Longitudinal and circadian variation in breastmilk macronutrient composition across the first six months of lactation - associations with maternal determinants and infant anthropometric development

Background. The composition of breast milk depends on the lactation period, the phase of single breastfeeding, time of day, and frequency of breastfeeding throughout the day, alongside other maternal and infant variables. This study aimed to assess the macronutrient and energy content of breastmilk during the first six months of lactation and evaluation of maternal determinants and association with infant anthropometric development. Material and methods. Breast milk samples were collected from 47 mothers from Warsaw in the first, third, and sixth month of lactation. Breast milk macronutrient composition was assessed using the MIRIS Human Milk Analyzer. Maternal diet was assessed using 3 -day food records in the third and sixth months of lactation. At each study visit, we assessed maternal and infant anthropometric parameters. Results. Protein content declined consistently from 1.3 g/100 ml in the first month to 1.0 g/100 ml in the sixth month (p < 0.001). However, fat, lactose content, and energy values showed no significant changes throughout lactation. Fat content and milk energy value varied over time of the day in the 1st and 3rd month of lactation. There were no significant differences in the protein and lactose content according to the time of the day. Breastmilk fat adversely correlated with pre -pregnancy BMI, but not current BMI. The energy values of the diet were 2180 +/- 614 and 2046 +/- 503 kcal (p > 0.05), respectively, in the third and sixth month of lactation. The majority of mothers had lower than recommended intakes of vitamin D, folate, vitamins B1, B2, B3, B6, B12, C, A, E, magnesium, zinc, and calcium for at least one of the study periods. However, maternal dietary intake and anthropometric parameters showed a limited correlation with breastmilk macronutrient composition. We did not observe any significant correlations between breastmilk macronutrient composition and infants' body mass, length, or BMI to age z -score, nor changes in these parameters between the third and sixth month of life. Conclusion. Our findings support the assumption that breastmilk offers chrononutrition to infants. It is important to consider diurnal changes in its composition, especially when the infant is fed expressed breastmilk. Furthermore, it is necessary to implement educational programs targeting maternal nutrition, as many women do not maintain adequate dietary intake.