http://lpi.oregonstate.edu/mic/vitamins/niacin#disease-prevention
Skin
Niacin deficiency can lead to severe sunlight sensitivity in exposed skin. Given the implication of NAD-dependent
enzymes in
DNA repair, there has been some interest in the effect of niacin on skin health.
In vitro and animal experiments have helped gather information, but human data on niacin/NAD status and skin
cancer are severely limited. One study reported that niacin supplementation decreased the risk of ultraviolet light (UV)-induced skin cancers in mice, despite the fact that mice convert tryptophan to NAD more efficiently than rats and humans and thus do not get severely deficient
(43). Hyper-
proliferation and impaired
differentiation of skin cells can alter the integrity of the skin barrier and increase the occurrence of pre-
malignant and malignant skin conditions. A protective effect of niacin was suggested by
topical application of myristyl nicotinate, a niacin derivative, which successfully increased the expression of
epidermal differentiation markers in subjects with
photodamaged skin
(44). The activation of the "niacin
receptors," GPR109A and GPR109B, by
pharmacologic doses of niacin could be involved in improving skin barrier function. Conversely, differentiation defects in skin cancer cells were linked to the abnormal cellular localization of defective "niacin receptors"
(45). Nicotinamide restriction with subsequent depletion of cellular NAD was shown to increase
oxidative stress-induced DNA damage in a precancerous skin cell model, implying a protective role of NAD-dependent pathways in cancer
(46). Altered NAD availability also affects sirtuin expression and activity in UV-exposed human skin cells. Along with PARPs, NAD-consuming sirtuins could play an important role in the cellular response to photodamage and skin
homeostasis (47).