N-acetylneuraminic acid.

Infant-associated bifidobacteria carry sialidases and transporters that let them use sialylated milk oligosaccharides as a growth substrate, which is part of why these organisms dominate the breastfed infant gut. Free Neu5Ac and sialyllactose are both potential substrates in that system. Whether adding either shifts an adult microbiome in a useful direction has not been settled. The enzymology is solid, the applied claim is not.

Sialic acid is almost always added onto a terminal galactose residue by a sialyltransferase, so the two sugars occupy neighbouring positions on the same glycan chain. Galactose availability is therefore part of what determines how much sialylation a chain can carry. This is structural biochemistry rather than a supplementation finding. It explains why the two co-occur constantly in the glycobiology literature.

Gangliosides are sialylated glycosphingolipids concentrated in neural membranes, and their lipid tails are enriched in long-chain fatty acids while their headgroups carry Neu5Ac. Infant formula work has combined the two on that structural logic. Sharing a molecule in tissue is not the same as a demonstrated benefit from taking both. The rationale is anatomical rather than clinical.

The carboxyl group of sialic acid is ionised at physiological pH and gives cell surfaces much of their negative charge, which binds calcium and other divalent cations. That charge behaviour is what underlies the mucus and cell-surface properties of sialylated glycans. It is a physical chemistry point about the molecule, not a claim that either ingredient changes the other's absorption in a person. Read it as mechanism.

Galactooligosaccharides and sialylated oligosaccharides are both fermentable substrates that shift bifidobacterial populations, and infant nutrition products routinely combine them to approximate the breadth of human milk oligosaccharides. They are not the same substrate and the organisms that use them overlap only partly. Combining them is a coverage strategy. The evidence for the combination is stronger in infants than in adults.

Ethanol appears alongside sialic acid in the literature mostly as an extraction and precipitation solvent, and the antagonism flag in the co-occurrence index reflects laboratory chemistry rather than an interaction inside a person. It is listed here so the flag is explained rather than silently dropped. There is no basis for presenting alcohol intake as an interaction with dietary sialic acid. This row is bookkeeping.