Major losses of fat, carbohydrates and energy content of preterm human milk frozen at-80°C

Objective:Long-term storage of human milk (HM) requires freezing at low temperatures, the consequences of which upon macronutrients are unclear. To test the null hypothesis that HM freezing and storage for a range of 1 to 10 weeks at-80 °C does not affect HM fat, protein, lactose and energy contents.Study design:Samples of HM were obtained from 20 mothers (60 samples) of preterm infants (25 to 35 weeks gestation), who routinely expressed their milk, every 3 h, using an electric pump, from the second to the seventh week after delivery. All samples were frozen at-80 °C for 8 to 83 days (43.8 days average). After thawing and homogenization, energy and macronutrient contents were measured using an HM analyzer.Result:Fat, carbohydrates and energy contents were significantly lower in thawed HM than in fresh HM (fat, fresh vs thawed: 3.72±1.17 vs 3.36±1.19 g/100 ml, P<0.001; carbohydrates, fresh vs thawed: 5.86±0.71 vs 4.09±0.96 g/100 ml, P<0.001; energy, fresh vs thawed: 64.93±12.97 vs 56.63±16.82 kcal/100 ml, P<0.0001), whereas protein content remained unchanged (protein, fresh vs thawed: 1.14±0.36 vs 1.15±0.37 g/100 ml, P=0.7). The decline in carbohydrates content but not in fat and energy correlated significantly with freezing duration.Conclusion:Freezing at-80 °C significantly decreases the energy content of HM, both from fat and carbohydrates. Since quantitatively the decrease in macronutrients was much higher than that published for HM storage at-20 °C, our results do not support freezing HM at-80 °C as the gold standard for long-term storage. We suggest that caloric intake calculations in preterm infants cannot be established based upon fresh HM data.