Why can’t some people digest milk?


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Dairy is a constant part of a person’s diet (except for vegans) from the moment they are born. For the first few months of infancy, a person solely depends on BREAST MILK for their nutrition. Breast milk not only gives an infant nutrition but also imparts lifelong IMMUNITY to them. As the child grows and is exposed to the various kinds of good groups, the sweet tooth that is observed in most of the children leads them to gravitate towards dairy products such as ice cream. A popular parenting technique which promotes the practice of rewarding a child for inculcating good habits gives out the dairy products as the reward, thereby increasing the craving of the child towards them.

As the phase of childhood passes with the overload of ice cream, the teenage phase opens a person up to other kinds of dairy – such as cheese, yoghurt, and butter. Milk is usually a part of a person’s diet throughout their life as it is present in beverages such as tea and coffee. Most Indian households revere the presence of milk in their diet and deem it essential regardless of the age of the person. However, a keen observation is that as people grow older, their ability to digest dairy product reduces. This condition is called LACTOSE INTOLERANCE – in simple terms, a person cannot tolerate (or metabolise) diary.


All dairy products are preliminarily sources from milk. Milk is included in one’s diet due to its nutritional value, owed to the presence of elements phosphorus and calcium, and vitamins such as vitamin B and D. These nutrients are essential for bone health and immunity. Other than these constituents, milk contains the carbohydrate LACTOSE. Lactose is a DISACCHARIDE – made up of two sugars, GALACTOSE and GLUCOSE – is the main carbohydrate present in milk and other dairy products. Lactose intolerant individuals are unable to digest lactose.

glucose formula

How is any biomolecule metabolised in the body?

The foods we are exposed to are made up of biomolecules such as lipids, carbohydrates, and proteins. Each of these materials must be broken into simpler particles as they diverge to enter energy production processes. Each of these biologically essential molecules are metabolised by their respective ENZYMES, which are the catalysts in our bodies. For instance, lipids are broken down by LIPASES and proteins are acted upon by PEPSIN.


Taking the example of pepsin, the respective protein lodges itself in the ACTIVE SITE of the enzyme and the resultant product is generated. This is the mechanism of action for nearly all the enzymes which are involved in metabolism. Lactose – which is present in milk and dairy products – has the same fate of being broken down into its constituents by the enzyme LACTASE. Lactase is an enzyme secreted by the small intestine and targets lactose to produce two molecules of glucose.


Why are some people unable to digest lactose?

Lactose intolerance is generally experienced by people who are of the Asian, African, and Latin American descent. It is common for people in these regions to experience symptoms such as bloating, nausea and diarrhoea when milk is consumed. These symptoms are brought by the absence or low production of the enzyme lactase. When milk is consumed, in the absence of lactase, the lactose is accumulated in the COLON. Due to the conditions of the colon, the sugar undergoes fermentation and produces a gas. This gas gives rise to the symptoms experienced by the majority of the lactose-intolerant individuals. About 60% of the population will experience intolerance to lactose at least once in their life.


The reason for this goes down to the genetics of the person. It has been found that people who can digest lactose possess a MUTATED gene. A gene codes for a protein – since enzymes are also proteinaceous, genes code for enzymes as well. In lactose tolerant individual, lactase is coded for by a mutated gene. This mutation came about due to the GENETIC ADAPTATION of humans to lactose.

Researchers have traced the lineage of the mutated gene to the Bronze Age, which was about 20,000 years ago. Biologically, infants are required to drink milk only from their mother’s breasts and initially, the levels of lactase in the body would rapidly decline after the infants stop feeding on the breast milk. However, since humans chose to consume the milk provided by other animals, an adaptation on the level of genetics was essential.

Hence, the genes coding for lactase have been switched on and active even years after switching to animal-derived milk. This is a consequence of evolution and biologically, consumption of milk was not essential for humans after a certain age. The production of lactase declines after a certain age but it is unpredictable to estimate when. A general observation is that people over the age of 50 claim difficulties in digesting milk due to unavailability of lactase in their body.

Scientists first thought whether the people with lactose possessed the mutated gene. However, studies revealed that the individuals who can digest lactose harbour the mutated genes. This debunked the myth of lactose intolerance being a ‘disease’ – which is why scientists are trying to popularise the term LACTOSE PERSISTENCE instead of intolerance, therefore referring to the mutants.


Does lactose intolerance need to be treated?

The simple answer to the question posed above is no. Since lactose intolerance is not a disease, it need not be treated. However, dietary restrictions and precautions must be undertaken to avoid the symptoms of the same. Various alternatives exist for milk – the most popular being almond milk or soy milk, which do not contain lactose. These plant-based milk can also be used to make desserts such as cake or ice cream.

Whether a person is lactose tolerant or persistent, there is a way of life comfortable for both. The lactose-intolerant individuals can choose the alternatives of diary and live peacefully, while the lactose persistent individuals are labelled with being mutants. Either way, the modern world can comfortably accommodate both kinds.

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