Impact of Short-Term Freezing at -20°C on Macronutrient Content in Human Milk from Preterm Infants

Objectives: Breast milk is an ideal food for newborns born term and preterm. The short-term storage of human milk (HM) involves freezing at low temperatures; however, its effects on macronutrients remain unclear. The macronutrients in HM are generally thought to be unaffected by short-term freezing. This study aimed to analyze the impact of freezing HM from preterm infants at -20°C for 5 days on macronutrient content. Methods: HM samples were collected from 54 mothers of preterm infants. Each sample was divided into three aliquots and stored at -20°C for 1 and 5 days. After thawing and homogenization, energy and macronutrient contents were measured using a HM infrared spectroscopy analyzer. Results: We analyzed 162 samples (from 54 mothers of preterm infants). Mean baseline concentrations in fresh milk were protein 1.55 ± 0.61 g/100 mL, carbohydrates 6.87 ± 0.81 g/100 mL, fat 4.29 ± 2.16 g/100 mL, and energy 75.9 ± 27.3 kcal/100 mL. Freezing at -20°C reduced protein to 1.33 ± 0.35 g/100 mL after 24 hours and 1.21 ± 0.38 g/100 mL after 5 days (14.2% and 21.9% decrease, p < 0.001), fat to 3.23 ± 1.15 g/100 mL and 3.04 ± 1.40 g/100 mL (24.7% and 29.1% decrease, p < 0.001), and energy to 64.3 ± 12.8 kcal/100 mL and 61.7 ± 16.1 kcal/100 mL, respectively (p < 0.001); carbohydrate content remained unchanged (p = 0.41). Conclusions: Freezing HM at -20°C for 5 days can significantly reduce critical nutrients, including fat and protein, suggesting the need for individualized fortification strategies to ensure optimal growth in preterm infants.