Discussion
We successfully nested a large multisite mechanistic study within a large RCT and used a multiomic approach and non-invasive sampling (stool and urine) to explore the impact of supplemental lactoferrin on the microbiome and metabolome. Using this broad, but in-depth approach, we did not demonstrate a significant impact of lactoferrin on the gut microbiome but did identify differences in some unidentifiable metabolomic features. This relative lack of impact of enteral lactoferrin (mean 3% variance) contrasts with other factors which had much larger impacts including infant age (mean 7% variance) and NICU site (mean 30% variance), which explained up to 40% of variance in VOC composition.Our findings are consistent with the lack of a clinical impact of lactoferrin in both the overarching RCT (ELFIN) and a second similar, large RCT.