B vitamins: folic acid and cyanocobalamin
Two essential vitamins with clear systemic roles and much thinner topical rationales. An honest look at both.
Folic acid (vitamin B9)
Essential for one-carbon metabolism, DNA synthesis and repair, and cell division. Deficiency has serious systemic consequences. The topical rationale is that skin is a rapidly dividing tissue and therefore folate-dependent, with some interest in folate's role in DNA repair after UV damage.
The evidence for topical folic acid improving skin appearance is limited, and most of it comes from studies of multi-ingredient products where attribution to folate is not possible. Youlief lists it in the Setup Cream at 0.0015% — 15 ppm.
Cyanocobalamin (vitamin B12)
Required for methylation reactions and myelin maintenance; deficiency causes anaemia and neurological effects. The topical interest is narrower and more specific: there is some evidence for topical vitamin B12 in atopic dermatitis, attributed to modulation of nitric oxide.
That is a therapeutic rather than cosmetic context. Youlief lists cyanocobalamin in the Cleansing Serum Collagen at 0.00074% — 7 ppm — where its most visible contribution is likely the characteristic pink colour B12 imparts.
The honest summary
Both are legitimate ingredients with established biological importance, present at trace levels, in a topical route that is not the natural way to address either. Neither should carry a claim. Where they appear, the accurate description is as part of a vitamin complex rather than as an active with a demonstrated cosmetic effect.
This is a useful page precisely because it is unglamorous: readers encountering a vitamin-complex claim benefit from knowing which vitamins in it are doing something.