Diane,
We have talked about bottle related risks, so I won't talk about those here,
but quickly some risks of formula use (even one bottle per day)  These risks
are all dose-dependent- the more formula, the greater the risk.  These are
just a few things off the top of my head:
1. excess iron overwhelms lactoferrin - increased risk of diarrhea, botulism,
NEC, tooth decay.
2. change in gut flora to e-coli rather than lactobacillus species leads to
greater chance of infections.
3.  change in bile acid conjugation and fat metabolism, change in cellular
energy availability (carnitine in human milk is necc to transport fuel into
mitochondria, especially in the first 3 weeks of life).
4. lack of lactose reduces calcium absorbtion and cerebroside (brain
constituent) formation.  soy formulas lack lactose.
5. breastmilk is optimized to maximize absorbtion of each nutrient.  giving
other foods, for example, decreases absorbtion of iron from breastmilk.
6. infants cannot digest fats well, human milk contains a bile salt stimulated
lipase, which digests fats in the small intestine, where they belong.  Formula
contains no lipases.
7. for the feeding in which baby gets formula, the trophic (growth promoting)
functions of human milk are not being supplied, nor are the hormones in human
milk.
We just don't know enough to quantify all the risks and disadvantages of
giving any non-human milk or any food other than breastmilk to kids until they
are ready for solids (when they can sit up, grab the food themselves, put it
in their mouths, and chew, IMHO).  It has been determined that there is a dose
response relationship to most of the benefits of breastfeeding - the more
breastmilk, the greater the benefit.  So exclusive bf is best, exclusive
formula feeding is least advantageous, and mixed feeding falls in the middle.
 If the only way a mother can breastfeed is to give a bottle or two here and
there, I will support her, but I will not lie and say it will make no
difference.
You can find a lot of info on the biochemistry of human milk in Ruth
Lawrence's text "Breastfeeding a Guide for the Medical Profession".  Chet
Berlin, MD does a lot of speaking about the biochemical advantages of human
milk as well.
Catherine Watson Genna, IBCLC  NYC  [log in to unmask]