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Gerontology. 2008;54(2):100-5. doi: 10.1159/000128269. Epub 2008 Apr 21.

Organic elemental composition in fingernail plates varies between sexes and changes with increasing age in healthy humans.

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Department of Human Biology, Zoological Institute, Christian Albrechts University of Kiel, Kiel, Germany.



Keratin, an alpha-helical fibrous protein, is the primary component of human nail plates. No data on age-related changes in healthy subjects are present.


This study investigated whether keratin amount and composition, as indicated by organic elemental composition of fingernails, varies with aging and between sexes.


Nail clippings from 225 healthy individuals (93 males, 132 females), aged 20-90 years, were analyzed for carbon (C), nitrogen (N), and sulfur (S) contents with an automatic elemental analyzer. C/N and N/S ratios were calculated. The C/N ratio is indicative of keratin composition, i.e. the ratio of alpha-amino acids and protein. The relationship of elemental composition with dietary intake was analyzed by standardized food record.


Females have in their nails more sulfur (p < 0.001) and less nitrogen (p = 0.001), and thereby, a lower N/S ratio than males, whereas their carbon content is not different. With aging, the carbon content increases (p < 0.01, both sexes) and the nitrogen content decreases (p = 0.05, females), both leading to an increased C/N ratio (p < 0.001). By contrast, the sulfur content and the N/S ratio do not change with aging (p > 0.05). The carbon content correlates positively with macronutrient intake in females. Nitrogen and sulfur contents are not related with dietary intake.


Results suggest that the N/S ratio is indicative of sex differences and the C/N ratio of aging in healthy humans. The increasing carbon content with ongoing age could be explained by loss of inorganic material from the nails, followed by a subsequent increase of organic material. The increasing C/N ratio gives evidence that keratin composition changes towards a higher amount of alpha-amino acids with aging.

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