What is it?
Lactose intolerance is the inability to properly digest the sugar found in milk called lactose. It is caused by the insufficient production of an enzyme called lactase. Symptoms appear if the intolerant person consumes dairy, and they include gas, a bloated stomach, and diarrhea. The rates of lactose intolerance vary significantly between areas: in some western countries it is about 5%, while in some areas of Asia and Africa it can be as much as 90%.
What causes it?
Being lactose intolerant is actually what’s natural. Milk is what mammals feed to their kids, and when they grow up, they move on to normal food and no longer need milk. Healthy humans produce the enzyme lactase when they are children to be able to digest their mother’s milk, and the production stops when they stop breastfeeding. In some human populations, however, a genetic mutation has caused the lactase production to continue throughout life: is has undoubtedly been evolutionally beneficial to be able to add this source of energy into our diets when food has been hard to find. Lactase deficiency can also sometimes be caused by infection, surgery or other health issues.
What happens during it?
Lactose is a disaccharide, which means that it consists of two smaller sugar molecules that are bound together. Our cells want the molecules separately to be able to utilize them in energy production. The bigger molecule, lactose, needs to be broken into two pieces, and that’s the job of the enzyme lactase.
The lactase production peaks at the time of birth. It is essential to newborns, and insufficient production can be fatal. In “normal” people this production then starts to decline, but in some it continues into adulthood.
Since the lactose cannot be broken down, it’s not absorbed to the blood stream. Water moves spontaneously from a solution with a lower concentration to a solution with a higher one, and since lactose is something that increases the concentration of the intestine “solution”, more water also moves there and increases the volume of stuff. This is a factor in the symptom of diarrhea.
We have bacteria living inside our intestines. They protect us from foreign microbes, break down things we can’t, and synthesize some vitamins for us. When they encounter lactose, the break it down into gases like hydrogen gas, carbon dioxide, and methane. This accounts for the bloating and the gas.
How can it be treated?
While there is no “cure” for lactose intolerance, the symptoms can be effectively managed through controlling one’s diet. This includes avoiding things like dairy products, or opting for lactose free dairy. It should be remembered though that suddenly cutting an important element from one’s diet could lead to nutrient deficiencies if not properly planned for. Taking an lactase substitute capsule can also be useful in some cases.
Sources: http://www.ncbi.nlm.nih.gov/pmc/articles/PMC4586575/, https://en.wikipedia.org/wiki/Lactose_intolerance#Signs_and_symptoms, http://www.nhs.uk/Conditions/lactose-intolerance/Pages/Treatment.aspx, http://www.webmd.com/digestive-disorders/chronic-diarrhea-16/diarrhea-lactose-intolerance