Background/purpose: This research aims at assessing the influence of baseline skin colour on the ability of reflectance spectrophotometry to detect cutaneous erythema induced by a low concentration of methyl nicotinate (2.5 mM) (first objective), and to detect tanning induced by ultraviolet rays (UVA+UVB) at infra-erythemal doses (second objective).
Methods: Two independent studies were conducted to reach their respective objectives, on 27 women for the first study and on 12 women for the second study. Skin colour measurements were expressed in two different ways: percentages of reflected light at increasing wavelengths lambda (400 nm<lambda<700 nm, at 10 nm intervals), and chromametric coordinates of the CIELab 1976 system and individual typological angle (ITA degrees). Partial least squares discriminant analysis was performed to identify percentages of reflected light that allow the discrimination of the observations obtained after methyl nicotinate application from those obtained after water application (control). The same method was used for the discrimination of the measurements obtained after UV irradiation from those obtained before UV irradiation (control).
Results and discussion: The cutaneous erythema induced by a low concentration of methyl nicotinate was detected only in subjects with fair to very fair skin defined by ITA> or =40 degrees. The assumption is that in the darkest skins, the emitted light is mainly absorbed by the melanin in the epidermis. Otherwise, after UV irradiation, the tanning was detectable only for individuals with fair to dark skin defined by ITA <50 degrees. This can be explained by the fact that UV stimulation of the fairest skin subjects, known to be melano-compromised individuals, can only produce a weak tanning that our study did not succeed in detecting.