The research also concluded that any human milk was better than none, and that the more human milk a preterm infant received, the better the outcome.
Human milk has been established as the best feed for preterm infants, but when mothers are unable to produce enough milk, supplementation with either preterm formula or donor human milk is used; the latter is considered the next best choice after mother's own milk.
Since the 1980s, pasteurised human milk has been reported to have numerous advantages for infants, including better cognitive health and protection against infection and necrotising enterocolitis, especially for those of very low birth weight.
However, this information is outdated. In countries like Norway, the donor human milk often used is unpasteurised, which is thought to increase its health benefits.
As such, a systematic review and meta-analysis led by the South Australian Health and Medical Research Institute was conducted to directly compare exclusive feeding with human milk to that with preterm formula.
The researchers also sought to determine if human milk was still protective in infants who were fed both human milk and preterm formula, and the dose-related effects of human milk. In addition, they aimed to elucidate the impact of human milk pasteurisation on infant morbidity.
The review covered post-1990 literature on human milk's effects on morbidity, specifically necrotising enterocolitis, late onset sepsis, retinopathy of prematurity, bronchopulmonary dysplasia, and neurodevelopment in infants born after gestation of 28 weeks or less.
This included papers that had reported mean infant birth weight of 1.5kg or lower. A total of 49 studies — six RCTs involving 1,472 infants and 43 observational studies involving 14,950 infants — were reviewed.
The researchers stated that human milk had a "clear protective effect" against necrotising enterocolitis, resulting in an approximate 4% reduction in incidence. It might also have led to a reduction in late onset sepsis, severe retinopathy of prematurity, and severe necrotising enterocolitis (2% reduction in the latter).
Additionally, a diet of exclusively human milk was linked to a possible 5% reduction in late onset sepsis, though no dose-dependent effect was observed.
When it came to necrotising enterocolitis in particular, any volume of human milk was superior to exclusive preterm formula, with dosage directly proportional to the level of protection.
With regards to pasteurisation, the evidence was inconclusive, but the researchers said it seemed to have no impact on certain outcomes. Instead, enhancing infants' consumption of mother's own milk and / or donor human milk led to small improvements in morbidity.
There was also inconclusive evidence that exclusive or any human milk influenced the incidence of bronchopulmonary dysplasia or retinopathy of prematurity, save for a possible effect of exclusive human milk — compared with formula — on severe retinopathy of prematurity (7.6% reduction).
The outcome of human milk on neurodevelopment was complicated by the variation in test timing, as well as the different tests used, leading to insufficient evidence in this aspect.
The researchers wrote: "This supports a policy of moving to 100% human milk for necrotising enterocolitis protection when mothers are unable to meet all their infant requirements."
However, they also said that the individual studies reviewed were not "sufficiently powered" to determine the mean differences between feeding groups.
They concluded: "We have shown that any human milk is better than none, that the more human milk the preterm infant receives, the better the outcome, and that for necrotising enterocolitis, there is an advantage in topping up infants who are already receiving quite large proportions of their enteral intake as human milk to exclusive human milk.
"From a clinical perspective, it would seem just as important to offer donor human milk to an infant who is getting nearly all mother’s own milk as it is for an infant who is getting none.
"The benefits of human milk-feeding are difficult to study, given that it is not ethical to randomise breastfeeding. However, there is a need for large and well-conducted studies, designed to answer specific questions, particularly in relation to the effects of donor human milk and pasteurisation.”
"A Systematic Review and Meta-Analysis of Human Milk Feeding and Morbidity in Very Low Birth Weight Infants"
Authors: Jacqueline Miller, et al.