Strengths and weaknesses
We successfully collected, stored and transported >30 000 samples and used existing high-quality clinical data making this study acceptable, cost-effective and clinically relevant. Successful pragmatic daily longitudinal sampling meant that samples in predetermined time windows were available for many infants, including those who developed serious morbidities. We controlled and adjusted for factors with an established impact on the gut microbiome namely gestation, health (LOS or NEC development), NICU site, postnatal age, MOM receipt and exposure to the intervention. Using a multiomic approach with multiple longitudinal non-invasive samples of urine and stool allowed us to explore fixed and variable exposures, and we conducted analyses blinded to the allocation group.The 13 NICU sites reflect a diverse geographical spread across the UK as well as a range of clinical practices. Our data emphasise the strong impact of NICU site on microbiomic and metabolomic outcomes, potentially driven by clinical practices including antibiotic and probiotic use as well as feeding practices, and the potential limitations of similar studies restricted to single sites. The biological activity of lactoferrin is affected by multiple factors including iron binding, surface glycans, processing such as pasteurisation, bovine versus human origin,