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.