I heard this on the radio, and they said the levels were so low that breast feeding still was better for the child than formula.
I do believe it is the added immuninty from breast milk and how kids who are breast fed don't get constipated.
It is really pretty cool, my kids were breast fed and they would go a week without a poopy diaper. Their body used everything it got. When we switched to formula (at 6 months,) we went to 3 poopies a day.
(Heh this should be in the poop thread, but I think it is fair discussion given who started the thread. 😛)