Macronutrient Concentrations in Human Milk Beyond the First Half Year of Lactation: a Cohort Study

Objective: Human milk composition is dynamic. While extensive research has focused on its macronutrient concentrations during the first 6 months of lactation, limited research exists for extended lactation periods. This study aims to examine the nutritional composition of human milk during these longer lactation phases.

Design: A retrospective longitudinal cohort study performed within the National Dutch Human Milk Bank.

Participants: We selected donors who had provided milk donations at least once after the 6-month postpartum mark.

Main Outcome Measures: The Miris Human Milk Analyser was used to analyse macronutrient concentrations in the milk samples. Linear mixed models were used for longitudinal analysis of these concentrations, factoring in time variables established for six sequential lactation periods.

Results: We analysed 820 milk samples from 86 women, collected between 5 weeks and 28 months postpartum. Initially, milk protein concentrations dropped over the first 8 months of lactation (diff = -0.19 g/dL, p<0.001) and stabilised between 8 and 18 months before increasing again by 0.21 (95% CI 0.06-0.21) g/dL. Carbohydrate concentrations remained steady throughout the study period. Fat concentrations were stable for the first 8 months but saw an increase afterwards. Post 18 months, the fat content saw a rise of 1.90 (95% CI 1.59-2.21) g/dL. The caloric density mirrored the pattern of the fat concentrations.

Conclusion: The nutritional content of human milk does not decrease after 6 months of lactation. Therefore, human milk banks may accept donations from mothers up to 2 years post-birth.