Background: The purpose of these studies was to evaluate the changes in osmolality which occur in human breast milk or formula after two modifications; these changes are sometimes used in preparing these milks for consumption by premature infants, namely, the addition of fortifiers and the addition of exogenous lactase enzyme.
Methods: The osmolality of expressed, previously frozen human breast milk, breast milk fortified with commercial fortifiers, liquid formulas, powdered formulas, or glucose polymers was measured. Osmolality was measured before and after warming (15 minutes at 37°C) or after refrigeration at 4°C for 12 hours with subsequent warming. In a second group of experiments, the osmolality of expressed breast milk and three lactose-containing formulas was measured before and after incubation with lactase (Lactaid ®) at 4°C for 2, 6, and 24 hours.
Results: Warming of breast milk mixed with some of the additives was associated with a significant increase in osmolality. The additives which increased osmolality included liquid glucose polymers, two commercially available powdered human milk fortifiers, and three formulas which contain glucose polymers (a protein hydrolysate infant formula powder, powdered lactose free formula, and a powdered preterm formula). Maximum increase in osmolality of breast milk occurred with the addition of 20 ml/100ml liquid glucose polymers (Polycose, Ross Laboratories) which resulted in a 21% increase in osmolality after refrigeration and warming. The addition of liquid glucose polymers and of powdered preterm formulas containing glucose polymers (Enfacare, Mead Johnson, 9.5 g/100ml and Nutramigen, Mead Johnson, 6 g/100ml) resulted in a final osmolality of over 425 mOsmol/L. The addition of lactase and subsequent incubation under refrigeration resulted in significant increases in osmolality that ranged from 25 to 66% in fortified breast milks and lactose-containing formulas. Incubation of these milks at 37° for 15 minutes produced about 50% greater increase in osmolality than observed after 2 hours of incubation under refrigeration.
Conclusions: Routine warming of breast milk with glucose polymer containing additives, or the addition of lactase enzyme to lactose containing feedings, can increase osmolality to levels that exceed current guidelines for premature infant feedings.