For clinical grading of pigmentation, pairwise comparisons between various zones presented in Table 1 illustrate no statistical difference between untreated zone and Product A on day 1 (after the 1st exposure), day 4 (24 hours after the last exposure), and day 14. Despite no significant difference between Product B and C, both showed statistically significant differences when compared to untreated zone and Product A at all timepoints.
For ΔL*, there was no statistical difference between untreated zone and Product A at day 1, day 3 (p value not shown), and day 4, and a statistical difference at day 14, favoring Product A. Product B demonstrated significant higher ΔL* compared to exposed zone and Product A at all-time points. Compared to untreated zone, Product C showed significant higher ΔL* at all-time points (less skin darkening), from day 2 (p value not shown) to day 14 when compared to Product A. These results suggest that both Products B and C were equally effective in blocking visible light*- induced skin pigmentation.
Similar results were observed for ΔE and ΔITA° parameters (Figure 3B, 3C, and Table 1). Product A was similar to untreated zone at all timepoints for ΔE, and same for ΔITA°, except at day 14. Both Products B and C were more effective in preventing skin color change compared to Product A from day 2 for ΔITA° and from day 3 for ΔE. No statistical differences in performance between Products B and C for ΔE, and same for ΔITA°, except a statistical difference at day 1, favoring Product B.
Skin redness (Δa*) and skin yellowness (Δb*) showed much smaller changes over time and inconsistent results for product performance distinction, indicating that the blocking of the visible light*-induced pigmentation by the products were specific for skin darkening (data not shown).
Similar to UVA, VIS elicits immediate and persistent pigment darkening (PDD) in subjects with skin phototype III and above; processes that are mediated via the photo-oxidation of pre-existing melanin and de-novo melanogenesis, respectively.4,20-22 The potential topical or oral use of antioxidants, molecules that scavenge free radicals, for VIS protection, has been proposed and tested by various research groups.23-26 However, clinical studies evaluating the efficacy of antioxidants to protect against VIS-induced pigmentation are scarce. One study demonstrated that topical application of an antioxidant mixture reduced the immediate erythema and pigmentation responses followed by VIS+UVA1 exposure in subjects with skin phototypes I-III and IV-VI, respectively.27 But, this protective trend was not observed at day 7, indicating that antioxidants may be more effective in reducing skin darkening mediated by melanin photo-oxidation, and less effective at preventing de-novo melanin synthesis, which constitutes the later phases of pigment formation. Our results show that both FeO-containing formulations tested efficiently prevented further skin darkening following each irradiation, which persisted up to 14 days, while the mineral SPF50+ sunscreen gave similar results as untreated skin. Due to their higher concentrations of metal oxides, particularly FeO, it is clear that these formulations provided a better physical barrier for the skin against VIS rays, defending against cumulative effects and inhibiting delayed tanning.
Interestingly, despite the difference in FeO and TiO2 levels (Figure 1A), there lacked statistically significant differences in performance between formulations B and C. This raises an important point concerning how to assess products photoprotective efficacy against VIS, as it is performed for UVB and UVA sunscreens under regulatory guidelines.28-30 Several papers have suggested different in vivo methods to evaluate the VIS protection factor (VL-PF) of products.9,12,31,32 For single dose exposure, the VL-PF is based on the minimal PPD of unprotected and protected skin, similar to the UVA protection factor method.12 More recently, Kholi et al proposed to use the spectral signatures of the VIS+UVA1-induced skin pigmentation by obtaining the ratio of the area under the curve of the differential apparent absorbance of untreated skin from 400–700nm to that of treated skin at specific timepoints.31 For multiple doses, Duteil et al determined the VL-PF by obtaining the ratio of the mean slope of the linear regression calculated between timepoints of the ΔITA° curves for untreated over treated.9 Using similar method, the calculated VL-PF of Product A (mineral SPF 50+) was 1.48, while Product B and Product C had a VL-PF of 7.07 and 5.4. In alignment with prior studies, it appears that products with FeO pigments present higher VL-PF, as compared to products without pigment.32 Despite the big differences in the FeO content between product B and C, in this study, we found both products demonstrated similar VL photoprotection. Future studies are needed to expand on the findings of this pilot study with bigger sample sizes, longer evaluation time beyond 14 days, determination of the minimal level of FeOs necessary for effective VIS