Niacinamide, also known as nicotinamide, is an amide compound of niacin (vitamin B3). 
Niacinamide is a relatively stable vitamin and is an essential component for the production of ATP in the body.

Source
Currently, niacinamide is mainly prepared by enzymatic synthesis.

Purposes
It is a common lightening ingredient in cosmetics. It can improve skin dullness, reduce hyperpigmentation and brighten skin tone.

Efficacy
Niacinamide acts on the skin mainly by inhibiting melanin transport, lightening the skin and brightening the skin tone; repairing the skin barrier and increasing the epidermal thickness; improving sebum secretion[1].

Niacinamide was found to reduce melanin vesicle transfer and skin pigmentation, treat melasma, and alleviate inflammation in in vitro and human models[2-3].

Another[4] human study showed that niacinamide thickens the stratum corneum and reduces transepidermal water loss.

Applications
Niacinamide is mostly used in skin lightening products.

References
[1] Diana D Z, Akira M, Kenneth S. The effect of 2% niacinamide on facial sebum production[J]. Journal of cosmetic and laser therapy: official publication of the European Society for Laser Dermatology, 2013, 2006, 8(2):96-101.

[2]Hakozaki T, Minwalla L, Zhuang J, et al. The effect of niacinamide on reducing cutaneous pigmentation and suppression of melanosome transfer[J]. British Journal of Dermatology, 2002, 147(1): 20-31.

[3] Navarrete-Solís J, Castanedo-Cázares J P, Torres-Álvarez B, et al. A double-blind, randomized clinical trial of niacinamide 4% versus hydroquinone 4% in the treatment of melasma[J]. Dermatology research and practice, 2011, 2011.

[4] Mohammed D, Crowther J M, Matts P J, et al. Influence of niacinamide containing formulations on the molecular and biophysical properties of the stratum corneum[J]. International journal of pharmaceutics, 2013, 441(1-2): 192-201.

Ascorbic acid (vitamin C), a water-soluble vitamin that is acidic when dissolved in water, is also known as L-ascorbic acid.

Source
VC is widely found in fresh vegetables and fruits. At present, ascorbic acid is obtained mainly by natural extraction, chemical synthesis and fermentation methods.

Purposes
It is a common ingredient in cosmetics for lightening, anti-oxidation, and promoting collagen growth.

Efficacy
The common application efficacy of ascorbic acid (vitamin C ) in cosmetics[1]:

▪ Anti-oxidation

Vitamin C is often used as an antioxidant in skin care products to reduce oxidative damage to key cellular components by scavenging free radicals and reducing their activity.

▪ Skin lightening

Vitamin C has been proven to be used as a skin lightening agent in skin care products, which can inhibit melanogenesis by reducing o-quinones (e.g., dopaquinone), preventing the formation of tyrosinase-dependent melanin.

▪ Promotion of collagen production

Vitamin C can also be used as an anti-wrinkle agent, as it can stimulate collagen production in the dermis and promote the proliferation of fibroblasts to produce more collagen.

Applications
It is mainly used in skin care products that lighten and brighten skin tone. Due to its antioxidant properties and inhibition of melanin production, vitamin C is also used in anti-photoaging skin care products to reduce the appearance of skin wrinkles and fine lines caused by photoaging.

References
[1]Stability, transdermal penetration, and cutaneous effects of ascorbic acid and its derivatives[J]. Journal of Cosmetic Dermatology, 2012, 11(4):310-317.