Whipped cream, milk, and ice cream: delicious dairy products that up to 65% of the world’s population cannot easily digest. Is your body sensitive to lactose? Learn about the science and genetics behind lactose intolerance and check your genes for this common trait!
A (brief) summary of lactose intolerance
The sugar found in dairy products like milk is called lactose. In order for it to be used as an energy source, it must be broken down into two simple sugars: glucose and galactose. The enzyme that accomplishes this is called lactase. Without the production of lactase in the small intestines, an individual cannot digest lactose and it passes undigested to the large intestines (or colon). Here an individual's bacterial colonies, or gut microbiota, make use of the energy source, but their metabolism of lactose produces gas and other byproducts. The diminished ability to produce lactase and the symptoms of bloating, abdominal cramps, nausea, and diarrhea are referred to as lactose intolerance.
The influence of genes
DNA contains regions that code for enzymes, as well as regions that regulate genes. The lactase enzyme is produced from the LCT gene and has at least one regulator — MCM6. The MCM6 gene that is upstream from, or ahead of, the LCT gene plays a role in regulating the gene. Several genetic variants (or SNPs) in the MCM6 gene have been linked to lactose intolerance.
Global genetic varaiations
Interestingly, the SNPs related to lactose intolerance vary widely by ethnicity.
In people of European descent, two genetic variants in the MCM6 gene that have been linked to lactose intolerance. [R1]
These variations suggest that the mutations developed separately, but for similar reasons — a process called convergent evolution. In other words, the ability to digest milk was incredibly valuable for populations that relied on domesticated animals for food. In times of famine and reduced food availability, those in pastoralist communities who could digest milk were more likely to survive and produce offspring. These strong evolutionary pressures existed primarily in the pastoralist populations in Africa and Europe.
Two types of lactose intolerance - Genetic vs acquired
While pastoralist populations developed the ability to digest lactose throughout the lifespan, all infants have the ability to digest the lactose in human breastmilk.
Primary lactose intolerance is the most common type, and is determined by genes. Normally, an infant’s body produces a substantial amount of lactase to break down all the milk being consumed. As other food begins replacing milk in a child’s diet, the production of lactase decreases. This sharp decrease results in too little lactase for the amount of dairy consumed by a typical adult.
Secondary lactose intolerance on the other hand occurs when the body decreases lactase production following an injury, illness, or surgery related to the small intestine.