GI Blog #2 Why milk drinkers are mutants and lactose intolerance is the new normal

Years ago while doing my training in gastroenterology, I performed the very first lactose breath test in a 32-year-old volunteer who often drank milk and ate ice cream. After an uneventful first hour, the test confirmed his lactose intolerance in dramatic fashion – a rapid dash to the bathroom that resulted in watery expulsion, along with a rise in hydrogen gas in his sampled breath. This was my first confirmed diagnosis.

So why milk? (Disclosure, I am not paid by the National Dairy Council.)

Milk is an abundant source of calcium and vitamin D. Vitamin D promotes calcium absorption and bone health and prevents rickets and osteoporosis (thinning of bones). In Florida, ultraviolet light activates vitamin D in your skin, but in Wisconsin, higher calcium intakes compensate for the lack of sun exposure in winter. But milk is also full of lactose, the milk sugar. 

Lactose absorption

Your small intestine is lined by villi, which are small, finger-like projections that absorb nutrients. These villi are lined at the tips by lactase enzyme, which is the protein that splits lactose, a double sugar, into glucose and galactose for absorption by the intestine. In someone with lactose intolerance, the unabsorbed double sugar traps water and passes on to the colon where it causes gas, cramps and diarrhea.

Testing for lactase deficiency

The original test was to give lactose sugar in water and measure rising blood levels of glucose. This has been replaced by the noninvasive breath hydrogen tests, which are great for kids and adults. We also can measure lactase enzyme in biopsies taken from the small intestine during endoscopy, but it is too invasive and costly to do when trying to diagnose lactase deficiency alone. 

Who gets lactose intolerance and who doesn’t?

All babies have lactase enzyme during breastfeeding, but in most of the world once they are weaned, the enzyme is turned down, and they eventually become lactose intolerant. The same pattern applies to the animal kingdom: Dogs, cat, cows … all mammals, in fact, become lactase deficient when there is no further milk exposure after weaning. In Asia, Africa and the Middle East, lactose intolerance can affect 80-90 percent of the population. 

It’s the other way around for those of northern European descent as 90 percent or so have lactase persistence and can drink milk as adults. They deviate from the normal biological pattern due to a mutation that allows the lactase enzyme to persist beyond the point of weaning. Yes, a mutation leads to milk drinking!  

What is the advantage of lactase persistence?

When a trait is common, there is usually some biological advantage to it. And so it is with lactase persistence. Over the past 10,000 years, it allowed hunter-gatherers to become stable dairy herders. Because calcium for bones is so important, it also allowed people to live in such inhospitable locales such as sunless Sweden – and even Wisconsin! Having lactase enzyme beyond infancy may have even prevented pelvic rickets and allowed normal babies to be born.

What symptoms do you get?

We mentioned the typical symptoms – gas (topic of another blog), bloating, grumbling, crampy abdominal pain, and diarrhea. However, a number of studies in kids and adults show that giving lactose sugar to lactase-deficient patients doesn’t always cause symptoms. There is also evidence to suggest that you get symptoms from milk when lactase deficiency is combined with irritable bowel syndrome (IBS). That is, the lactose in the colon causes gas, triggers spasms and pain from IBS. Hmmm, so it is complicated, less clear than one would think.

So you are lactose intolerant, what to do?

The latest National Institutes of Health consensus panel (of which I was an early member) suggests continuing to drink/eat some milk/dairy due to its high nutritional value, rather than eliminating it. In fact, most lactase-deficient individuals can drink up to one-half to 1 glass of milk without symptoms – you have to find your tolerance level and stay below it. Cheese and yogurt (because it contains bacterial lactase) are better tolerated than milk or ice cream. And there are over-the-counter lactase enzyme pills you can take with dairy.

That volunteer subject with lactase deficiency? That was me. Now I know I am quite normal and don’t have a disease! Caucasians who are lactase persistent, on the other hand, are mutants. I had to spill the milk and speak the truth. See you at Culver’s.

Terminology                                                                                          

Lactose (sugar) intolerance – you develop GI symptoms when drinking milk

Lactase (enzyme) deficiency – you have low lactase levels but may or may not get symptoms when drinking milk

Lactase (enzyme) persistence – you have normal lactase enzyme levels from a mutation

- B Li, MD, gastroenterologist, Children’s Hospital of Wisconsin

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