Lactose intolerance

What is lactose anyway? 

Lactose is a type of sugar found in milk (and milk derivatives such as cheese) that is broken down by an enzyme called lactase [1] Jump to reference section: [1] . Lactase is an enzyme that breaks down lactose into glucose, which serves as an important fuel for our body. It is present in the small intestine, and this is where it does its work. However, when lactase is not present it leads to lactose fermenting in your small intestine and bowels, producing hydrogen, methane and carbon dioxide. These gasses often cause bloating, diarrhoea and stomach aches. 

In the majority of mammals the ability to produce lactase, and therefore the ability to digest lactose, is lost shortly after weaning. Weaning is the process of introducing an infant mammal to its adult diet while withdrawing the supply of its mother's milk. This likely happens to allow the mother to transfer more of her energetic resources towards fertility again after her offspring has gained enough nourishment and is able to fend for itself. 

The genetics of lactose intolerance

The loss of ability to produce lactase, scientists believe, happens because of a process called ‘methylation’ [2] Jump to reference section: [2] . Methylation is when, over time, chemicals in our body attach themselves to our DNA. In doing so, these chemicals stop our cells from being able to read the instructions from our DNA correctly, and can even cause the gene to ‘switch off’.  

Many years ago, some medical groups referred to lactose intolerance as a ‘genetic fault’. But really lactose intolerance is the normal human state and 68% of humans are thought to not be able to produce lactase after weaning [3] Jump to reference section: [3] . Evolutionary geneticists uncovered that ancient humans (more than 10,000 years ago) would not have had the ability to digest lactose at all [4] Jump to reference section: [4] . But, for reasons we’ll explore, some humans have managed to keep their ability to produce lactase after weaning. This is known as ‘lactase persistence’. 

The ability to produce lactase is thought to be mostly genetic, but the ability to digest lactose can be affected by external forces. For example, those whose lactase gene (LCT) ‘switches off’ after weaning can sometimes regain the ability later in life through picking up bacteria which help to break down lactose and decrease hydrogen production, therefore alleviating some symptoms [5] Jump to reference section: [5] . On the other hand, some people with the ability to produce lactase after weaning can lose it if their small intestine becomes damaged.

Geneticists have managed to pinpoint certain variants near the lactase gene which grant lactase persistence. But they also found that many single variants appear to have popped up all over the world and that each - on their own - allows for the continued production of lactase. For example, the majority of individuals from certain populations from Europe, Saudi Arabia, and Sub-Saharan Africa all have their own separate variant that allows them to produce lactase [6] Jump to reference section: [6] ,[7] Jump to reference section: [7] ,[8] Jump to reference section: [8] . 

Why are some people lactose intolerant and some not?

Evolving the ability to digest lactose by chance is very, very rare,  so scientists started to investigate if other forces were at work that may have caused greater lactose tolerance in these populations. The answer is quite interesting! Evolutionary geneticists and anthropologists noticed that the same populations from Europe, Saudi Arabia, and Sub-Saharan Africa which were lactase persistent all had strong histories of milking [9]. However, other populations that had never adopted the practise of dairying their cattle had never evolved the trait. 

Scientists now believe that the domestication of cattle and the process of milking them leads to the humans of that population evolving the ability to digest lactose. But this doesn’t mean that if you keep on drinking milk that you’ll eventually be able to digest it. What it does mean is that, in populations where dairying was practised thousand of years ago,  being able to digest lactose meant you were far more likely to successfully pass on your genes [9] Jump to reference section: [9] .

An evolutionary edge?

There are many theories as to why this might be, but a proposed explanation is that it leads to improved nutrition. For example, in a population that milked cows, being able to get calories from the breakdown of lactose into glucose and the absorption of important nutrients like calcium, likely means that people were able to live longer and have more children than those who were lactose intolerant [10] Jump to reference section: [10] . However, this effect likely isn’t as strong anymore, as many more sources of nourishment are now immediately available to many of us. Not to mention that you can now buy lactase to take as a pill if you’re lactose intolerant but still want to enjoy eating a pizza or having a milkshake!