Lactose is a disaccharide (a sugar composed of two simpler sugars) found in milk that has gotten a bad rap. Anti-dairy folk like to point out that as much as two-thirds to threequarters of the world’s adult population don’t produce much lactase, the enzyme that breaks down lactose.
And they often go on to paint a scary story about the implications of lactose intolerance and the negative health consequences. As a result, some people avoid dairy altogether. Lactose naturally occurs in all milk, including human breast milk. In fact, breast milk contains significantly higher quantities of lactose when compared to cow’s milk.
Infants naturally secrete an enzyme called lactase that breaks lactose apart into simpler sugars, glucose and galactose, both of which the body can absorb and use. So for anyone who produces lactase, lactose is perfectly easy to digest.
The (imagined) problem occurs when a person does not create lactase. In certain areas of the world (Europe, East Africa, and West and South Asia [the Middle East and Indian subcontinent]), humans evolved to continue to produce lactase into adulthood. These are regions in which humans have long relied upon milk as a significant part of their diet.
So the adaptation to be able to digest lactose into adulthood was advantageous. In other parts of the world where milk didn’t make up a large part of the traditional diet, less than 30 percent of the population retains the ability to produce lactase into adulthood.
Superficially, it would seem simple: for those who make lactase, milk and dairy is good, and for those who don’t make lactase, milk and dairy aren’t so good. But it turns out that lactose isn’t always a problem, even for those who don’t produce lactase.
Why would that be? Because if you cannot enzymatically digest a carbohydrate it becomes food for the bacteria in your large intestines. In other words, for people who don’t produce lactase, lactose is effectively fiber. It feeds the bacteria in the large intestine. Not only that, but it preferentially feeds one of the most beneficial classes of bacteria in the human digestive system: Bifodobacterium (Parche, et al., 2006).
Those who do not produce the lactase enzyme fall into two categories. One group eats dairy without problem. The other group avoids dairy because it can cause digestive discomfort for them. What is the difference between these two groups? The answer would seem to be their gut microbiome (i.e. their “beneficial bacteria”), which is something that is modifiable by way of diet.
Those who experience digestive discomfort don’t have large numbers of the bifidobacteria that digest lactose (and produce short chain fatty acids that improve the health of the intestines). It’s the very same reason many people who are accustomed to eating a low fiber diet will experience digestive problems if they suddenly eat large amounts of fiber.
Remember, for those who don’t produce lactase, lactose is effectively fiber. And a cup of milk contains about 12 grams of lactose “fiber”. That means that for a person who doesn’t produce lactose, a pint of milk contains 24 grams of fiber!
But there’s good news. Those who cannot tolerate large amounts of lactose can usually develop tolerance simply by regularly eating lactose (Szilagyi, 2015). The most comfortable way to achieve that is to introduce lactose in small amounts and increase slowly as the number of bifidobacteria in the digestive system increase.
In summary, lactose is a natural sugar that is perfectly healthy for those who can digest it. For those who are unable to produce lactase, large amounts of lactose without digestive support (lactase and/or bacteria that digest lactose) can be problematic.
However, fermentation, and aging (cheese) can greatly improve the digestibility of milk, even for those who don’t produce lactase. And regular consumption of lactose feeds bifidobacteria that digest lactose, producing health effects and increasing lactose tolerance in most people.