TLDR Older people's hair becomes less shiny because it gets more uneven and curved.
In a study involving 230 Japanese females aged between 10 and 70 years, researchers investigated the effects of aging on hair properties from macroscopic to microscopic levels. They found that hair luster, as perceived through sensory tests, decreases with age. However, changes in hair diameter and the ellipticity of the hair cross-section were not sufficient to explain the reduction in luster due to aging. The study identified that an irregular increase in hair fiber curvature with age contributes to the loss of luster. Detailed structural analysis using synchrotron radiation microbeam X-ray diffraction showed that the inhomogeneity in the hair's microstructure lateral distribution increased with age, which is associated with the irregular curvature increase. This curvature leads to misalignment of hair fibers, resulting in reduced luster and contributing to the appearance of aging hair.
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