Heidi asked if there was a reference regarding the mixing of breastmilk and
formula. The one I use to illustrate the down side of fooling with mother
nature is the following:
Quan R, Yang C, Rubinstein S, et al: The effect of nutritional additives on
anti-infective factors in human milk. Clinical Pediatrics 1994; June:325-328.

Mixing of breastmilk and cow's milk based formula decreases lysozyme
activity. In this study it was between 41%-74% reduction depending on which
formula was used. Human milk fortifier reduced activity by 19%.

Hope this is helpful