Question:  I grew up in South Korea, and one thing that struck me as interesting once I arrived in the United States for college is the abundance of choice for milk. Wherever I go to Wal-Mart or Target, I can easily choose lactose-free milk. Also, at Starbucks, people choose milk as low or lactose free. What is lactose free milk and how does lactose in our body work?

Answer:  Normally, a glass of milk contains 11g of lactose. The human body breaks down the lactose, a disaccharide into its monosaccharide components, galactose and glucose, during the digestion process. The enzyme called lactase in the intestine is an integral part of this degradation. To be specific, intestinal lactases stick to the brush border inside the intestine and digest lactose into two sugars – galactose and glucose (Figure 1); thus, the two sugars can be absorbed into our body effectively. However, for the people who lack intestinal lactase, the lactose cannot be fully degraded and moves to large intestine from small intestine. Consequently, the bacteria in large intestine ferment the non-degraded lactose. During the process, the bacteria make gas; as a result, the people have some kind of uncomfortable feeling and diarrhea due to this process. If the sugar can be absorbed effectively in small intestine, people will not feel uncomfortable. Since, people have different amount of lactase in their body as children or adolescents, milk companies make various types of milk. Interestingly, the amount of lactase highly depends on the ethnicity. Only 5% of Caucasians have lactose intolerance and the African or Latino population has a higher percentage of lactose intolerance. Over 90% of East Asians have lactose intolerance (Figure 2). Genetically, the reason why ethnicity shows different lactose intolerance is that a mutation occure few times followed by natural selection for the persistence of the lactase gene (Wade, 2007). “As late as 5000 BC most ancient Europeans could not have digested milk as adults and that they only later evolved into milk-drinking societies (Understanding Evolution, University of California at Berkeley, 2007).” Aside from choosing lactose free milk, another way to prevent the lactose intolerance is a lactase digestive aid or taking foods that contain lactase. If you are suffering from diarrhea or uncomfortable feeling after drinking milk, lack of lactase will be the one possibility that causes the symptoms.


  • Deok Yong K.


Why are people lactose intolerant? (Super Scienced) URL:

Burger, J., Kirchner, M., Bramanti, B., Haak, W., and Thomas, M. G. (2007). Absence of the lactase-persistence-associated allele in early Neolithic Europeans. Proceedings of the National Academy of Sciences USA 104(10): 3736-3741.

Got Lactase? April 2007, University of California at Berkeley, URL:

Lactose Intolerance, MedlinePlus, U.S. National Library of Medicine, URL:

Wade, N. (2007). Lactose tolerance in East Africa points to recent evolution. The New York Times. Retrieved March 27, 2007 from