Objectives
The safety and feasibility of human milk fortification with bovine colostrum (BC) were investigated in very preterm infants (FortiColos trial, NCT03537365). The BC product contained lower calcium, phosphate, and iron levels compared to the conventional fortifier (CF). We tested whether fortification with BC plus extra phosphate was sufficient to support the infants' mineral status assessed by blood biochemistry.

Methods
In a randomised controlled trial (FortiColos, NCT03537365), mineral status was compared after fortification with BC versus CF. Blood calcium, phosphate, and haemoglobin were determined before and up to 3 weeks after the start of fortification (at the mean age of 8–9 days). The maximum supplemental doses of calcium, phosphate, and iron given were retrieved from patient medical records. Results were adjusted for gestational age, birth weight, and enteral nutrition with the mother's own milk and/or donor human milk.

Results
Blood values of calcium, phosphate, and haemoglobin were similar between groups. Infants in both groups required supplementation with calcium and phosphate, but infants fed BC required higher maximum doses of phosphate and calcium (p < 0.05) to maintain acceptable blood values. Regardless of fortification groups, the most immature (<29 weeks of gestation) and small for gestational age infants showed a higher risk for requiring additional phosphate (OR: 3.9, p < 0.001; OR: 2.14, p = 0.07, respectively).

Conclusions
The use of BC as a fortifier for human milk requires additional phosphate and calcium relative to a CF. Regardless of the fortification product, the most immature and small infants require additional mineral supplementation.