"Early C and Late A" actually refers to a collocation method of skin care, that is, skin care products containing vitamin C(VC) in the morning and vitamin A(VA) in the evening are used together in the morning and evening to achieve whitening and anti-aging effects. It should be noted that cosmetics containing VC are not necessarily used during the day, but cosmetics containing VA can be used at night.

The "c" in "early C and late A" refers to VC and its derivatives. VC, also known as L- ascorbic acid, is one of the water-soluble vitamins with strong antioxidant capacity. In its molecular structure, there are unstable "C = C" and "-OH" (see figure 1), so VC is a very unstable vitamin. VC has many important biochemical functions. On the one hand, VC is an effective water-soluble antioxidant. The dienol structure in its molecular structure (see figure 2) makes VC have strong reducibility and can scavenge free radicals in the body. In the synergistic antioxidant effect with vitamin E(VE), the tocopherol hydroxyl radical compound generated by the reaction between VE and free radicals in vivo is reduced, so that it becomes VE again, the antioxidant activity of VE is enhanced, the damage of free radicals to cells is resisted, and the damage of ultraviolet rays to skin is alleviated.

On the other hand, VC is a coenzyme of hydroxylase, which can participate in the reaction of hydroxylation of lysine and proline into hydroxyproline and hydroxy acid in collagen synthesis, promote the synthesis of collagen and resist skin aging. In addition, VC can also inhibit the production of melanin by inhibiting the activity of tyrosinase, thus playing a whitening role.