Early Feeding in PKU

At the first consultation with your specialist team, you will have been advised on how to bring your baby’s Phenylalanine (Phe) level down as quickly as possible into the safe range. This usually involves feeding solely with a protein substitute (Phe-free infant formula) for a set time period with frequent blood monitoring. Breastmilk or infant formula will then have been reintroduced under close supervision.

In these early stages of feeding, the essential Phe in the diet is provided by breastmilk or infant formula. The amount is adjusted according to blood test results which your dietitian will be calling you with. A protein substitute is then used to provide all of the other building blocks of protein (amino acids) and key nutrients needed for normal growth and development.

To remove the Phe from the protein substitute, the protein needs to be broken down into individual amino acids. The result is a slightly different smell and taste, but your baby should take this readily and build taste acceptance from early on. Being positive and firm with the protein substitute from the very beginning is important to build a strong foundation for your baby’s dietary management in the future.


Having a baby with PKU does not need to hinder successful breastfeeding; if breastfeeding is an option, it can provide many benefits to you and your baby. The essential Phe is provided by breastmilk; but needs to be limited to ensure that blood levels do not exceed the safe range.

Your team will help you to achieve this by recommending your baby take a measured amount of protein substitute (Phe-free infant formula) before breastfeeds. The baby can then be put to the breast and can feed to demand, unless advised otherwise by your healthcare professional. The protein substitute should displace some of the breastmilk taken, and keep blood levels under control.

It is important that the protein substitute is always offered before breastfeeds, and spread evenly throughout each day . If your doctor or dietitian has advised otherwise, please discuss this at the next opportunity.

Bottle feeding

If you choose to bottle feed your baby, the essential Phe will be provided by a standard infant formula. This needs to be measured carefully to ensure that blood Phe levels do not exceed the safe range.

Your dietitian will help you to achieve this by prescribing a volume of infant formula; which should be spread over several feeds throughout the day. A protein substitute (Phe-free infant formula) can then be used to ‘top up’ feeds, and you dietitian will likely advise that your baby can drink as much of this as they need.

It is important that infant formula feeds are offered first, so that you and your dietitian can keep track of the amount of Phe that your baby has taken each day. Try to give the infant formula and protein substitute in separate bottles; this also helps you to keep an accurate record of what your baby has taken, and ensures that your baby will get used to the taste of the protein substitute.

More Information

For more information on the Phe-free infant formulas, visit the Products section of the website.

We also have a useful parent’s guide.

NOTE: The dietary management for PKU varies for each person so all information presented here is for guidance purposes only. The information provided on this page is in no way intended to replace the care, advice and medical supervision of your healthcare professional. Your own dietitian and/or doctor will advise you on all aspects relating to management of PKU for you and your family. Always consult your healthcare professional before making any changes to your child’s low-protein diet.

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