PKU And Pregnancy: A Diet Worth Following

PKU Pregnancy

As a society, we are obsessed with “eating right.”  All you have to do is spend a few minutes on your Internet provider’s home page before some ad or other pops up with advice to “eat this” or “don’t eat that.”  Advertisers know that people are concerned about what they put into their mouth and how it affects their health.

Pregnant women can be doubly concerned about diet; after all, they are eating for two (or more).  And diet and nutrition are two subjects they should talk about with their obstetrician.  However, there is one group of future mothers for whom a strict diet is critical for their baby’s health:  those with phenylketoruria (PKU).

PKU Basics

Individuals with PKU are lacking a critical enzyme that converts one of the body’s amino acids, phenylalanine, to tyrosine, another amino acid.  Amino acids are the building blocks of proteins, and we need 20 different ones including 11 the body doesn’t make on its own.  Phenylalanine is one of these latter amino acids, known as essential amino acids.

However, we only need a certain amount of phenylalanine.  Normally, the enzyme breaks down the rest.  However, between 1 in 10,000 to 20,000 people lack this enzyme, and phenylalanine builds up in the body.  And this buildup causes problems: for reasons not well understood, it appears normal nerve growth is inhibited.  This can lead to severe intellectual delay, behavioral problems, movement problems, and seizures.  (A more detailed discussion of phenylalanine and PKU is featured in this blog.)

Although PKU is an inherited disease and the baby has about a 1 in 120 chance of having it—and although he will be a carrier of the PKU gene—the majority of these babies will not be affected at birth and will be able to eat a normal diet.

The Dietary Fix

Fortunately, two breakthroughs have occurred that allow those afflicted with PKU to develop relatively normally.  It has been found that most of the complications of PKU can be avoided by restricting the amount of phenylalanine in the diet.  And since the 1960s, screening of newborns for PKU has been available.  This development was particularly important since it’s critical to lower the level of phenylalanine early on in order to produce healthy nerves.

Now done in all 50 states, the PKU test was one of the first to be included in the newborn screen—that drop of blood taken from newborns right before discharge.  That drop now tests for dozens of diseases, but perhaps in homage to the practice’s beginnings, many still refer to the screen as the “PKU test.”

As for dietary management, there is some work involved.  Basically, it involves elimination of high-protein foods, a watch on foods with some protein, and a special formula with the right balance of amino acids.  Moreover, although initially the diet was only continued until age 5, we now know that subtle learning and behavior issues can persist even in adults who fail to follow the diet, so it’s now recommended indefinitely for those afflicted.  This is a time when a support group with tools to help follow the diet is useful.

And for Pregnant Moms with PKU…

Thanks to a landmark research study, we now know a lot more about how to manage maternal PKU.  It turns out that dietary control of phenylalanine becomes even more critical in pregnancy.  There are two primary reasons for this.  First, phenylalanine concentrations are up to 70% to 80% higher in an unborn baby than in the mother.  Second, early pregnancy is a time of critical development in the fetus.  This means that more things can go wrong.  In uncontrolled maternal PKU this means that in addition to developmental delays, an infant may suffer from poor fetal growth, small head (microcephaly), heart defects—even miscarriage.

Because of this critical window, it’s recommended that any woman with PKU who might possibly become pregnant—taking into account that many pregnancies are unplanned—follow the PKU diet.  This is by far the safest action to take for the baby.  If for any reason the diet isn’t followed prior to conception, the American Academy of Pediatrics (AAP) Committee on Genetics recommends getting on that diet by eight weeks of gestation at the latest.

The AAP also recommends that pregnant women with PKU have their phenylalanine level measured at least twice weekly, and that they be followed by a provider knowledgeable in PKU treatment.  As with so many chronic diseases, this can pose transportation, time, and economic problems.  But however moms-to-be manage their disease, it’s worth consulting with their obstetrician, local laboratory—and their insurer—to make sure their pregnancy can and will be handled acceptably.  Ideally, these conversations should take place before becoming pregnant.

Yes, dietary management of PKU does require effort.  It’s probably even more daunting for adults who haven’t followed the diet and then must do so to protect their baby.  But there is some good news for most moms.  Although PKU is an inherited disease and the baby has about a 1 in 120 chance of having it—and although he will be a carrier of the PKU gene—the majority of these babies will not be affected at birth and will be able to eat a normal diet.

Stan Sack
Dr. Stan Sack has 29 years’ experience as a primary care pediatrician in Massachusetts and Florida. A medical writer since 2015, he enjoys blogging on topics that are on parents’ minds but are covered less often in books and on websites. He lives in the Florida Keys with his family and enjoys healthy cooking, fitness activities and singing in his spare time.

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