Lil - Formula milk has certainly gone through rigourous testing over the years. In the past it has had various things missed out and too much of other things added causing illness but more recently it doesn't appear to cause any specific illness. Certainly the fact that it is heavier (casein protein which turns to curds) means that it is less digestible, and some mothers may overfeed their babies and they mix the feed too strong etc etc. Also bottles may cause more wind because of air being trapped in the bottle. Formula in itself is of course not poison and it is not going to harm your baby feeding it or doing a mixture of both.
However to say which is better ... well you cannot really compare the two because you are not comparing the same thing. Formula milk cannot exactly replicate breastmilk because the latter is specifically designed for humans and the latter for cow's. Therefore breastmilk will always be 'better' than formula. This is not to say that humans cannot thrive adequately on formula.
See the following:Infant Formula:
Second Best but Good Enough
www.fda.gov/fdac/features/596_baby.html
"While greater knowledge about human milk has helped scientists improve infant formula, it has become "increasingly apparent that infant formula can never duplicate human milk," write John D. Benson, Ph.D, and Mark L. Masor, Ph.D., in the March 1994 issue of Endocrine Regulations. "Human milk contains living cells, hormones, active enzymes, immunoglobulins and compounds with unique structures that cannot be replicated in infant formula."
The main differences are that cow's milk contains larger amounts of protein designed for the need of calves to grow rapidily. Human milk has higher levels of lactose and specific long-chain fatty acids which are important to the development of the brain specifically DHA and AA which are important in the process of myelination (b/fed babies do better on IQ tests).
Also human milk contains live cells which protect baby from disease and enzymes and hormones that support physiological development.
Recent studies have shown that human milk helps to reduce cholesterol and heart disease (this was posted on the Mumsnet board a few weeks ago). Breastfed babies are less likely to develop diabetes.
Breastmilk is lighter and more quickly absorbed (human milk has whey protein) hence why baby wakes more quickly but this does mean it is better digested and therefore less stomach problems. Milk sugar (lactose) promotes the growth of Lactobacillus bifidus in the infant gut which thwarts the growth of bacteria which can cause diarrhea. Human milk contains one and a half times more lactose than cow's milk. The increase in milk sugar in formulas is made up by sucrose.
Formula milk produces more waste so it does mean more bulky (and smelly!) nappies. Also if a child is ill and likely to be dehydrated on formula you have to give them a special mixture for rehydration. This is not usually necessary for breastmilk as the water content can vary as indeed any of the contents can over a feed or a day.
Human milk is specially designed for your baby alone whereas formula is all the same. If you and baby are ill your milk will produce antibodies which will mean you will recover quicker. Breastfed babies have much fewer ear infections...
You've probably seen all this stuff on posters anyway but as you asked.
So you can see it is not just a case of nutrition but of how the milk itself is digested and excreted and of the many features of the milk itself. Milk for a child has to feed the body and brain and other foods cannot replicate this. then there is the immunological properties which I have mentioned. No matter how healthy our society is we still have lots of minor illnesses to deal with. Some workplaces would argue that we all take too much time off sick! Breastfed children tend to be healthier with less sickness, diahorrea, and ear infections.
Yes indeed many of us have been bottle fed and are perfectly healthy. However it depends on how you look at it - there are plenty of people around with heart disease for example. As already stated there are links to formula feeding and this condition.
Breastfed babies are rarely overweight (see www.bmj.com/cgi/content/full/319/7224/1576 - this states that the longer the child is breastfed the less likely they are to be obese in later life.
Breastfeeding lowers your risk of breast cancer.
And so on - See 101 Reasons to Breastfeed your child for more info: promom.org/101/index.html