3.3. Compositional characteristics of breast milk triacylglycerols
In this study, UPLC-Q-TOF-MS was used to identify the species and content of TAGs in 180 breast milk, and a total of 108 TAGs were identified based on the m/z values of precursors and fragments. As shown, Table 2 shows the TAG in breast milk and 95% confidence intervals (95% CI) in 3 groups, of which 41 TAG were significantly different (P < 0.05). The highest content of TAG in mature breast milk is O-P-L (median of 17.09%; 95% CI: 16.83, 17.63), followed by O-P-O (median of 12.84%; 95% CI: 12.40, 13.20), which is consistent with many previous research conclusions. Studies on TAGs in breast milk in Western countries have shown that the highest content of TAG is O-P-P, which may be related to the different preferences of various countries for edible oils, genetic and environmental factors. Pons et al. (Morera Pons et al., 2000) detected the highest content of two TAGs in Spanish breast milk as O-P-O and O-P-L, which accounted for more than 49 % of the total, and dominated in transition milk (34%), and mature milk (42%). And Ten-Domenech et al. (Ten-Domenech et al., 2015) detected 42 TAGs in mature breast milk in Spain, of which O-P-O had the highest content, followed by O-P-L and O-P-La. Zou et al. (Zou et al., 2013) detected 22 TAGs in Danish breast milk, of which the highest content was O-P-O (about 22%), followed by O-P-L (about 17%) and O-P-La (about 10%). Different from Western countries, Zhu et al. (Zhu et al., 2021) studied breast milk from 8 different cities in China. A total of 66 TAGs were detected in mature breast milk, and the maximum values of O-P-O and O-P-L were 15.7 % and 11.88 %, respectively. The difference of TAG content in breast milk in different countries may be related to the detection methods used by researchers and the dietary habits and genes of mothers in different regions, which also reflects the diversity and complexity of TAG composition in breast milk.
This study shows that the highest content of TAG is O-P-L in Chinese breast milk which fits the previous studies (Zhang et al., 2021; Xinghe et al., 2019). This may be due to the high consumption level of soybeans and soybean products in Chinese mothers (Wang et al., 2023). Soybean oil ranks first in the total consumption of Chinese cooking (Fang and Beghin, 2002). The content of O-P-L in breast milk of protein group (~18%) was significantly higher than that of carbohydrate group (~17%) (P < 0.05). In addition, the content of O-P-O in breast milk of the protein group was 13.52%, which was significantly higher than that of the carbohydrate group (~12%) (P < 0.05). It can be seen that the breast milk of the protein group is rich in saturated-unsaturated-unsaturated (SUU) type, especially 1,3-diunsaturated fatty acid-2-palmitate (UPU) type TAG. However, because there are few studies on the relationship between the composition of the three major nutrients and the composition of triglycerides in breast milk, it is not clear why the content of SUU type TAG in protein group is high.
Among the 41 TAGs with significant differences, 22 were MLCT type. L-L-Ca (median of 0.19%; 95% CI: 0.19, 0.22) and O-L-Ca (median of 0.64%; 95% CI: 0.62, 0.68) are typical MLL-type TAG. As shown in Table 3, the contents of MLCT for example L-L-Ca and O-L-Ca in breast milk of carbohydrate group were 0.24% and 0.71%, respectively, which were significantly higher than those of fat group and protein group (P< 0.05), and their contents were 0.17% and 0.60%. Similarly, the contents of L-L-La (median of 0.70%; 95% CI: 0.73, 0.85), O-L-La (median of 2.06%; 95% CI: 2.05,2.28) and L-Po-M (median of 0.12%; 95% CI: 0.12, 0.14) in breast milk of carbohydrate group (0.96%, 2.46% and 0.15%) were significantly higher than those of fat group (0.75%, 2.1% and 0.13%) and protein group (0.66%, 1.93% and 0.12%) (P <0.05). In addition, it can also be seen that the content of MML type TAG such as L-La-Ca (median of 0.22%; 95% CI: 0.22, 0.25), L-M-Ca (median of 0.16%; 95% CI: 0.16, 0.18) and L-La-La (median of 0.36%; 95% CI: 0.36, 0.42) and UUU such as L-L-L (median of 1.71%; 95% CI: 1.83, 2.20) and O-L-L (median of 3.82%; 95% CI: 3.94, 4.79) in the breast milk of the carbohydrate group was also significantly higher than that of the other two groups (P< 0.05). This study confirmed that the higher the proportion of carbohydrate energy supply in mothers in Nanjing, China, the higher the content of MLCT in breast milk.
The contents of SSU such as O-P-P (median of 2.89%; 95% CI: 2.96, 3.28) and S-O-P (median of 3.58%; 95% CI: 3.44, 3.86) in breast milk of the fat group were slightly higher, which were 3.31 % and 4.01 %, respectively. Table 2 shows that the composition and composition of TAGs in breast milk fat are correlated with maternal diet. This observation should inform future research and can be taken into account when discussing diet with breastfeeding mothers.