Phototherapy breaks down bilirubin to a water-soluble molecule that can
indeed be excreted in urine.  
This makes the excretion of bilirubin go faster than it would if the baby
were left to its own physiologic mechanisms for excretion.  It has no effect
whatsoever on the underlying process causing the elevated bilirubin.  Not
until the baby's capacity to get rid of bilirubin is greater than the rate
at which bilirubin is produced, does the jaundice fade.  
Total serum bilirubin consists of two types of bilirubin, unconjugated and
conjugated.  Unconjugated bilirubin is not water soluble; it circulates in
serum and if levels are higher than normal, it is deposited in fatty tissue,
causing visible jaundice.  Unconjugated bilirubin is conjugated in the
liver, and then sent down the bile duct to the gut, after which it is
excreted in stools, giving them the yellow color typical of a healthy,
thriving baby.  It is the excess unconjugated bilirubin resulting from the
dying off of surplus fetal red blood cells that is responsible for the
visible jaundice seen in *normal* newborns.  Conjugated bilirubin is
water-soluble and can be excreted in urine, but it is only if gut excretion
is hampered in some way that this becomes a clinically significant route. 
Normally about 90% of bilirubin is excreted in stools, and 10% in urine. 

An otherwise healthy baby who is not feeding well, will not stool as often,
and the conjugated bilirubin in its gut can be re-absorbed across the
intestinal mucosa, adding to the existing total bilirubin load and
prolonging the period in which visible jaundice is present.  It was formerly
believed that the urinary route was more important than we know it to be
today, and that is likely why babies were given extra fluids as treatment
for jaundice in the past.  We know now that the baby needs food, not fluids,
in order to clear the bilirubin from its system.  However, if the baby is
treated with old-fashioned phototherapy, which involved large fluid loss
from evaporation via the skin, they may need more fluids than they are
getting from milk alone.

In looking for the details I needed to refresh to write this post, I found
the following site, and I thought it was pretty good.  The info for parents
was quite clear, meaning even *I* understood it.
http://www.childliverdisease.org/education/yellowalert

Rachel Myr
Kristiansand, Norway

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