Bioavailability of 6'-Sialyllactose and 3'-Sialyllactose: Dynamic Interplay between Intestinal Absorption and...

Sialyllactose (SL) is the most abundant acidic human milk oligosaccharide. This study investigated the differences in absorption, distribution, metabolism, and excretion between 3'-sialyllactose (3'-SL) and 6'-sialyllactose (6'-SL). After oral administration, SLs were hydrolyzed into free sialic acid (SA) in the large intestine. The concentration of SA in the intestine and conjugated SA in the brain was higher in the 6'-SL group than that in the 3'-SL group. In contrast, the level of conjugated SA in the serum and urine of the 3'-SL group was higher. In Caco-2 cells, 3'-SL was absorbed in an energy-dependent manner, with its absorption efficiency higher than that of 6'-SL. During in vitro fermentation, SA was the primary metabolite of SLs. The hydrolysis rate of 6'-SL was lower than 3'-SL, suggesting that 6'-SL may increase the brain bioavailability of its active form, SA, via slow hydrolysis and absorption as well as reduced excretion.