Deteriorating sleep quality and physical or mental fatigue in older adults leads to decreased quality of life and increased mortality rates. This study investigated the effects of the time-dependent intake of nicotinamide mononucleotide (NMN) on sleep quality, fatigue, and physical performance in older adults. This randomized, double-blind placebo-controlled study evaluated 108 participants divided into four groups (NMN_AM; antemeridian, NMN_PM; post meridian, Placebo_AM, Placebo_PM). NMN (250 mg) or placebo was administered once a day for 12 weeks. Sleep quality was evaluated using the Pittsburgh Sleep Quality Index. Fatigue was evaluated using the “Jikaku-sho shirabe” questionnaire. Grip strength, 5-times sit-to-stand (5-STS), timed up and go, and 5-m habitual walk were evaluated to assess the physical performance. Significant interactions were observed between 5-STS and drowsiness. 5-STS of all groups on post-intervention and drowsiness of the NMN_PM and Placebo_PM groups on mid- and post-intervention showed significant improvement compared with those in pre-intervention. The NMN_PM group demonstrated the largest effect size for 5-STS (d = 0.72) and drowsiness (d = 0.64). Overall, NMN intake in the afternoon effectively improved lower limb function and reduced drowsiness in older adults. These findings suggest the potential of NMN in preventing loss of physical performance and improving fatigue in older adults.

Melanin, the principal component of skin pigmentation, is produced through tyrosinase activity. Recently, supersulphides have been identified in human cells and found to play a role in maintaining cellular activities. Glutathione trisulphide (GSSSG), a supersulphide with antioxidant properties, has various biological effects. However, the influence of melanin production on intracellular persulphide levels remains unclear, and no studies have reported the anti-melanogenic effects of GSSSG. Therefore, we aimed to elucidate the impact of melanogenesis on intracellular persulphide abundance and synthesis, as well as the efficacy of GSSSG in inhibiting melanogenesis in melanoma cells. In this study, B16-F0 cells were used to evaluate the melanin and intracellular persulphide levels by alpha-melanocyte-stimulating hormone (α-MSH). Moreover, the effects of GSSSG on melanin production were studied. The results revealed that α-MSH-induced melanogenesis significantly increased melanin production and decreased intracellular persulphide levels. Furthermore, the expression of persulphide synthesis genes, including Cars2 and Cbs, was significantly downregulated by α-MSH. In contrast, GSSSG significantly suppressed α-MSH-induced melanin production. Notably, GSSSG restored the intracellular persulphide levels reduced by α-MSH and upregulated Cars2 and Cbs expression. These findings suggest that GSSSG exerts anti-melanogenesis effects, maintains intracellular persulphide levels, and improves skin brightness.

Ultraviolet B (UVB) irradiation induces hyperpigmentation and contributes to skin photoaging. Glutathione trisulphide (GSSSG), a naturally occurring supersulphide with anti-inflammatory and antioxidant properties, has been shown to suppress α-melanocyte-stimulating hormone-induced melanogenesis and improve skin brightness. However, its efficacy under physiologically relevant UVB-induced pigmentation conditions and its skin-brightening potential in human skin-like systems remain unclear. Therefore, this study evaluated the anti-melanogenic effects of GSSSG in UVB-stimulated melanogenesis and assessed its skin-brightening efficacy in a three-dimensional (3D) skin model while investigating the relationship between melanogenesis and intracellular endogenous supersulphide levels. UVB irradiation increased melanin production while decreasing intracellular supersulphide levels. GSSSG suppressed UVB-induced melanogenesis and restored intracellular supersulphide levels. Furthermore, Cars2 knockdown reduced intracellular supersulphide levels and melanin production, suggesting a role for endogenous supersulphides in pigmentation. In addition, 0.1% GSSSG significantly improved skin brightness in a 3D skin model. These findings demonstrate the efficacy of GSSSG in both UVB-induced melanogenesis and a human skin-like model and suggest that supersulphides play context-dependent roles in melanogenesis, suppressing excessive pigmentation while supporting physiological melanin synthesis.

Ultraviolet B (UVB) irradiation induces oxidative stress and inflammation in keratinocytes, contributing to skin damage and photoaging. Although supersulfides have recently emerged as important regulators of cellular redox homeostasis, their role in UVB-induced skin damage remains poorly understood. In this study, we investigated the effects of glutathione trisulfide (GSSSG), a representative supersulfide, in UVB-exposed normal human epidermal keratinocytes (NHEKs). UVB irradiation significantly reduced cell viability and intracellular supersulfide levels while increasing reactive oxygen species (ROS) production and the expression of IL-1α, prostaglandin E2, and p21. Pretreatment with GSSSG significantly restored intracellular supersulfide levels, improved cell viability, suppressed ROS accumulation and inflammatory mediator production, and attenuated p21 induction. Furthermore, GSSSG significantly promoted nuclear factor erythroid 2-related factor 2 nuclear translocation and increased heme oxygenase-1 production, indicating activation of antioxidant defense pathways. Knockdown of cysteinyl-tRNA synthetase 2 (CARS2), a key enzyme involved in endogenous supersulfide biosynthesis, increased cellular susceptibility to UVB-induced cytotoxicity, whereas GSSSG restored the viability of CARS2-deficient cells. Collectively, these findings suggest that GSSSG protects keratinocytes against UVB-induced oxidative stress, inflammation, and cytotoxicity. This protective activity may be mediated, at least in part, through restoration of intracellular supersulfide levels and activation of the NRF2-mediated antioxidant response.