Phenylketonuria (PKU) and the Breastfeeding Dilemma

Phenylketonuria Breastfeeding

Over the past several decades, breastfeeding has been encouraged increasingly and more new mothers have chosen to do it. However, a genetic condition in the baby called phenylketonuria (PKU) has led doctors to recommend immediate weaning of the newborn from the breast. There is a strong reason for this. Babies with PKU develop severe medical and mental abnormalities if exposed to more than a small amount of an amino acid called phenylalanine. Breastmilk contains phenylalanine in levels high enough to cause permanent damage, if the breast milk accounts for all of the child’s nutrition. Since the goal for mothers who decide to breastfeed is, in fact, to supply all of the child’s nutrition through the breast, breastfeeding would seem to be harmful for a child with PKU.

As for the possibility  of allowing the child to nurse a small amount, this is also challenging because it is difficult to measure precisely how much breastmilk a nursing child receives. Management of PKU in a newborn depends greatly on keeping the child from getting too much phenylalanine, yet still allowing him to receive a small amount. This leads to a concentration of phenylalanine in the infant’s blood that should be like a window –not over a certain level but also not below a certain level. You can think of it as a kind of “Goldilocks Zone” for phenylalanine. Like the level for blood sugar in people with diabetes, it’s quite challenging to keep the patient within the Goldilocks Zone, but the situation is more manageable if uncertainties in the diet can be eliminated or at least reduced. Consequently, mothers of PKU babies have traditionally been recommended to give specific amounts of commercial infant formula that does contain phenylalanine in addition to a very special formula that does not contain phenylalanine. In this way the doctors can figure out exactly how much phenylalanine is going to the infant. Knowing this, the doctor can calibrate the ingested amount of phenylalanine to the infant’s measured blood levels of phenylalanine and adjust the diet as needed. In recent years, however, pediatricians have become more open to allowing breast milk into the mix, even though it does add a higher level of uncertainty.

What is phenylalanine, what is PKU, and why is dietary control so important?

The answer to these questions has to do with proteins, on of the principal categories of biological chemicals of which your cells and body are composed. Proteins contribute both to the structure of cells and what is in them as well as to their function. We cannot live without proteins, so our cells are programmed to build them. Building proteins requires building blocks and these building blocks are called amino acids.

Now, we need 20 different amino acids, although many others exist. Many of these 20 amino acids we can manufacture in our cells, because we have genes that encode enzymes, special proteins that help build them from parts of other amino acids and parts of other kinds of molecules. But there 9 amino acids that we do not have the enzymes to build. Certain animals can build some of them but, as mammals, we humans cannot. We must get them in our diet –by ingesting protein from plants, animals, or other types of organisms, such as algae or mushrooms. They are called essential amino acids, and phenylalanine is one of them.

Now, if everything is working well, you are just fine if you eat more phenylalanine than you need. Your cells make an enzyme, called PAH, that converts the excess phenylalanine into another amino acid, called tyrosine. Normally, tyrosine is one of the amino acids that you do not need in your diet (it is not an essential amino acid), since you can just make it from phenylalanine. But the problem comes when the gene that encodes PAH is defective. You have 2 copies of this gene, one from your Mom and the other from your Dad on chromosome number 12. Having one normal copy of the PAH gene and one defective copy makes a baby a carrier for PKU, but having both copies defective means that he has PKU.

If PKU is not recognized immediately after birth, the consequences are devastating. The baby will develop serious heart problems and in other internal organs, and the brain will not develop properly, leading to severe mental disabilities. However, if PKU is diagnosed early, dietary control can allow everything, including the brain, to develop normally. Screening of newborns for PKU has been occurring since the 1960s, so there are many adults living with PKU worldwide. In the meantime, a medication called sapropterin has come into use, as it can help to control phenylalanine levels in 25-50 percent of people with PKU.

Because monitoring of phenylalanine blood levels is now highly advanced, quick, and reliable, the attitude about breastfeeding of infants with PKU is shifting. This does not mean that you can simply support your PKU newborn purely on breastmilk, but it does mean that you can work in concert with your pediatrician to get some breastfeeding mixed in. In some cases, you may be able to breastfeed your infant directly, and in other cases pump your milk so as to be able to give measured quantities in a bottle. Either way, if this is something that you want to do, it’s going to require a great deal of care in terms of measuring and calculations and generally paying attention to minute details. Things get even more complicated for mothers who have PKU themselves, and that’s a topic that you can explore in a report that we are preparing specifically on maternal PKU.

David Warmflash
Dr. David Warmflash is a science communicator and physician with a research background in astrobiology and space medicine. He has completed research fellowships at NASA Johnson Space Center, the University of Pennsylvania, and Brandeis University. Since 2002, he has been collaborating with The Planetary Society on experiments helping us to understand the effects of deep space radiation on life forms, and since 2011 has worked nearly full time in medical writing and science journalism. His focus area includes the emergence of new biotechnologies and their impact on biomedicine, public health, and society.

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