This is one of my favorite posts and favorite topics. I like it because it is so immediate: everyone knows someone whose belly is troubled by milk and dairy. And it’s an example of evolution that we can touch right here and now. Over a couple of thousand years, equal to about fifty human generations, the human population is changing via selection in response to their environment. This is an important change, too, because it’s a change to our environment brought about by culture. Cro-magnons didn’t keep cows or goats for milk. There was no need for adults to process lactose. When culture changed and humans began to keep milk animals, some people fared better than others – those who can process lactose. It’s interesting because we are no longer simply at the whims of natural selection but are now spinning in a feedback loop of culture influencing behavior which influences culture. Fascinating stuff!
Copyright Dennis Mitton
The Science of Lactose Intolerance
My daughter has a neat trick. Give her a meal and then a nut to eat – maybe a pecan on a salad – and have her chew on it. Within about thirty seconds of chomping down and swallowing the nut, the entire meal comes up. It’s a hyperactive immune response. Antigens in her immune system recognize nut protein and instantly alert the body that something is amiss and it’s time to clean the pipes. She throws up, makes tons of saliva, and her nose starts running. Surprisingly it’s not much of a bother to her and, as her doctor reminds us, it’s a far better response than having her throat close up.
She also has a dairy sensitivity. As the family observer and a scientist, I kept arguing that her mom was making it up. But each time I sneaked the daughter a sliver of ice cream or milk she would inevitably end up with an upset stomach in ten or twenty minutes. We wondered about modern evils like wheat or gluten and finally bought some lactose-freeNote1 ice cream which she ate without effect. Then we tried lactose-free milk with the same effect. My wife wagged her finger as I finally came ‘round to agree that our daughter has a sensitivity to lactose.
I bristle at the so-called health advice sold on the shelves at Barnes and Noble or the local vitamin store. It irritates me when people buy into superfoods or GMO poisoning. I am deeply bothered when people don’t vaccinate their children. I have some of the same ideas about lactose intolerance. I filed it loosely under what I consider a pseudo-pseudo-science: yes, there are real cases of medical intolerance based on genetics but much of what is reported and sold is just plain old hucksterism. And I still think this is true. But in my daughter’s case, it appears to be part of a general sensitivity. What is the science behind it?
Lactose intolerance is well understood and easily explained. Lactose is a sugar found in milk and dairy products. It is a disaccharide, meaning that it is a carbohydrate, in this, case a sugar, made up of two parts called monosaccharides. These are simple sugars. Lactose is made of glucose and galactose and, like all sugars, these are used to provide energy to power cellular functions. But there’s a hitch: your body can’t metabolize and absorb lactose. In order to release the usable sugars, lactose must be split in half by another molecule, an enzyme called lactase.
See that ‘-O-‘ in the image sitting right in the middle of lactose? That is oxygen and it holds the two simple sugars together. When this molecule slips through the stomach and into the small intestine it alerts the body to secrete lactase to cleave or chop the two sections to make two simple sugars. It’s a bit like your nasty brother taking a hatchet to your tinker toy creation. Both simple sugars are easily absorbed through the intestinal wall to provide energy to your cells and organs to keep you going. But when held together by an oxygen atom they won’t digest.
If the lactose isn’t cleaved it won’t absorb and will pass through to the large intestine whole. Bacteria in the intestines love this stuff and gorge themselves like high schoolers at a cheap pizza buffet. Unfortunately for the intestine owner, the bacterial make a whole lot of gas. It’s this gas that causes the symptoms of bloating, abdominal pain, and cramping.
Evolution and Lactose
How does evolution help to explain lactose intolerance? We are born with the ability to process lactose. In this regard, humans are just like any other lactating mammal. About a third of the calories from both human and cow’s milk comes from lactose so, if we drink milk, the ability to process this sugar is essential. It is generally understood that lactase expression in the mother is down-regulated after weaning and continues through adulthood. This makes sense: if you aren’t drinking mother’s milk you have no need of the means to process it. So lactose intolerance is the normal condition for adult human beings.
Johns Hopkins reports that:
- Seventy-five percent of all African-American, Jewish, Mexican-American, and Native American adults are lactose intolerant.
- Ninety percent of Asian-American adults are lactose intolerant.
- Lactose intolerance is least common among people with a Northern European heritage.
These Northern Europeans – Modern Scandinavians and their ancestors – are generally lactose tolerant and use dairy products without painful side effects. Why? Lactase expression appears to be governed by a couple of different genes. These genes are turned off in normal lactose-intolerant individuals (called lactase non-persistence) but remain functioning in those who can use dairy (lactase persistence). Northern Europeans have a recently emerged specific haplotype – a group of genetic variations – that maintains lactase expression. We know that this is a recent mutation as it is nearly identical across the populationNote² and aligns well with the timeline of the emergence of dairy as a food source about 8,000 years ago. Interestingly enough, studies of Asian and African Americans show that groups who are more prone to lactose-intolerance carry a lactose haplotype similar to more ancestral Europeans. There are small pockets of people in Asia and Africa who live largely on dairy who do not show such strong genetic evidence. It could be that there are other factors that aren’t accounted for by genetics alone. It’s an ongoing area of research. But the recent emergence of human lactose-tolerance is a clear example of the co-evolution of a genetic trait selected for from within an emerging culture.
References and Notes
- I was surprised to learn that most lactose-free dairy products are made not by removing the lactose – milk would lose much of its taste without it – but by adding lactase which pre-processes the milk sugars. If you buy products that allow an intolerant person to ingest dairy you area buying lactase. It doesn’t appear to work with my daughter. You can try lactase enzyme pills before easting diary. This adds the lactose that you are missing to process lactose. They are considered safe by the FDA and many nutritionists suggest their use to gain the nutrition that comes from milk
- Why does an identical haplotype indicate a recent emergence? Think of a penny made by a press at the mint. When the press is brand new, every surface is clean and perfect. But as it is used daily to pump out a million pennies it will, over time, begin to accumulate nicks and scratches that will appear on every penny made. This happens all the time and collectors use these accumulated defects to determine what plates coins were made from and in what order. A penny without these defects indicates that it was pressed from a brand new plate.Genes work in much the same way. A new mutation is passed through a population like a new penny. But as it is reproduced through generations and thousands of years, it accumulates nicks and variations. These mutations occur so regularly that they can be used as a kind of clock as in the case of Mitochondrial Eve. So just like in coins, cars, or furniture more defects or differences indicate a longer time in circulation.
- Composition of human milk. Read here from the NIH.
- Evidence of emerging lactase-persistence in modern humans.
- Nature Genetics: Gene culture co-evolution between cattle milk protein genes and human lactase genes, 2003. Georg Erhardt. (PubMed paywall)
- European Journal of Human Genetics: Evolutionary Genetics: Genetics of Lactase Persistence – Fresh Lessons in the History of Milk Drinking, 2005. Edward Hollox
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I haven’t read this so cannot comment on its contents. It is, however, the highest rated book on Amazon related to reintroducing wheat and dairy back into your diet.