B2 and B6 have phosphate groups attached to them
The active forms do (hence the names riboflavin 5 phosphate, pyridoxal 5 phosphate).
In the gut, the phosphate gut is clipped off so the vitamin can cross cell membranes and enter cells. Once in the cell the phosphate group is put back on again.
Some of the active vitamin is also absorbed directly, by a different mechanism. For any individual, the usefulness of swallowed active B2 and B6 depends on how much of this latter mechanism is operating as well as the effectiveness of the enzymes in the first mechanism which remove and replace phosphate groups.
This is the variability I was meaning - independent of other digestive processes. The sublingual route bypasses this variability.
B12 in foods and swallowed supplements requires intrinsic factor (IF) produced in the stomach for absorption. Problems with production of IF, such as in pernicious anaemia, result in B12 deficiency, even if there are adequate dietary sources.
Even under normal circumstances, production of IF is limited and thus so is absorption of B12 in the gut.
There is some controversy about absorption of B12 via the sublingual route. There have been a couple of studies which suggest that it is not really absorbed across mucous membranes but rather is dissolved very slowly and absorbed in the gut via the normal IF route. The slow dissolution makes absorption more efficient than a single bolus dose into the gut as would happen if swallowed.
Actually I think there is far too little research to know what is really happening, plus there are a lot of other unknowns in B12 uptake and processing.
Very slow dissolution of sublingual B12 does seem to give better results but even so only a small proportion is absorbed - perhaps 5-10%.
Studies with the topical route show about 80% absorption.
