Babies are typically able to digest lactose, but in early humans, this ability quickly declined after breast-feeding years. Around 75% of modern humans lose full ability to digest lactose as adults. The continued ability to digest lactose is due a relatively new mutation in a single gene called LCT. Roughly 10,000 years ago, mutations emerged that allowed for increased LCT gene activity in adulthood, which is technically called ‘lactase persistence’.
In other words, rather than the traditional human trait of being breastfed then losing the ability to digest milk sugar, some people persisted. Breastfeeding might be over, but some humans benefited from the ability to still digest milk sugar.
Why would that be? In a word, pastoralism. Raising livestock can provide for a steady supply of meat and milk, and certain geographies were set up well for animal husbandry.
One of those regions, Sweden, provides some interesting data. Nearly all modern Swedish people can digest lactose, but as little as 4,000-5,000 years ago, there was a stark divergence in that area: DNA from Swedish hunter-gatherer populations of that era show only 5% to have had lactase persistence, while DNA from cattle-raising populations of the same era show 75% with lactase persistence. If you’re a modern-day Swede, you probably owe your milk-digesting ability to the success of the latter population.
Roughly 10,000 years ago, the first humans to digest lactose in adulthood started handing down their genes. After centuries of dairy animal raising, dairy products have become ubiquitous in modern society.
Note that none of these options matter for vegans or those avoiding dairy for other reasons.
Here are the three most common routes to take:
Eat dairy anyway, and sometimes too much ice cream. This option is likely to result in gas (from intestinal bacteria eating lactose and releasing gas), along with borborygmus (a delectable word that equates to tummy rumbling from gas and liquid sloshing around), and for some, a possible extended stay on the toilet.
Avoid it at all costs. Commonly, lactose tolerance sufferers will avoid any hint of dairy, whether milk, cream, or cheese. Even though cheese and butter have nearly no lactose. Dairy alternatives abound though, from oat milk to soy milk to coconut milk.
There is actually a fourth option. Eating a bit of dairy on occasion may foster the growth of lactose-digesting bacteria in your gut, and enable better lactose tolerance than if you never ate dairy then suddenly ate some.
Some people are okay eating the occasional dairy product, but many either completely avoid it or take lactase enzyme pills or pre-digested lactose-free milk. You can use a one of dozens of dairy alternatives, or simply just eat the plethora of other foods that are digestible to humans.
Processed dairy is generally lower in lactose. Whereas a cup of 2% milk contains 11 grams of lactose, cheddar cheese only has around 0.5 grams per serving. The very thing that makes cheeses solid, straining out of the watery whey protein during processing, also eliminates most of the lactose with that liquid.
Yogurt has a bit lower lactose content than milk, as some of the lactose is fermented by bacteria, producing the lactic acid that gives yogurt its characteristic tartness. These bacteria also help our lactose digestion in an interesting way. The lactase enzyme they themselves employ to digest lactose is protected within bacterial cells, and enjoys safe transport through our acidic digestion process, buffered by the alkaline yogurt that surrounds it.
Fuller-fat yogurts, like greek yogurt, have less lactose than other types due to having less of the lactose-containing whey.
Some people can't handle milk but can handle yogurt or cheese, due to lower lactose content from human processing or bacterial processing.
Now that we’ve looked at the history of lactose persistence, let’s take a step back and think about the substance in question: lactose. Why is there even lactose in milk, to start with?
All mammals feed their young with milk, but that milk is rich in lactose rather than sugars we see in other foods (namely sucrose, or table sugar). This isn’t likely to be an accident: lactose is made up of galactose and glucose, and that galactose could play crucial roles in infant development.
For example, galactose is part of complex structures called galacto-oligosaccharides (GOS), which serve as food for beneficial bacteria in infant guts. This role as a prebiotic (a substance that promotes beneficial bacterial growth) has made GOS a highly researched compound these days even for adults.
Galactose is also part of complex molecules that are essential to the nervous system and immunity. Given that infants have increased nutritional needs, in order to bolster their developing physiology, their intake of lactose makes sense.
So why, then, is glucose our primary carbohydrate fuel, accounting for nearly all the sugar in our blood? The same characteristics that allow galactose to form complex molecules also makes it too reactive to be used as a main fuel source. Feeding lots of galactose to rodents actually makes them age faster, which may give you pause when considering your dairy intake. Luckily, research doesn’t seem to show adverse effects for human disease.
Galactose in milk may provide health benefits to babies.
Don’t presume that the ability to digest lactose with ease also means that you’re meant to consume tons of dairy. And don’t presume dairy is either “good” or “bad”.
We do know that humans are unique in drinking milk past childhood, and also drinking the milk of other animals routinely.
This arose due to physiological benefits, such as the ability of dairy products to provide water, vitamin D, and calcium in times of low food availability. Nowadays, we no longer have to rely on dairy as a main source of these nutrients. Thus, it might be prudent to test whether you feel better with or without dairy, if you suspect that one or more dairy products may not agree with you.
Dairy is a common allergen and may be linked to health detriment (or benefit, the evidence is mixed). A trial period without dairy may reveal sensitivities you didn't even know you had.