My father is lactose intolerant. This means that his body no longer produces the enzyme lactase, which breaks down lactose, the sugar in milk. This is incredibly inconvenient and limiting for him. If he consumes pizza, mac and cheese or buttery cookies he has some serious digestive problems that leave him isolated in the bathroom for a few hours. Diarrhea and fatigue are part of the equation. His condition is mitigated to some extent by taking lactase pills but carrying the pills around and remembering to take them are a real hassle.
I wondered why my father has this condition while most of the people I know do not. My mother doesn’t. Most of my friends don’t. My sister and I don’t. In school, we had been using Google Scholar to look up scientific articles. So, I inquired online and here’s what I learned.
Ten thousand years ago, everyone had lactose intolerance. Human beings were not designed to drink milk after early childhood, so after the age of three or so the production of the lactase stopped. So, in order to consume dairy, people had to ferment it. The fermentation process converts lactose to lactic acid (Battcock, 1998).
Then, certain people domesticated livestock such as cattle or camels in Europe, East Africa, and the Middle East. Those populations evolved to be able to digest milk during their entire life, not just early childhood.
Lactose tolerance gave populations that had it a huge competitive advantage. Fermenting milk eliminates between 20 to 50 percent of its calories. With the ability to digest milk, lactose tolerant people could access much more energy than their competitors. So, following the rules of evolution, more and more people had lactose tolerance over time (Smithsonian Magazine, 2009).
Currently, more than 90 percent of humans on the planet have some form of lactose intolerance. The degree to which people are lactose tolerant depends on which gene variants and the number of copies of those genes they possess. Some people, mostly of African, Asian, or Mediterranean descent, are not able to digest lactose at all (Smithsonian Magazine, 2009).
“BACTERIAL FERMENTATIONS.” Fermented and Vegetables. A Global Perspective. Chapter 5., https://www.fao.org/3/x0560e/x0560e10.htm.
Magazine, Smithsonian. “Lactose Tolerance and Human Evolution.” Smithsonian.com, Smithsonian Institution, 7 Apr. 2009, https://www.smithsonianmag.com/arts-culture/lactose-tolerance-and-human-evolution-56187902/.
Evolutionary Anthropology 