| Summary: | Free amino acids (FAAs) in human milk are indicated to have specific functional roles in infant development. Studies have shown differences between human milk that is expressed at the beginning of a feed (i.e., foremilk) and the remainder of the milk expressed (i.e., hindmilk). For example, it is well established that human hindmilk is richer in fat and energy than foremilk. Hence, exclusively feeding hindmilk is used to enhance weight gain of preterm, low birthweight infants. Whether FAAs occur differently between foremilk and hindmilk has never been reported, but given their bioactive capacities, this is relevant to consider especially in situations where hindmilk is fed exclusively. Therefore, this study analyzed and compared the FAA and total protein content in human foremilk and hindmilk samples donated by 30 healthy lactating women. The total protein content was found to be significantly higher in hindmilk (<i>p</i> < 0.001), whereas foremilk contained a significantly higher total content of FAAs (<i>p</i> = 0.015). With regards to individual FAAs, foremilk contained significantly higher levels of phenylalanine (<i>p</i> = 0.009), threonine (<i>p</i> = 0.003), valine (<i>p</i> = 0.018), alanine (<i>p</i> = 0.004), glutamine (<i>p</i> < 0.001), and serine (<i>p</i> = 0.012) than hindmilk. Although statistical significance was reached, effect size analysis of the milk fraction on FAA levels in milk revealed that the observed differences were only small. To what extent these differences are of physiological importance for infant development remains to be examined in future research.
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