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Why Are Some People Lactose Intolerant?

Why are people lactose intolerant?

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Why Are Some People Lactose Intolerant?

Milk. Some people love it, some hate it, and some simply have trouble digesting it. Today, we’ll explore why some people are lactose intolerant and some aren’t.

Why Are Some People Lactose Intolerant?

When we were younger, milk was a big staple in most of our lives. What would our breakfast cereal be without it? Letting your Cocoa Pebbles soak and turn your milk brown, then drinking the resulting ‘chocolate milk’ was (and for some still is) a morning delight. At some point, usually in the mid-teens to early 20s, a percentage of the population started to feel increasing discomfort or bloated after consuming a few glasses of milk. They became lactose intolerant.

The Missing Lactase

The reason for this is a lack of the enzyme ‘lactase’ which is responsible for breaking down the lactose (a sugar in milk) in your digestive track. Scientists have discovered that most people develop at least some degree of intolerance as they age.

lactase lactose enzymeWhy we lose this ability still remains a bit of a mystery. Most scientists believe that we all originally lost the ability to digest lactose as we got older, and thanks to a fairly recent genetic mutation, the lactase producing gene was left “switched on” throughout adulthood.

Those with the intolerance will still continue to produce some lactase, but at a substantially lower rate. The decreased production will then reach a critical point where your digestive tract can no longer break down lactose and it will pass undigested through your intestine until it reaches the colon. Once there, the lactose undergoes a kind of fermentation by the bacteria that hang out there. As the bacterium interacts with the lactase, gas is produced. This gas is what causes the discomfort, pain as well as cramping and diarrhea.

Should We Cut Out All Dairy?

For all but the most severe cases, being lactose intolerant doesn’t mean you should permanently cut out all dairy. Several studies have shown that those who are intolerant can drink one to two glasses of milk a day without a problem. According to the studies, the negative symptoms can actually improve when milk is consumed regularly, but in lower levels. Researchers have speculated this is because the bacteria in the colon build up over time, which causes a stronger reaction than if the bacteria had been continually exposed to lactose — and the bacteria also becomes more efficient.

Got Milk?Just don’t forget the most important reason to drink milk: calcium. If children don’t get the amount of calcium they need, their growth and skeletal health will be put at risk. A New Zealand study showed that kids who cut dairy completely out of their diet only get one-third of their needed daily calcium which was directly responsible for a substantially higher bone fracture rate than children who consumed dairy products. Milk does in fact, do a body good.

While it’s certainly possible to get calcium from other food sources, you would have to consume unreasonable amounts of it to equal the same amount found in many common dairy products. If you miss the days when you could enjoy dairy products without a care or worry, consider giving lactose digestive-aids a try. Many with light to mild symptoms have reported great results and a sizable portion can now consume dairy products again without ill effects.

Bonus Science Fact: Milk is white because of the casein protein. Thanks to its spherical shape and being suspended uniformly throughout the liquid, the light is deflected throughout most of the visual spectrum. This causes milk to be fairly opaque and appears white to our eyes.

Itan Y, Jones BL, Ingram CJ (2010). “A worldwide correlation of lactase persistence phenotype and genotypes”. Evolutionary Biology 10:36.
Nature Genetics “Identification of a variant associated with adult-type hypolactasia”. Nat. Genet. 30: 233–7. doi:10.1038/ng826. PMID 11788828.
Flatz G, Rotthauwe HW (1971). “Evidence against nutritional adaption of tolerance to lactose”. Humangenetik 13 (2): 118–25.
Coles Harriet (2007-01-20). “The lactase gene in Africa: Do you take milk?“. The Human Genome, Wellcome Trust.

Ingram CJ, Itan Y, Mulcare CA (January 2009). “Lactose digestion and the evolutionary genetics of lactase persistence”. Hum Genet 124 (6): 579–91.
Swallow DM (2003). “Genetics of lactase persistence and lactose intolerance“. Annual Review of Genetics 37: 197–219.



  1. Eric84

    July 12, 2011 at 1:24 am

    To throw some more details at the matter, this tolerance is a result of a genetic mutation that occurred in people living in the Middle East and Europe some time in the past 10,000 years. It’s a good example of the human species adapting to an agricultural lifestyle.

  2. Glen

    July 12, 2011 at 1:24 am

    If it’s a “new” thing then wouldn’t North American First Nations people be mostly lactose intolerant? To my knowledge that isn’t the case?

    • Lucy

      July 13, 2011 at 7:54 am

      I read in a National Geographic that Native Americans are mostly lactose intolerant…

  3. Rein62

    July 12, 2011 at 1:25 am

    Basically anyone who didn’t think it was a good idea to go tug on that cow over there. And then let it rot. And then eat it.

  4. JackMion

    March 11, 2012 at 6:45 pm

    Most people who claim they’re lactose intolerant, really aren’t. They just want attention. It was a huge trend in Los Angeles 10 years ago, making people feel and believe that they were “unique”. Hell, I even know a girl who said she “caught this illness” from a close friend. So you had a wicked fart after a scoop of ice-cream. That DOES NOT make you lactose intolerant. Fact is, most who claim to be have NEVER been evaluated by a doctor. Sorry to break the news, but you’re probably not lactose intolerant.

  5. Johana

    August 21, 2012 at 7:36 pm

    Maybe we are lactose intolerant because we are the only species that keeps drinking milk after we should have been weaned off. It isn’t normal to keep drinking milk after a certain period of times. Babies usually wean off after a year or so, animals after 7 months, but we are so crazy we actually think that milk should be part of our diet. It all started a long time ago when we decided that we were better than anything and anyone else… Milk should only be drank from our mothers when we are born, then after we wean off, that’s it, no more, just like everything else in nature.. Wake up people

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