In this study, the researchers compared extremely premature babies receiving human milk fortified with either a human-milk-based or a cow’s-milk-based fortifier. The babies who received no cow’s milk-based products had lower rates of necrotizing enterocolitis (NEC).
Sullivan S, et. al. An Exclusively Human Milk-Based Diet Is Associated with a Lower Rate of Necrotizing Enterocolitis than a Diet of Human Milk and Bovine Milk-Based Products. J Pediatr. 2010 Apr;156(4):562-7.e1.
CONCLUSIONS: For extremely premature infants, an exclusively human milk-based diet is associated with significantly lower rates of NEC and surgical NEC when compared with a mother’s milk-based diet that also includes bovine milk-based products.
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Here is some information on the use of breastmilk fortifiers for very low birthweight (VLBW) premature babies (p. 33 of the document is a good place to start):
Best Practice # 3.4: VLBW infants fed human milk should be supplemented with protein, calcium, phosphorus and micronutrients. Multinutrient fortifiers may be the most efficient way to do this when feeding human milk. Formula fed infants may also require specific caloric and micronutrient supplementation.
And a bit more from Lucile Packard Children’s Hospital at Stanford: