Contents of linoleic acid, <i>α</i> -linolenic acid and their ratio in human milk, influence factors, and effects on infant health

Both linoleic acid (18:2 n-6, LA) and α-linolenic acid (18:3 n-3, ALA) are essential fatty acids for infants. The contents of LA and ALA, and their ratio exhibited significant changes in human milk over the past four decades, which were not well summarized. Here, we summarized these values in 9,898 human breast milk samples of 6,664 mothers from 50 countries in 81 studies. A literature search was conducted using PubMed, Embase, and Web of Science between January 1980 and October 2023. The 95% CI of LA/ALA ratio across lactation and gestation ranged from 14.24 to 31.26. The LA content was higher in China and Turkey (&gt; 20%) whereas the ALA content was below 1% in Africa. The LA/ALA ratio in countries along the Mediterranean coast exceeded 20 or even 30. LA and ALA contents increased significantly (<em>P &lt; </em>0.01) while the ratio remained stable over the last 40 years. Multivariate meta-regression results showed that regions significantly (<em>P &lt; </em>0.01) determined the LA, ALA, and LA/ALA ratio. Especially, maternal diet could definitely explain the variation while the effects of gestational age, lactation period was not significant. Clinical trials demonstrated that decreasing the LA/ALA ratio increased docosahexaenoic acid (22:6 n-3, DHA) status, reduced arachidonic acid (20:4 n-6, AA) contents, exerted no effect on the visual function of infants, and reached no consensus on growth. The current review aimed to provide an overview on the LA and ALA contents and their ratio in human breast milk to raise concern in infant formula.