The participants were divided into four groups based on breastfeeding periods: 1-6 months (n = 43), 6-12 months (n = 47), 12-18 months (n = 50) and 18-24 months (n = 44). There were no significant differences between the groups with respect to demographics or baseline characteristics (Table 1).
Table 1. Characteristics of the Study Population.
|
|
Breastfeeding 1-6 months
(n = 43)
|
Breastfeeding 6-12 months
(n = 47)
|
Breastfeeding 12-18 months
(n = 50)
|
Breastfeeding 18-24 months
(n = 44)
|
P-value
|
Maternal age 1 (mean ± SD, median, years)
|
32.6 ± 3.6,
32.0
|
30.9 ± 4.6,
31.0
|
32.2 ± 4.8
32.0
|
32.0 ± 4.5
32.0
|
0.279
|
Maternal BMI 2 (mean ± SD, median, kg/m2)
|
23.2 ± 4.5,
22.5
|
22.6 ± 4.8,
21.3
|
22.3 ± 4.1,
21.6
|
22.3 ± 3.3,
21.7
|
0.777
|
Educational level 3 (n, %)
|
|
|
|
|
|
|
Undergraduate degree
|
9 (20.9)
|
10 (21.3)
|
20 (40)
|
16 (36.4)
|
0.085
|
Graduate degree
|
34 (79.1)
|
37 (78.7)
|
30 (60)
|
28 (63.6)
|
|
First ANC 2 (mean ± SD, median, months)
|
2.0 ± 1.3,
2.0
|
2.3 ± 1.6,
2.0
|
2.3 ± 1.3,
2.0
|
2.1 ± 1.2,
2.0
|
0.784
|
Gestational age 2 (mean ± SD, median, days)
|
270.6 ± 6.4,
272.0
|
271.3 ± 8.1,
272.0
|
271.5 ± 6.4,
273.0
|
272.3 ± 8.3,
272.5
|
0.840
|
Birth order 4 (n, %)
|
|
|
|
|
|
First born
|
27 (62.8)
|
31 (66)
|
33 (66)
|
25 (56.8)
|
0.711
|
Second born
|
15 (34.9)
|
15 (31.9)
|
13 (26)
|
16 (36.4)
|
|
Third-fifth born
|
1 (2.3)
|
1 (2.1)
|
4 (8)
|
3 (6.8)
|
|
Parental status 4, couple (n, %) 4
|
|
42 (97.7)
|
47 (100)
|
48 (96.0)
|
44 (100)
|
0.458
|
Breastfeeding frequency 2,
mean + SD, median, times per day
|
6.4 ± 5.6,
6.0
|
6.0 ± 4.8,
5.0
|
5.7 ± 3.8,
6.0
|
6.1 ± 3.9,
5.0
|
0.984
|
|
|
|
|
|
|
|
|
|
1 One-Way ANOVA test, 2 Kruskal-Wallis test, 3 Chi-square test, 4 Fisher’s exact test were used for statistical calculations, and a p-value lower than 0.05 was regarded as significant. BMI, body mass index; ANC, antenatal care; SD, standard deviation
All the participants were Thais.
Correlation of Human Milk Composition with Duration of Lactation
Macronutrients
The concentrations of fat, and energy in human milk expressed by mothers who had been lactating from 1-24 months showed a positive correlation with the duration of lactation (r = 0.229, p < 0.01 and r = 0.229, p < 0.01, respectively) (Figure 1b,1c.)
In the subsequent lactation period (Table 2), the protein concentration in human milk after 18 months postpartum (2.84 ± 0.90 g/dL) increased significantly compared with human milk collected from 6-12 months and 12-18 months postpartum (2.39 ± 0.52 g/dL and 2.40 ± 0.75 g/dL, p < 0.01, respectively). The fat concentration and energy content were significantly higher in human milk collected after 18 months (4.64 ± 1.61 g/dL and 94.64 ± 16.13 kcal/dL, respectively) than those in the other groups (1-6 months and 12-18 months of lactation, fat concentration 3.67 ± 1.30 g/dL and 3.90 ± 1.32 g/dL, respectively, p < 0.01; energy content 84.86± 12.93 kcal/dL, and 87.91 ± 13.23 kcal/dL, respectively, p < 0.01).
There was no significant correlations of protein and carbohydrate concentrations with the length of lactation (r = 0.106, p = 0.15; r = -0.032, p = 0.67, respectively) (Figure 1a,1d).
Table 2. Comparison of Macronutrients, IgA and Total Antioxidant Capacity of Human Milk by Month of Lactation.
Breastfeeding
duration
|
1-6 months
n=43a
|
6-12 months
n=47b
|
12-18 months
n=50c
|
18-24 months
n=44d
|
P- value
|
Mean + SD
|
Median
|
Mean + SD
|
Median
|
Mean + SD
|
Median
|
Mean + SD
|
Median
|
Protein (g/dL) 1
|
2.56 ± 0.62
|
2.53
|
2.39 ± 0.52
|
2.23
|
2.40 ± 0.75
|
2.36
|
2.84 ± 0.90
|
2.66
|
0.002** bd,cd
|
Fat (g/dL) 1
|
3.67 ± 1.30
|
3.79
|
3.96 ± 1.36
|
3.53
|
3.90 ± 1.32
|
3.85
|
4.64 ± 1.61
|
4.61
|
0.008**ad,cd
|
Energy (kcal/dL) 1
|
84.86 ± 12.93
|
86.09
|
87.77 ± 13.61
|
83.54
|
87.91 ± 13.23
|
86.70
|
94.64 ± 16.13
|
94.28
|
0.008**ad,cd
|
Carbohydrates (g/dL) 1
|
9.62 ± 1.04
|
9.43
|
9.34 ± 0.59
|
9.27
|
9.31 ± 0.84
|
9.35
|
9.39 ± 0.81
|
9.42
|
0.650
|
IgA (mg/dL) 2
|
110.82 ± 14.06
|
111.48
|
129.59 ± 16.67
|
130.33
|
124.29 ± 10.80
|
125.07
|
127.16 ± 14.59
|
126.07
|
< 0.001**ab, ac, ad
|
TAC 2
|
1.61 ± 0.94
|
1.62
|
1.61 ± 0.67
|
1.64
|
1.84 ± 0.85
|
1.84
|
1.60 ± 0.97
|
1.27
|
0.117
|
1 Kruskal-Wallis test, 2 One-Way ANOVA test were used for statistical calculations, and a p-value lower than 0.05 was regarded as significant. * p < 0.05, ** p < 0.001
Total Antioxidant Capacity (TAC) unit is mM, Trolox equivalent.
Immunoglobulin A (IgA)
The concentration of IgA in human milk showed a positive correlation with lactation duration (r = 0.304, p< 0.001) (figure 1e). The mean IgA concentration was lowest from 1-6 months (110.82 + 14.06 g/dL) compared with that of longer duration groups (6-12 months, 12-18 months and 18-24 months of lactation, 129.59 + 16.67 g/dL, 124.29 + 10.80 g/dL and 127.16 + 14.59 g/dL, respectively, p < 0.001).
Total Antioxidant Capacity (TAC)
Similar to carbohydrate content, there was no significant correlation between the antioxidant capacity of human milk and the length of lactation(r = -0.056, p > 0.05) (Figure 1f).
Factors Affecting Human Milk Composition
Correlations among maternal BMI, maternal age, breastfeeding frequency, and milk composition were tested using Spearman’s Rank correlation coefficient and Pearson's correlation coefficient (Table 3). Maternal BMI was positively correlated with fat concentration and energy content in human milk (r = 0.233, p < 0.001 and r = 0.233, p < 0.001, respectively) and negatively correlated with carbohydrate content (r = -0.193, p < 0.05). Maternal age was positively associated with changes in the carbohydrate concentration (r = 0.148, p < 0.05), while breastfeeding frequency was negative associated with carbohydrate content in human milk (r = -0.182, p < 0.05).
Table 3. The Correlation Coefficient of Maternal BMI, Maternal Age and Breastfeeding Frequency with Human Milk Composition.
|
Protein
(g/dL)a
|
Fat
(g/dL)a
|
Energy
(kcal/dL)a
|
Carbohydrate (g/dL)a
|
TAC
(mM)b
|
IgA
(mg/dL)b
|
Maternal BMI
|
0.081
|
0.233**
|
0.233**
|
-0.193*
|
-0.033
|
-0.116
|
Maternal age
|
0.02
|
-0.112
|
-0.112
|
0.148*
|
0.094
|
-0.082
|
Breastfeeding frequency
(times per day)
|
-0.116
|
0.002
|
0.003
|
-0.182*
|
0.068
|
-0.048
|
a Spearman’s Rank correlation coefficient was used for statistical calculations, b Pearson correlation coefficient was used for statistical calculations, p-value lower than 0.05 was regarded as significant, *p < 0.05, ** p < 0.01