chapter 5, "Cultural Mediation: The Evolution of Adult Lactose Absorption" which gives an in-depth discussion of the biology of lactose malabsorption, distribution of absorbers and malabsorbers and theories to explain the phenomenon.