Mild Hyperphe

The Young Woman with Mild Hyperphenylalanine (BPH)

A Publication of
Department of Pediatrics, Maternal PKU Study
University of Texas Medical Branch
Galveston, TX 77555-0319
Copyright (C) 1992

The purpose of this booklet is to inform you and your family of the importance of going to a physician skilled in treating metabolic disorders for information on Mild Hyperphe (BPH) and pregnancy. This booklet contains:

  • An explanation of Mild Hyperphe (BPH) 
  • A description of the possible effect of Mild Hyperphe (BPH) on pregnancy
  • An explanation of a research project studying the risks of Mild Hyperphe (BPH) and pregnancy
  • A list of Metabolic treatment centers (which includes PKU)

The authors encourage the widespread reproduction and distribution of these materials for any educational and/or patient care related purpose and ask only that the source of the materials be acknowledged.

We hope this booklet will be a useful guide for you.

What is Mild Hyperphe (BPH)?

Mild Hyperphe (BPH) is an inherited condition in which a person's body is unable to properly utilize one part of protein found in food. This condition causes mildly increased levels of phenylalanine in the blood. There are no known problems associated with having Mild Hyperphe (BPH) as a child so dietary treatment is usually not required then. However, research suggests special attention might be needed for the woman with Mild Hyperphe (BPH) during pregnancy to protect her babies from birth defects.

Hyperphe is an abbreviation of the medical term Hyperphenylalaninemia (pronounced hyper-fenel-al-ah-ninemia). This long medical term can be divided into three parts and defined as follows:

  1. Hyper: means high
  2. Phenylalnine: is an amino acid (a part of protein) found in food
  3. Emia: refers to the blood

Phenylalanine will be called PHE in this booklet.

To understand why the PHE is increased in the blood of a person with Mild Hyperphe (BPH), we need to explain how the body handles proteins found in foods. Proteins are made up of amino acids fastened in a chain, as shown in Figure 1.

One of these amino acids is called PHE. The body takes food protein, digests it into amino acids to construct all organs and tissues in the body. The process is as follows: when protein is eaten, it is digested into the individual amino acids. The amino acids are then absorbed from the intestinal tract into the blood stream and carried around the body to where they are needed to build body proteins. The amino acids remaining after sufficient protein has been formed are broken down into waste products. PHE, for example, is converted into another amino acid (tyrosine) in the liver.


Amino Acids

amino acids

Figure 1: Proteins are made of Amino Acids fastened in a chain. Phenylalanine (PHE) is an amino acid.

The person who does not have Mild Hyperphe (BPH) changes PHE to Tyrosine. This change is caused by the action of an enzyme in the liver. An enzyme causes this change to occur as shown in Figure 2. The person with Mild Hyperphe (BPH) cannot change PHE into Tyrosine well because the enzyme does not work properly. Excess PHE builds up in the blood (Figure 3).

In a person who does not have Mild Hyperphe, an enzyme in the liver changes PHE into Tyrosine.

Figure 2: In a person who does not have Mild Hyperphe (BPH), an enzyme in the liver changes PHE into Tyrosine.

In a person with Mild Hyperphe, the enzyme does not work properly causing PHE to build up in the blood.

Figure 3: In a person with Mild Hyperphe (BPH), the enzyme does not work properly causing PHE to build up in the blood.

Are There Different Types of Hyperphe?

Mild Hyperphe (BPH) is one of three known types of Hyperphe. Each type is classified by the amount of PHE that can be measured in the blood. Blood PHE is reported by the laboratory as mg/dl (milligrams per deciliter) or mmol/L (micromoles per liter) of blood. They are known as Benign Persistant Hyperphenylalania (BPH), MILD Hyperphenylalania and Classical PKU as listed on the chart below.

Unlike PKU, the child with Mild Hyperphe (BPH) usually does not need a special diet. However, women with Mild Hyperphe (BPH) should have their blood levels checked during pregnancy because they may need to be on a diet low in PHE during the pregnancy to protect the unborn baby from birth defects.

Standards Used by the Texas Department of State Health Services
Standards Used by the Texas Department of State Health Services
Blood Level of PHE
< 4
< 240
Normal None
4 - 6
Mild Hyperphe (BPH) A low PHE diet is usually not prescribed during childhood.

A low PHE diet may be needed during pregnancy.

MILD Requires a low PHE diet.

Women must have a low PHE diet before and during pregnancy.

20 and higher
1200 or higher
Classical PKU A low PHE diet is needed to prevent intellectual disabilities.

Women must have a low PHE diet before and during pregnancy.

How is Mild Hyperphe (BPH) Detected?

There are two times in a person's life when Mild Hyperphe (BPH) is usually discovered; when a child is born with Mild Hyperphe (BPH) and when a woman has a baby with Mild Hyperphe (BPH).

A child with Mild Hyperphe (BPH) is usually discovered shortly after birth when a screening test is done by collecting blood from the baby's heel. If the results of that test show a slightly elevated blood PHE level, Mild Hyperphe (BPH) is diagnosed. The PKU treatment center will decide if a special diet is needed.  


How Did I Get Mild Hyperphe (BPH)?

Genes are responsible for your characteristics, such as eye color and height. Genes are also responsible for instructing the cells in the liver to make the enzyme that changes PHE to tyrosine. When the gene responsible for making this enzyme is altered, Mild Hyperphe (BPH) occurs. Below is a simple illustration of how the altered gene was passed from your grandparents to your parents and to you.

Genes are units of inheritance found in body cells. Genes come in pairs in your cells with each parent contributing one gene to the pair in each cell. A person who has one "normal" gene and one altered Mild Hyperphe (BPH) gene is called a carrier.

Generation I represents the possible genetic makeup of your grandparents. One of your maternal grandparents and one of your paternal grandparents are carriers of the Mild Hyperphe (BPH) gene. One in 70 persons are carriers of the Mild Hyperphe (BPH) gene. 

Generation I

Generation II represents your parents both of whom are carriers of the Mild Hyperphe (BPH)  gene. The chances of two carriers marrying is one in 5,000. 

Generation II

Generation III represents your genetic make-up and the possible make-up of the other children in your family. For each pregnancy there is a one in four chance the child will have Mild Hyperphe (BPH). You inherited the Mild Hyperphe (BPH) genes from each of your parents. Mild Hyperphe (BPH) occurs once in 20,000 births. 

Generation III

Your parents and grandparents probably were not aware that they were carriers of the PKU gene. This is because the carrier's "normal" gene tells the liver cells to make enough of the enzyme to break down PHE. Your parents and grandparents carry the Hyperphe gene but do not have elevated blood levels.

What Are the Effects of Mild Hyperphe (BPH) on an Unborn Child During Pregnancy?

There are no known problems associated with having Mild Hyperphe (BPH) as a child so treatment is not needed then. However, research suggests special attention may be needed for the woman with Mild Hyperphe (BPH) during pregnancy to prevent birth defects.

In 1984, the Maternal PKU Collaborative Study began collecting information on Hyperphe during pregnancy. Women with various types of Hyperphe, including Mild Hyperphe (BPH), were enrolled and followed during pregnancy by the Study. The results show that women with Mild Hyperphe (BPH) should be followed by a PKU center before and during pregnancy. The center monitors the woman's diet and blood PHE levels and, if necessary, puts her on a diet to avoid possible damage to the baby.

The potential damage for the unborn baby during pregnancy works this way. As blood flows from the mother through the placenta during pregnancy, nutrients, including PHE, are transferred to the baby. The placenta concentrates some nutrients, like PHE, to help the baby grow. The blood PHE level may increase 1½ times in the baby. For example, a mother's blood level of 7 mg/dL (420 mmol/L) may be concentrated by the placenta to 11 mg/dL (660 mmol/L) before passing on to the baby as illustrated in Figure 4. Blood PHE levels that are too high place the baby at risk for birth defects.  


Figure 4: The mother's blood PHE is concentrated by the placenta, causing the level actually reaching the baby to be higher.

The special care needed for women with Mild Hyperphe (BPH) during pregnancy includes evaluating the foods eaten to be sure the diet is proper for pregnancy and checking blood PHE levels. The exact blood PHE level at which dietary treatment should be started by the pregnant women with Mild Hyperphe  (BPH)is not yet known. The Maternal PKU Collaborative Study was designed to determine the level of blood PHE "safe" for the unborn baby. If dietary treatment is recommended, it may be necessary to replace part of the protein in the diet with a Medical Food Product free of PHE and to measure the amounts of low protein food eaten.

Image Courtesy of ATL Ultrasound

What If I Decide to Have a Baby or Think I Am Already Pregnant?

Call the Metabolic treatment center nearest you for information about Mild Hyperphe (BPH) and pregnancy. See Appendix for the listing of clinics.

Metabolic treatment centers specialize in treating persons with Hyperphe. The staff consists of a physician, dietitian, and consultants such as a nurse, psychologist, and social worker. They will explain Mild Hyperphe (BPH) and the concerns of Hyperphe during pregnancy.

At your initial visit, a schedule of tests and evaluations will be arranged for you before and during pregnancy. The staff at the PKU center will inform your obstetrician or family doctor of the recommended schedule. Care provided by the PKU center and your local doctor will give you a better chance of having a healthy baby.  


  1. Mild Hyperphe (BPH) means you have a mild elevation of Phenylalanine (PHE) in your blood.
  2. To protect your baby during pregnancy, you should be followed by a PKU treatment center and your obstetrician or family doctor.
  3. To obtain further information, contact the nearest PKU center, see Appendix

Written by:
Lois Castiglioni, M.S., R.D., L.D.

Bobbye M. Rouse, M.D.
Professor, Pediatrics

Cindy Bauman-Frischling, M.P.H.
Barbara Gross, M.S., R.D.
Deborah Lobbregt, B.S.
Wanda Schoonheyt, R.N.

Edited by:
Cathleen Connolly, R.D., L.D.

This booklet was made available for use in the Maternal PKU Collaborative Study and funded in part by the National Institute of Child Health and Human Development (contract #NO-1-HD-2-3156).

N.I.H. Project Officer: Felix de la Cruz, M.D. Contracting Officer: Harvey Shifrin 

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Last updated January 20, 2011