A baby that has only had breastmilk actually has a very different environment in their intestines.
A young infant actually has cells that are farther apart then an adult, which let larger molecules of food into thier blood stream (I believe this is to help all those live germ fighting cells you are producing actually get a chance to fight whatever is attacking her, and there have been studies that show that exclusively breastfed say 7 month olds have less sickness then 7 month olds that get breastmilk and solids).
Which is why foods that have a higher risk for allergies are not recomended until over 1, because those larger pieces of peanut or wheat protiens that get into her blood stream can cause her body to over react.
Infants don't have the same digestive jucies that we have (that's why carrots look the same coming out), they really are not ready to digest many foods yet. (many of these aren't at high enough levels until after 1 year old).
Other changes that happen lessen the intestines ablility to digest iron, so an exclusively breastfed baby is really at less of a risk for iron deficency then one that starts solids early.
Early solids increase the risk for obesity.
Starting solids can have bad effects on your milk production.
It can cause more fussyness, night waking, spitting up, ect.
It is a mess- more dishes, laundry, baths, ect.
I don't think my 4 month olds are really prepared to make these kind of decsions about thier health.
How was your trip? How is that baby? We are leaving next saturday, but I hope we can get together when we get back!