Vomiting in Infants with Lactose Intolerance

The history of lactose intolerance (LI) is odd in almost every way.

The condition was first recognized in the 1950s, in babies who were born without the ability to digest lactose, a condition now called congenital lactose intolerance. This is an extremely rare, not even one-in-a-million births, event.

So naturally doctors were rocked to the tips of their stethoscopes when they found that not only was LI fairly common, it is the natural condition of all mankind (and all other mammals), occurring in about 70% of all humans alive today.

When studies and surveys were conducted on almost every ethnic group and national population on earth in the 1970s, an interesting side fact emerged. The populations with the smallest overall percentage of LI - which turned out to be, not surprisingly, the northern Europeans whose doctors had never discovered the problem - also were the populations in which the onset of LI occurred at the latest age. In fact, the condition was called adult-onset lactose intolerance for a while to distinguish it from congenital lactose intolerance.

But with time and investigation, researchers finally realized that in those populations in which almost everyone had the normal version of the gene, the one that turned off lactase production sometime after weaning, most children became LI before adulthood.

And then came another surprising finding. Almost any ailment that impacted the gastrointestinal system could temporarily knock out the lactase making ability, since that happens at vulnerable places on the inside of the small intestine. This became known as temporary lactose intolerance.

Today, three types of LI are recognized, and the names have more or less stabilized at congenital LI, secondary (rather than temporary) LI, and primary (rather than adult-onset) LI. The last two can occur at any age.

Even babies, who aren't supposed to be LI at any time - that interferes with breastfeeding, since human milk has the highest lactose content of any mammal's milk - can get secondary LI. They can, and do. A lot. Any ordinary "stomach flu" - which is really a gastrointestinal ailment not located in the stomach - can stop lactase production for weeks until the intestines heal.

One other oddity. Even though vomiting is so rare a symptom of primary LI that most people, even me, tell anyone who vomits that LI isn't the cause, babies with secondary LI can and do vomit as a symptom. (Here's one of the first medical journal articles to report this, Severe lactose intolerance with lactosuria and vomiting, by A. Hosková, et al., Arch Dis Child. 1980 April; 55(4): 304–305.)

This tends to confuse everyone. You can search - as I did, which is why I'm writing this - and finds lots of non-academic sites on the net which will tell you flatly that vomiting is never a symptom. Or that babies who vomit must be allergic to milk rather than LI. I don't know for sure which is more common, but my first thought with a baby who was never bothered by milk but now vomits because of it would definitely be secondary LI.

Talk to your pediatrician and check this out if this happens to your child.