Evolution in the composition of human skin surface lipids during their accumulation on scalp and hair.

Synopsis Qualitative examination of human surface lipids collected using a surfactant solution shows that there are no significant differences between individuals when the samples are collected after one day, starting from clean hair and scalp. On the contrary, samples collected after 4 days reveal discrepancies within the population, related to the oily hair symptom. An evolution in the composition of the recovered mixture occurs during the 1-4 days accumulation period, which mainly results in an increased unsaturation of the total (free + glycerides) fatty acids fraction isolated after saponification. A two-step mechanism is proposed to account for the observed evolution: in a first step, free fatty acids are cleaved from glycerides by the well known enzymatic hydrolysis. Analytical determinations show that this process is structure dependant, the straight chain saturated acids being more readily hydrolyzable than the unsaturated and branched chain species. In a second step, free fatty acids are immobilized on the keratinic fibers, by a process which involves the formation of Ca salts. This evolution results in a decreased viscosity of the skin surface lipids and in an increase of the low temperature melting fraction of the mixture. Its extent is more or less pronounced, depending upon the individuals. Subjects who exhibit extensive evolution are affected by the oily hair symptom.