What Caused Lactose Intolerance?

Author: Renee Cao

Editor: Liane Xu

Artist: Jenny Li

Around the world, about 65% of adults are unable to consume milk or milk products without having symptoms such as cramping and excessive gas. Their condition is called lactose intolerance. When someone is lactose intolerant, they are unable to digest lactose, a sugar naturally found in milk that is normally broken down by the enzyme lactase.

Enzymes are proteins that work as biological catalysts, allowing chemical reactions to occur in organisms at rates necessary for the maintenance of life. Catalysts speed up chemical reactions by lowering the energy needed to start a reaction, also known as activation energy. In the case of consuming milk, lactase allows the breakdown of lactose into the sugars glucose and galactose. Glucose is the most abundant monosaccharide (single sugar unit) in our body, and it is also an important energy source in living things. Galactose itself does not necessarily have a function, but it can be converted into glucose and serve as a cell’s source of energy.

At birth, almost all individuals produce the lactase necessary to consume the milk they need as a baby, through the small intestine. However, as we age, our lactase levels start to decrease as we start replacing milk with other foods. Other times, there may be illnesses or injuries involving the small intestine that disturb lactase production.

Luckily, scientists have developed ways that allow lactose intolerant people to enjoy milk and milk products like cheese and ice cream. Lactose-free milk can be produced using lactase from Kluveromyces Lactis, a yeast that grows naturally in milk. Biotechnological companies can extract and purify this yeast for food manufacturing companies. Lactose-free milk is also beneficial in other ways. Because galactose and glucose are sweeter than lactose, less sugar is needed when making sweet foods. Lactose crystallizes during the production of ice cream, which results in a rough and gritty texture; since galactose and glucose are more soluble than lactose, they give ice cream a smoother texture when they are dissolved.


Allott, Andrew, and David Mindorff. IB Biology Course Book: 2014 Edition: Oxford IB Diploma

Program. 2014th edition. Oxford: Oxford University Press, 2014.

“Lactose Intolerance by Country 2020.”


“Mayo Clinic Q and A: Lactose Intolerance Can Develop at Any Age.”


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