Everything about lactose intolerance

    Prevalence, origin, and everything else

    One of our new condition pages is on lactose intolerance, which is arguably the most common nutrition-related condition in the world, with around 75% of people experiencing it.

    As Jerry Seinfeld might say, “What’s the deal with lactose?” What’s so special about lactose, as opposed to glucose or fructose? When, where, and why did humans start drinking milk from animals?

    If you want to give your family the gift of random nutrition facts at the holiday dinner table, check out some choice ones I’ve gathered below.

    How important was milk to our ancestors?


    Consider this: the first two beings created in Norse mythology were not man and woman, but a massive dairy cow named Auðumbla and a giant named Ymir. Ymir’s sole source of nourishment was suckling milk from Auðumbla, and Auðumbla’s was licking away at salty ice blocks, eventually revealing the first man.

    So milk has quite an important role in certain cultures, even popping up in creation myths. In other areas, animal milk has little to no cultural significance, partly due to lactose intolerance, which also happens to be the most concrete example of regional ancestry influencing how we currently eat.

    Milk, milk, everywhere, nor any drop to drink

    Milk and other dairy products are everywhere, but if you can’t digest lactose (the sugar in milk), your options are limited. For the lactose intolerant among us, here are the three most common routes to take when confronted by a bowl of delicious ice cream:

    • Eat it anyway, or eat less of it. This is the most enjoyable option. But it’s also likely to result in gas (from intestinal bacteria eating the lactose and releasing gas) and borborygmus (a delectable word for tummy rumbling from gas and liquid sloshing around) — along with an extended stay on the toilet, for some.
    • Avoid it at all costs. Lactose-intolerance sufferers commonly avoid any hint of dairy, be it milk, cream, or cheese. But there is often some misunderstanding of lactose going on here, which we’ll get into in a bit.
    • Take a lactase enzyme pill. Lactose intolerance is, by definition, a lack of sufficient lactase enzyme. So lactase enzyme pills reliably help with lactose digestion.

    In some parts of the world, lactose intolerance probably wasn’t a big deal in decades past. Home refrigerators only became widespread in the 1930s. Before then, if you didn’t have a cow for fresh milk, you might not encounter it that often. Longer-lasting dairy products that weren’t as dependent on refrigeration, including cheese and yogurt, had the upper hand.

    It turns out that processed dairy (not in the ultraprocessed sense of modern life but simply processing milk into something else) is also 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 cheese solid — straining it out of watery whey — also eliminates most of the lactose along with the liquid.

    Yogurt also has a 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 the bacteria themselves employ to digest lactose is protected within bacterial cells. It enjoys safe transport through our acidic digestion process, buffered by the relatively alkaline yogurt that surrounds it. Fuller-fat yogurts, such as Greek yogurt, have less lactose than other types because of containing less lactose-laden whey.

    Pastoral cultures and the rise of lactase persistence

    Babies are typically able to digest lactose, but in early humans, this ability quickly declined after breastfeeding years. The continued ability to digest lactose, termed “lactase persistence”, is due to a relatively new mutation to a single gene called LCT (with mutations allowing for increased activity, emerging roughly 10,000 years ago).

    Why would that mutation be beneficial? In a word: pastoralism. Raising livestock can provide a steady supply of meat and milk, and certain regions were well set up for animal husbandry.

    One place that falls in that category, 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% had lactase persistence, whereas 75% of cattle-raising populations did. If you’re a modern-day Swede, you probably owe your milk-digesting ability to the success of the latter population.

    Why lactose?

    Now that we’ve looked at the history of lactase 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 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 an accident: lactose is made up of galactose and glucose, and that galactose could play a crucial role in infant development.

    For example, galactose is part of a complex structure called galactooligosaccharide (GOS), which serves as food for beneficial gut bacteria. 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, to bolster their developing physiology, their intake of lactose makes sense.

    So why, then, is glucose our primary carbohydrate fuel, accounting for nearly all of the sugar in our blood? The same characteristics that allow galactose to form complex molecules also make 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 humans.

    That being said, don’t presume your ability to digest lactose with ease also means you’re meant to consume tons of dairy. Nobody knows all the effects of dairy for sure, either positive or negative.

    We do know that humans are unique in drinking milk past childhood, and also in routinely drinking the milk of other animals. Milk consumption arose due to its physiological benefits, such as providing 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. So, your choice of how much, if any, dairy to consume can be influenced by a variety of factors, from how it may affect your own health, acutely or long term, to your ethical stance on dairy production methods.

    If you’ve managed to read allllll the way down here, I applaud you and wish you the happiest of holidays!


    Kamal Patel
    Co-founder, Examine