HM is considered the “gold standard” for an infant’s nutrition, to which infant formula manufacturers strive to emulate in both nutrient composition and performance. HM composition varies considerably between individual mothers and over lactation, and so could be considered to be personalised to each infant that is breastfed. Various factors such as diet, geography, ethnicity, milk collection time, and genetics have been implicated to have a significant influence on the HMO, PL, and GA composition in HM, but most of the data obtained to date indicate that the stage of lactation is perhaps the primary factor that has the greatest influence on HM composition [27
]. There are several recent studies reporting the composition of HM, trying to gain a better understanding of the changes in the HMOs and complex lipids (PLs and GAs) through lactation of different geographical population cohorts [28
]. However, this is the first study that looks at the transitional and mature milk from UAE mothers, helping to address geographical variation in HMOs, PLs, and GAs.
4.1. Human Milk Oligosaccharides
The UAE mothers’ transitional breast milk samples had significantly higher average total HMO concentrations (8204 ± 2389 mg/L) compared with the mature milk samples (3876 ± 1403 mg/L). The largest decrease in the HMOs over these two lactation timepoints was observed with the acidic oligosaccharide, LSTc, which decreased by 98% from 488 ± 224 mg/mL in transitional milk to 11 mg/L in the six months post-partum mature milk (Table 1
). The only HMO to increase across this period was 3FL (Table 1
). This trend is in line with lactational trend data reported in the literature [27
] (Figure 1
). While only five neutral oligosaccharides (2’FL, 3FL, LNT, LNnT, and total LNFP) were measured in this study, they made up a significant proportion (82% and 93% for transitional and mature milk, respectively) of the total HMOs measured in this study, with the acidic HMOs making up 18% and 7% for transitional and mature milk, respectively. This finding is consistent with those reported by Ma et al. [30
] for their Malaysian and Chinese cohort of 89%–91% and 8.5%–11% of neutral and acidic HMOs, respectively, and other similar studies [27
], despite differences in the range of HMOs and respective concentrations being different.
The individual HMO levels measured in this study for the UAE breast milk samples were also in a range similar to that reported by Ma et al. [30
] for Chinese and Malaysian mothers (Table 3
and Figure 1
); and Larsson et al. [41
], Coppa et al. [42
], Bao et al. [43
], and Austin et al. [29
] for the common HMOs measured for the corresponding time points, except for 3’SL, LNT, and LNnT where Austin et al. [29
] reported lower HMO lactational results (Table 3
and Figure 1
HMOs have been implicated not just to provide anti-infective protection for the infant, but also as being involved as immune modulators, and may play a key role in gut maturation of the rapidly growing infant. Higher HMO concentrations in colostrum and transitional milk may be the consequence of increased protection required for the vulnerable infant during the early few days of life. The changes in HMO levels over the course of lactation [29
] may reflect changes in the development stages of the growing infant, and a requirement for specific compositions of these HMOs.
On the basis of the 2’FL concentrations, 26% of the UAE mothers in this cohort expressed a non-secretor phenotype, having a 2’FL concentration <50 mg/L in their breast milk samples [29
]. 2’FL (and LNFP I) are products of α-(1,2)-fucosyltransferase 2, which is coded by the FUT2 gene that is supposedly non-functional in non-secretor mothers. However, in the study of Austin et al. [20
], 2’FL was found to be not completely absent in the non-secretor mothers, as was the case in this study. The secretor/non-secretor frequency is known to vary with geography and racial difference, with 22.5% non-secretor phenotype reported for the Han Chinese population, Eastern China region [29
]; 37% reported for the Chinese cohort (Guangzhou); and 17% reported for Malay mothers [30
]. The typical frequency of non-secretor phenotype reported by Azad et al. [39
] was 28% for Caucasian mothers, while Asian mothers had higher non-secretor frequency at 40%; however, the Asian mothers’ sub-ethnic groups were not defined. There is no current information as to the non-secretor frequency for the UAE population. Azad et al. [30
] also reported that the non-secretor group had significantly less HMOs than the secretor group. For the HMOs measured in this study, however, there was no significant difference (p
> 0.05) in the average total HMOs between secretor and non-secretor mothers for either their transitional milk (8292 ± 2516 mg/L versus 6994 ± 1905 mg/L, respectively) or their mature milk (4289 ± 1791 mg/L versus 4317 ± 1857 mg/L, respectively) (Table 1
One important consideration in assessing crude population figures for HMO species is the impact that the percentage of non-secretors in each study population has on the average or mean 2’FL and 3FL concentrations that would be reported for the full cohort; the higher the percentage of non-secretors in a population, the lower the average 2’FL and higher the average 3FL concentrations for the cohort as a whole. Furthermore, it is not only the crude 2’FL and 3FL figures that are impacted, but also the figures for other oligosaccharides such as LNFP I and 3’S3FL. For example, in this study, the mature milk results for the full cohort showed that 2’FL and 3FL concentrations are relatively similar, which was also shown by Ma et al. [30
] and Austin et al. [29
], but if the non-secretor percentage was lower, then 3FL would be lower than 2’FL at later points in lactation, as evidenced in the limited data of Larsson et al. [41
], which is the analysis of secretors alone, and the results of this study. Figure 1
also emphasises the impact of lactation on interpreting which oligosaccharides are in highest concentration, because, at six months in all four cohorts (two Chinese, one Malaysian, and one UAE), the concentrations of 3FL and 2’FL are very similar.
The average total PL concentrations observed with HM samples of UAE mothers were within the typical ranges reported for human breast milk for other geographical population cohorts (Table 4
). In this study, the UAE transitional milk had an average total PL concentration that was significantly higher (p
< 0.05) than that measured for the six-month mature milk. While this trend is consistent with the majority of the published data [31
], the trend reported from some studies showed the total PL concentration in colostrum and transitional milk was either much lower than [49
] or the same as [52
] that in mature milk (Table 4
At the individual PL class level, changes in the relative distribution of the individual PL classes were observed over the transitional milk and mature milk periods (Figure 1
). There was a significant increase in the relative amount of PE from 25% in transitional milk to 36% in the UAE mature milk samples, while PC and PS both decreased, the former from 25% to 14% and the latter from 11% to 7%. SM increased only slightly, while PI decreased slightly. Similarly, changes in the relative distribution between transition milk and mature milk was observed for a Malaysian HM cohort [31
] (Figure 2
). In contrast, however, the PL distribution was relatively constant for Spanish [34
] and Chinese [24
] breast milk cohorts (Figure 2
However, across the mature milk period, three recent studies [31
] and two earlier studies [49
] showed that the individual PL class distribution remained relatively constant through the mature milk period, despite changes in their absolute concentrations.
Variation in absolute PL concentrations may be attributable to a variety of factors, such as time of sampling protocols (full breast expression, time of sampling, and breast variation [53
]), diet, geographic, and even metabolic stage and gestational age at birth, in addition to different analytical methods used [54
]. However, the fact that the relative distribution of the individual PL class remains constant through the mature milk period indicates that some metabolic controls are maintained over the biosynthesis of these PL classes, perhaps to maintain the integrity of the MFGM structure.
Changes in the individual PL distribution observed between early milk and mature milk may be the consequence of the changing structure. It is reported that colostrum and transitional milk has much larger fat droplet size than that of mature milk [57
]. In fact, Cohen et al. [58
] reported the phospholipid composition of the mammary epithelial cell regulated the lipid droplet size, rather than the cellular triglyceride content; this phospholipid composition is critical in maintaining membrane structure integrity as the lipid droplet size changes.
In HM, GAs are present predominantly as the GM3 and GD3 classes (which have the same polar head group, but different sphingosine and fatty acid). Typically, GD3 is present at relatively high concentrations in colostrum and transitional HM, making up approximately 30%–80% of the TGAs, but decreasing to 8%–25% by four to six months post-partum. Conversely, the relative proportion of GM3 is higher in mature milk [33
However, there is no clear consistent lactational trend observed for TGA concentration. While a few studies show colostrum and transitional milk to contain the highest TGA concentration [34
], other studies show the opposite [60
]. Similarly, the TGA lactation trend over the mature milk period is also not clear. While the studies of Thakkar et al. [32
] and Ma et al. [34
] showed a gradual increase in the TGA level over the mature milk period (of four months and eight months duration, respectively), from the data published for the Malaysian cohort [34
], this gradual increase did not appear to be sustained: the average TGA levels dropped from ~25.3 ± 15.7 mg/L at 6 months to 16.6 ± 8.5 mg/L at around 12 months post-partum.
In this study, there was no significant difference between the average TGA results measured between the transitional milk (21.18 ± 11.46) and the mature milk period (20.18 ± 9.75), with GD3 making up 55% of the TGAs in transition milk, decreasing to 8% at six months post-partum. The change in the GA class distribution is consistent across other published cohort studies [34
] (Table 5
), suggesting that the biosynthesis of GAs is under metabolic control, and different classes may be required for different stages of the development and growth for the breast fed infant. Interestingly, Thakkar et al. [32
] reported that HM from mothers with male infants had higher energy content and lipids compared with that mothers of female infants; these differences may be because of differences in nutritional requirements to support specific growth and development patterns between the two sexes [23
]. However, in the current study, there were no significant differences in the GA concentrations of either the transitional milk or mature milk breast milk from mothers who had male or female infants. The average TGA levels observed with the UAE cohort were similar to those reported for the Malaysian and Chinese cohorts (25.3 ± 15.7 mg/L and 22.9 ± 9.9 mg/L, respectively [34
]), but higher than those reported by Thakkar et al. [32
] for their Singapore cohort (4.6–5.6 mg/L) and Giuffrida et al. [33
] for their Chinese cohort (11.0 ± 5.0 mg/L).