The title is from one of those bad punny jokes that you never forget, about taking a bath in milk (pasteurized? No just up to my chin. Boom, boom). For the readers of this blog who are not native English speakers, just ignore this first paragraph and go on to the next.

This is I think a unique study, (Cossey V, Vanhole C, Eerdekens A, Rayyan M, Fieuws S, Schuermans A: Pasteurization of Mother’s Own Milk for Preterm Infants Does Not Reduce the Incidence of Late-Onset Sepsis. Neonatology 2012, 103(3):170-176) the authors from Leuven in Belgium randomized 303 babies (<32 weeks or <1500 g) who were getting maternal breast milk to either get it raw or to have it pasteurized first. This is something which has often been discussed, does pasteurizing milk reduce its anti-infective properties? We have often asked the question because donor milk from milk banks is pasteurized, and we wonder if it will still have the benefits that human milk has before you pasteurize it. We know that pasteurized milk still reduces NEC, as the studies by Alan Lucas from many years ago used pasteurized donor milk in some babies, and they still had the benefits. The data about breast milk and hospital acquired infections aren’t as strong, much less good data in the literature, and the Cochrane review comparing breast milk to formula only includes data from one study, which showed no effect. But we think that human milk feeding probably also reduces some late-onset infections in the preterm baby in an NICU.

We also know that breast milk is not sterile, it is often contaminated with staphylococci, and other organisms which have on occasion been implicated as causing invasive infections. So the authors compared systemic infections between babies that got the pasteurized or raw milk.  The way the title is written is a bit strange, as in the introduction the authors state that they hypothesized that pasteurizing breast milk reduced its benefits as an anti-infective agent, but the title refers to the opposite effect. And, if their hypothesis really was that pasteurization was bad, then there is an ethical issue: why would you test something that is difficult to organize and study, and is not standard practice, if your hypothesis is that it is bad? I think more realistically, that there was real equipoise (which hasn’t been expressed properly), that pasteurizing breast milk may have good effects (reducing the risk of bacterial colonization) and/or bad effects (inactivating immunoglobulins and adversely affecting other important proteins).

The findings of the study were generally negative, that is there was no big difference in any outcome between the groups. What they did show was the there were more infections in the group that received pasteurized milk. A modestly sized increase, compatible with a chance effect, 15% of the babies who received raw milk had at least one infection, compared to 20% of the group who got the ‘past-your-eyes’ stuff. The p value was 0.23. Which is not significant by conventional thresholds, but here again we must question these conventions. Wouldn’t it be more informative for everyone reading this article to state, rather than ‘not significant’ something like the following: ‘there is a 2 in 9 probability that the increase in infections in the pasteurized milk group could have arisen by chance’. Which means in fact, that this ‘non-significant’ result is more likely to be real than not, there is a 7 in 9 chance that the increase in infections is a real difference between the groups.

I am not certainly not saying that this is strong evidence to do anything different, or that we should change practice based on data that have a p value like this, but it is enough evidence for us to think about the issues, and design further, larger, studies to figure out what we should do with breast milk before giving it to our babies.

Of interest the frequency of severe NEC also looks possibly different between the groups, although again, not ‘significant’ (4 cases with pasteurized milk, 0 with raw milk).  Pasteurizing breast milk, as well as killing pathogens, also will inhibit the growth of good bacteria, so we might really be better off to give raw breast milk, as a source of bifidobacteria and lactobacilli. Or maybe we should give pasteurized donor breast milk with extra probiotics, if the mother can’t provide her own milk.

There is another issue with this study which isn’t fully explained, the authors withheld maternal breast milk if the cultures, of the milk, were positive, which sounds like they were doing routine cultures, and then waiting for the results before giving the milk. They don’t describe their protocol for doing this, and we certainly don’t do that. Maternal milk is not routinely cultured in most units in North America. Some babies in the raw milk group received pasteurized milk when the cultures were persistently positive, which is appropriate as part of their research design, but reduces the power of the study.

This study provides no evidence that pasteurizing mother’s milk is beneficial, and suggests that maybe it is better to use raw milk.