A longitudinal study of fatty acid profiles, macronutrient levels, and plasmin activity in human milk

Abstract

Introduction: Human milk provides nutrients essential for infant growth and health, levels of which are dynamic during lactation.

Methods: In this study, changes in macronutrients, fatty acids, and plasmin activities over the first six months of lactation in term milk were studied.

Results: There was a significant influence of lactation stage on levels of protein and plasmin activities, but not on levels of fat and carbohydrate in term milk. Concerning fatty acids in term milk, levels of caproic acid and α-linolenic acid increased significantly (p < 0.05), whereas those of arachidonic acid and docosahexaenoic acid decreased, in the six months after birth. Significant impacts of maternal pre-pregnancy BMI and infant gender on fatty acid profiles were also found. Multivariate statistical analysis showed that protein level, plasmin activity, and several fatty acids (α-linolenic acid, lignoceric acid, and docasadienoic acid) contributed strongly to discrimination of milk from different lactational stages.

Discussion: The study demonstrates that not all but some fatty acids were influenced by lactation, whereas protein and protease levels showed clear decreasing trends during lactation, which may help in understanding the nutritional requirements of infants.

Keywords: fatty acids; human milk; infant nutrition; macronutrients; plasmin.

Figure 1

Study flow chart indicating subject recruitment and sample collection for analyses in the study.

Figure 2

Changes in macronutrient levels [(A) fat; (B) protein; (C) carbohydrate], energy content (D), and plasmin activities (E) in human milk during the first 24 weeks of the lactation. Horizontal lines inside boxes indicate the median value. Lines extending vertically from the boxes indicate variability outside the upper and lower quartiles are indicated. Red points indicate the individual values in each lactational time group.

Figure 3

GC-FID chromatograms of (A) Supelco 37-Component fatty acid methyl esters (FAME) standard mix and (B) FAME from a human milk sample after one-week post-partum. The chromatogram of the milk sample is scaled to show all significant peaks.

Figure 4

PCA and PLS-DA of subgroup human milk samples (n = 50) from term deliveries collected during the first 24 weeks of lactation. (A) PCA scores scatter plot of observations colored according to lactational stages. (B) Corresponding PCA loadings scatter plot of variables measured. (C) PLS-DA scores scatter plot of observations collected colored according to lactational stages (one extreme outlier was excluded). (D) Corresponding w*c loadings scatter plot applies both the X-weights (w*) and Y-weights (c) to identify associations between the collected data (X variables) and lactational stages (Y variables). (E) X variables (macronutrients, plasmin activities, and fatty acids) able to discriminate between lactational groups, ordered by VIP score. VIP scores >1 are significant and indicate important X variables that predict Y responses (lactational stages). (F) Permutation test of the PLS-DA model, indicating the absence of over-fitting in the model. The tolerance ellipse in scores scatter plots were drawn based on Hotelling's T² with 95% confidence interval. PCA, principal component analysis; PLS-DA, partial least squares—discriminant analysis; Carbo, carbohydrate; PL, plasmin; SFA, saturated fatty acids; MUFA, monounsaturated fatty acids; LCPUFAs, long-chain poly-unsaturated fatty acids.