Incidence of Lactose Intolerance Among Young Children

Simple questions deserve simple answers. How I wish that were true. So many of the simple questions I get can only be answered by "maybes" and "possiblys" and "we need more facts."

Take this simple question.

What is the incidence of Lactose Intolerance among young children?

You might think I could just check my sources and give a quick answer. But no.

First, how young is young? The answer will differ greatly at five months, five years, and fifteen years. Second, while I can usually assume my questioners are asking about the U.S., I don't know this for sure. The answer differs greatly around the world. Heck, it differs greatly among various populations in the U.S.

Start with very, very young children. The ability to manufacture lactase in the intestine is one of the last functions to develop in a fetus. Most premature babies, therefore, can't handle lactose very well. Fortunately, the intestines mature rapidly after birth. Within a few days they catch up to other babies and should be fine after that with proper care.

A very few full-term babies are born without any ability to manufacture lactase at all. This condition is called congenital lactose intolerance. It is extremely rare. And by rare I mean in the tens or hundreds worldwide ever. Today this problem is caught very rapidly and the baby is put on a non-dairy formula. Congenital LI never goes away. The individual must go dairy-free for life.

Humans, like almost every other mammal, are programmed to drink their mother's milk until weaning. That means the percentage of children under three who are naturally LI is near zero. You can become unnaturally LI, though. The common gastrointestinal illness known incorrectly as the "stomach flu" can knock out the lactase-making ability temporarily. This is known as Secondary LI. A pediatric gastroenterologist I spoke with said that at any moment, 10-15% of the children in his practice are experiencing GI problems and 10-15% of those become LI as a result. Doing the math, if I could freeze time and just sample the population for that split second, about 1% of young children, under three, would be temporarily LI.

What happens after weaning? Humans, again, are programmed to lose their lactase-making ability after weaning. This is Primary LI, the LI that usually gets talked about. Medical researchers have scoured the globe doing tests on every possible population. The results are very mixed, partially because of small sample sizes, different lactose loads, and older testing methods, and partially because humans themselves are mixed. Pure populations are rare to non-existent, and this is doubly so in the U.S., that mongrel melting pot.

Overall, the most that anyone can say is that populations that tend toward having a very high percentage of their members LI as adults are also populations that tend toward losing the ability to make lactase as a very early age, some with high percentages even by age three.

Populations, like northern Europeans and their descendants, that tend toward having a low percentage of their members LI as adults tend to lose that ability at a later age, many not until adulthood. That's why Primary LI was mistakenly called Adult-Onset LI for a while, until researchers learned better.

A very long and complex answer to a seemingly simple question. I hope you found it better than my first try, which boiled down to "nobody really knows." That's still true, but now you know why that's true. And that's the big step forward.