From La Leche League's Breastfeeding Answer Book:

Phenylketonuria (PKU) is a rare metabolic disorder in which a liver enzyme
is lacking.  As a result an essential nutrient, the amino acid
phenylalanine, is not broken down and accumulates in the blood, interfering
with normal brain development.  If not detected and treated, PKU can cause
mental retardation.

The test for PKU will not be accurate unless the baby has had some
phenylalanine in his diet.  So to be effective, the test should not be
administered until the baby is at least 24 hours old and has been
breastfeeding well.  It is not necessary for the baby to receive cow's milk
or formula before being tested for PKU.

Many babies who test positive for PKU do not have the disorder.  However,
continuing to exclusively breastfeed a baby with PKU can cause serious
problems, so if a baby tests positive for PKU, encourage the mother to have
the baby tested again as quickly as possible to confirm the diagnosis.
Sometimes several retests are needed before PKU can be ruled out.

By being assertive with the doctor and the testing agency, a mother may be
able to greatly reduce the time needed to get results.  For example, the
mother can ask the baby's doctor to call the testing facility and request
special handling of the test.  If the mother is within driving distance of
the testing facility, one option would be for her or someone she trusts to
drive her baby's blood sample to the testing facility.  Overnight delivery
would be another option.  Once the sample arrives at the testing facility,
results may be obtained within a day or two.

A baby with PKU cannot get enough protein from his diet without also getting
too much phenylalanine.  But continued breastfeeding is possible because the
baby with PKU also needs some phenylalanine for normal growth.  This means
that in addition to being fed a special low-phenylalanine formula, the baby
also needs some other protein in his diet.  The mother can continue
breastfeeding while supplementing her baby's diet with low-phenylalanine
formula.  *Human milk is lower in phenylalanine than cow's milk formula*.

A baby with PKU needs to be carefully monitored to be sure the amount of
phenylalanine in his blood does not rise above safe levels.  Some mothers
use a special electronic scale to weigh their babies before and after to get
an accurate measure of how much milk the baby consumed, however, research
has found this procedure to be unnecessary.

In one study (Greve 1994), estimates of average daily human milk intake were
used to calculate how much low- phenylalanine formula should be given per
day to maintain safe blood levels, and mothers gave this amount of special
formula, either offering it before nursing or alternating feedings with
breast and bottle.  Twice weekly blood tests were used to monitor blood
levels and the results used to adjust the amount of special formula given.
Because human milk is lower in phenylalanine than regular formula, less low-
phenylalanine formula was needed for the breastfed babies than the
artificially fed babies, saving money and providing improved nutrition.  The
researchers concluded that although health professionals must spend more
time at first monitoring blood levels and assessing weight gains while
breastfeeding is being established, "eventually breastfeeding decreases the
need for complicated formula mixtures and can make overall management
easier."

A simpler approach is used in Norway (Motzfeldt 1999), where a baby is taken
off the breast when PKU is diagnosed (on average around day eight) and given
special phenylalanine-free formula for one to three days to bring down blood
levels.  Thereafter, the mother gives a set amount of special formula at
each feeding and may breastfeed unrestrictedly after the formula is given.
The volume of formula is set at 65 percent of the baby's total 24 intake and
then divided by the number of feedings.  Of the 74 babies followed in this
study, all had normal growth.  [Even simpler would be giving the low-
phenylalanine formula in certain set 'doses' during the day and just
breastfeeding on cue the rest of the time - this may be what they mean, but
it's not totally clear to me.]

One study found that children who were breastfed before their PKU diagnosis
and treatment (which usually occurred before six weeks of age) scored an
average of 14 points higher on intelligence tests during elementary school
than the children who were fed formula from birth (Riva 1996).  The
researchers concluded that breastfeeding may offer a "positive nutritional
benefit on later childhood neurodevelopmental performance", in part because
human milk contains less phenylalanine than formula, resulting in lower
levels of phenylalanine in breastfed babies before treatment begins.  Also,
human milk contains long-chain polyunsaturated fatty acids, which have been
found to enhance neural functioning.  Artificially fed babies have been
found to have decreased levels of these fatty acids in their brain membranes
after a few weeks of life.

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