The Science of Lactose Intolerance

While looking for something to write about this month, I tried to come up with something smart, something related to my research, or perhaps what it’s like to spend a month at sea (seeing as I just spent a month in the North Atlantic). Instead I came up with my inability to digest dairy and how special I feel whenever it gets referenced in shows and movies. It’s almost like hearing someone reference your favourite obscure book or hear that they went to the same summer camp you did; you’re automatically part of a club of “like-stomached” people who all feel terrible after eating ice cream.

While I was walking around Ireland, listening to one of my favourite podcasts (How Did This Get Made – download it now!) somehow a conversation about the Schwarzenegger film “The Running Man” turned into a conversation about the amazing dairy-digesting pill, lactaid. For anyone who isn’t lactose intolerant or blessed with lactose intolerant friends, this is a pill that includes the lactase enzymes that our bodies have stopped producing, thus giving us the ability to eat pizza, ice cream, and chocolate (to name a few delicious dairy-including foods). Bonding over a weird stomach problem is one of my favourite things to do – it really humanizes people – and I’ve never heard so many great lactaid-related jokes.

So between my experiences as a lactose-intolerant person (yes, it’s a club, we’re getting t-shirts), the questions I get, and the number of times I hear it referenced in shows and movies, I figured it deserved a scientific pop culture take-down.

New York city has no power
And the milk is getting sour
But to me that is not scary
‘Cause I stay away from dairy

– Phoebe Buffay (FRIENDS, Episode 1.07)
I think this song may imply that Phoebe is lactose intolerant

So here is my lactose intolerance takedown:

  1. Is it an allergy?
  2. Is it true that humans aren’t supposed to drink milk?
  3. It’s more common in Asia and Africa, right?
  4. My friend can eat cheese and yoghurt but they’re also lactose intolerant?
  5. Eww, all that dairy-free stuff tastes like crap, right?

(1) First and foremost lactose intolerance is not an allergy. It may seem easier to link the two together but someone who has a potentially deadly allergy does not appreciate you calling your sore stomach (no matter how sore it might get) an allergic reaction. An intolerance, by medical definition, is a reaction that does not involve the immune system and therefore will not cause hives or swelling, but instead often involves digestive distress (don’t worry – no gory details here). Lactose intolerance in particular, is the inability to digest lactose, a sugar present in milk, because you do not produce the lactase enzyme. This enzyme is found within the duodenum and its demise can either be due to a genetic, primary intolerance (your body doesn’t have the capabilities to continue making the enzyme) or an environmental, secondary intolerance (as a result of damage, for instance Crohn’s disease).

(2) When people tell you that “humans aren’t supposed to drink milk past childhood” it’s not entirely wrong and helps to explain the prevalence of lactose intolerance. Basically, the ability to digest milk past childhood is actually a genetic mutation; at some point during humans’ evolution we were able to keep the LCT gene turned on with a little help from a mutation in the MCM6 gene, which is responsible for breaking down lactose into more easily digestible sugars. As a result, anyone who can digest lactose has this mutation, and instead of lactose intolerance being an oddity, really anyone who is “lactose persistent” is actually a mutant.

(3) The rise of this lactose persistence is the real mystery. One single mutation within the European population helps to explain the spread and distribution of lactase persistence in Europe and North America, but there have also been additional, different mutations in Africa and the Middle East that can explain the smaller percentage of lactose persisters there (Gerbault et al., 2011). Lactose persistence was likely spread following the mutation of this gene because of selective pressure and the beneficial ability that digesting lactose had on a population. While it’s not entirely proven, it’s likely that this persistence was accompanied with the spread of pastoralism and dairying, when being able to consume milk products would have given a population an evolutionary advantage (Gerbault et al., 2011).

Lactose Intolerance Worldwide – from Encyclopedia Brittanica

So, yes you are basically an x-men (cough cough mutant) if you can digest dairy, but there has certainly been a beneficial pressure for this mutation since it has evolved in different populations and different genes worldwide. Therefore, the occurrence of lactose intolerance varies depending on different ethnic groups. In fact, even the timing of lactose intolerance varies worldwide, with most Chinese and Japanese people losing their ability to digest lactose 3 – 4 years post weaning, while the 10% of lactose-intolerant white Northern European descendants (the category I fall into) can take 18 – 20 years to stop producing lactase (I was 21!) (Matthews et al., 1995)

So to answer (4), why some people are more susceptible and sensitive than others, even when they might all label themselves as lactose intolerant, we’re not entirely sure. First, dairy products contain different amounts of lactose; fermented dairy products (i.e. cheese and yoghurt) naturally contain less lactose and are often well-tolerated by lactose intolerants (Gerbault et al., 2011). This means that lactose intolerant people can often eat dairy to varying degrees, depending on how sensitive they are. Lomer et al., 2008 found that this variability in the amount of lactose a lactose intolerant person can digest may actually lie within the gut flora. However, the more you wean your body off any form of lactose, the harder it is for your body to digest. So, some people can eat dairy because they continue to eat it despite minimal discomfort, while others have completely cut it out of their system and now can’t eat anything with even the teensiest bit of dairy without having some major complications.

So why are we seeing so many “lactose-free” or “soy-based” alternatives nowadays? Well, mostly because our culture is developed enough to understand what our bodies are doing and create alternatives for the discomfort we feel. If you think about the life-saving medication (antibiotics, vaccinations, sanitation) that has evolved over the last 100 years, it’s not surprising that the last 10 years have been filled with lots of new food discoveries (good and bad, fad or not fad?) that can help our tummies feel better.

(5) This is of course subjective, and I have to rep my brand, but no it does not all taste like crap. First – there’s lactose free and dairy free. Lactose free is a product where the manufacturers have added in the lactase enzyme during the production. Basically, instead of taking a lactaid pill that helps to breakdown lactose in your body, it’s already a part of the milk, ice-cream, butter, cheese, etc. And yes. It tastes normal because the presence of the lactase enzyme doesn’t change the taste – you mutants do not have to be afraid, I’m not feeding you crap. Dairy free products, like soy milk, almond butter, or tofu cheese do taste different – some are bad, but some are really good. In fact, I much prefer chocolate almond milk to chocolate milk and there are some cashew-based ice creams that are incredible. So don’t knock it until you’ve tried it.

So, even if you’re a mutant who can digest lactose, or a lowly lactaid popper like me, hopefully you are now more enlightened into this world of lactose intolerance. In any case, you can now enjoy this clip from the Big Bang Theory complete with every lactose joke out there, and whenever you hear lactose mentioned in shows and movies you can also be a part of this secret, like-stomached, club.


Gerbault, P., Liebert, A., Itan, Y., Powell, A., Currat, M., Burger, J., … & Thomas, M. G. (2011). Evolution of lactase persistence: an example of human niche construction. Philosophical Transactions of the Royal Society B: Biological Sciences, 366(1566), 863-877.

Global Lactose Intolerance Image:

Lomer, M. C. E., Parkes, G. C., & Sanderson, J. D. (2008). Review article: lactose intolerance in clinical practice–myths and realities. Alimentary pharmacology & therapeutics, 27(2), 93-103.

Matthews, S. B., Waud, J. P., Roberts, A. G., & Campbell, A. K. (2005). Systemic lactose intolerance: a new perspective on an old problem. Postgraduate medical journal, 81(953), 167-173.


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