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Lactose intolerance varies with race and region, but, comparing the percentage levels of intolerance vs. non-intolerance, it could be inferred that it's more normal for most adults to be lactose intolerant than not.

Wikipedia quotes some referenced journal article from 2007 as follows (which itself appears to reference another article from 1994, behind a paywall, so, I have no idea whether they've misinterpreted it or not):

Adult-type hypolactasia is characterized by a fall of lactase activity levels to 5 to 10% of birth levels occurring during childhood and adolescence. The condition affects more than 75% of the population worldwide

Is the claim above true?

Can it be interpreted that it is normal for adults to be lactose intolerant, and abnormal to not be lactose intolerant?

  • Thanks. Please give some examples of people making the claim - e.g. the percentages. I could find one, but that would just answer the question, making it pointless. – Oddthinking Jan 4 '15 at 5:42
  • @Oddthinking, done. – cnst Jan 4 '15 at 6:06
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    Okay, re-opened/ So the question now is... if you don't believe a peer-reviewed journal, what evidence would you accept? – Oddthinking Jan 4 '15 at 6:53
  • @Oddthinking, they reference another study, so, no idea whether they misinterpreted it or not! As well all know, misinterpreting studies (even by the scientists themselves) is hardly unheard of. – cnst Jan 4 '15 at 16:26
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    you may want to check out the refs I put here: biology.stackexchange.com/questions/7341/lactose-intolerance/… – nico Jan 4 '15 at 23:25
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Yes. Most adults, worldwide, are lactose-intolerant.

I found many papers explaining this, but perhaps the most convincing that I found is this one:

First, some background:

The ability of adult humans to digest the milk sugar lactose - lactase persistence - is a dominant Mendelian trait that has been a subject of extensive genetic, medical and evolutionary research. Lactase persistence is common in people of European ancestry as well as some African, Middle Eastern and Southern Asian groups, but is rare or absent elsewhere in the world.

So, this has been studied carefully; it isn't just a misunderstanding.

This paper attempts to identify to what extent the known genetic causes can explain the known differences in prevalence of lactase persistence (i.e. milk tolerance)/

An estimated 65% of human adults (and most adult mammals) downregulate the production of intestinal lactase after weaning. Lactase is necessary for the digestion of lactose, the main carbohydrate in milk, and without it, milk consumption can lead to bloating, flatulence, cramps and nausea. Continued production of lactase throughout adult life (lactase persistence, LP) is a genetically determined trait and is found at moderate to high frequencies in Europeans and some African, Middle Eastern and Southern Asian populations.

So, most adults become incapable of digesting milk as they get older.

The paper includes a link to a spreadsheet with 227 locations, and the prevalence of lactase persistence, so the raw data is there to check their estimates.

They also provide a figure (of the "Old World" - excluding the Americas) to help you visualise it. (Note: This diagram shows the proportion of people who CAN digest milk.):

Interpolated Map of Old Words LP phenotype frequencies

As can be seen, vast areas of the old world are green and blue - where the majority of adults are lactose intolerant. Nonetheless, larger parts of Europe have a higher prevalence of lactase persistence.

Wikipedia explains that this understanding has been fairly recent.

until the 1960s, the prevailing assumption in the medical community was that tolerance was the norm and intolerance was either the result of milk allergy, an intestinal pathogen, or else was psychosomatic (it being recognised that some cultures did not practice dairying, and people from those cultures often reacted badly to consuming milk). Two reasons were given for this perception. Firstly, many Western countries have a predominantly European heritage, so have low frequencies of lactose intolerance, and have an extensive cultural history of dairying. Therefore, tolerance actually was the norm in most of the societies investigated by medical researchers at that point. Secondly, within even these societies, lactose intolerance tends to be under-reported: genetically lactase nonpersistent individuals can tolerate varying quantities of lactose before showing symptoms, and their symptoms differ in severity. [...] Subsequent research revealed intolerance was more common globally than lactase persistence


As to the question about whether lactose intolerance can be interpreted as "abnormal", I reject the categorisation as having a moral component that the data cannot support. It is not abnormal to be taller than 6 foot (1.83m), despite that describing a minority of people. It is not abnormal to not be born in Asia, despite that describing a minority of people. It is not abnormal to have a phenotype that allows the production of lactase into adulthood, even if those adults are in the minority.

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    Wikipedia also has a nice table (for those of use without Excel) ... It's also a great example of evolution at work in humans, since it evolved only 10,000 years ago at the earliest... – Martin Tournoij Jan 5 '15 at 7:30
  • I knew that intolerance was genetic and that tolerance was the "deviation from the norm", despite it being so common in the US, but I didn't know it was a dominant trait. Interesting! – Bobson Jan 5 '15 at 18:57
  • I am very puzzled. In Indonesia, milk is a very popular drink. Indonesian government even encourage drinking milk for kids way above 5 years old. It's called "4 sehat lima sempurna". Ice cream vendors are popular. And yet all this white people say we gonna die if we eat milk. Well, I am still alive. In fact, there are some popular milk drink like "susu soda gembira" which is "milk with soda with syrup" that's only in Indonesia. – user4951 Jul 5 '18 at 5:17
  • @J.Chang: that is an anecdote. – Oddthinking Jul 5 '18 at 14:16
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    Lactose tolerance is the norm in societies descended from the parts of the human race that developed widespread use of milk-derived products early in their agriculture when the ability to digest it conferred significant survival advantages. So the heterogeneity is both expected and explicable and neither "normal" nor "abnormal". – matt_black Jul 16 '18 at 17:01

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