And why its perfectly natural
Lactose is a form of sugar, or disaccharide, found in animal milk.
Like all mammalian milk, human milk, the natural food for human babies, contains lactose. In fact human milk contains more lactose than cow’s milk (but less protein).
In order to fully digest milk, an enzyme called lactase is produced by cells in the wall of the small intestine which helps the body to break down the lactose.
Weaning is the process of feeding an infant the types of food that they will eat as an adult, whilst reducing the amount of milk they consume. A child is fully weaned when it is no longer fed any milk.
In human babies, weaning age is from about 2½ years up to about 7 (although 5 is often considered a better upper limit).
Once a child is weaned, in theory it no longer needs to produce lactase to be able to digest lactose because it would not be consuming any, and this is in fact the case for the majority of people – the production of the lactase enzyme switches off into adulthood, known as “lactase nonpersistence”.
However, about 7,500 years ago some human civilisations began to drink the milk of other animals beyond weaning age. This would have produced the classic symptoms of lactose intolerance (such as flatulence, diarrhoea, bloated stomach, stomach cramps and pains, stomach rumbling, feeling sick), but at some point a genetic mutation occurred which allowed the continued production of lactase into adulthood, allowing the continued consumption of milk without suffering intolerance symptoms (for those interested in learning more, a cytosine (C) nucleotide in a person’s DNA is replaced with a thymine (T) nucleotide).
So what happens to some vegans is that after they have not consumed dairy products for some time, they stop producing the lactase enzyme, as is perfectly natural, and thus they become intolerant to cow’s milk.