The Lipid Contents of Hair Fibers: The lipids present in the hair and on the surface of the hair are around 1 – 9% of the hair mass. There are two types of lipids, one being the internal lipids of hair and other being the external lipids. The internal lipids are present in the cell membrane complex, epicuticles, and free lipids. The external lipids are the lipids supplied by the scalp where the sebaceous glands are secreting these lipids and these lipids travel along the hair shaft gradually[i]. The internal lipids are around 1-3% of the hair mass and consist of fatty acids and polar lipids including ceramides. African American hair tends to have a lesser amount of lipids as compared to Caucasian hair [ii]. The external lipids vary according to the aging of humans. The sebaceous glands are not as active in children until they reach puberty. Therefore, the sebaceous secretion of lipids is low in children, as shown in Figure 2.12.
Figure 2.12: The children from the age of 9 to 13 tested twice a year for sebum secretion. Source: Pochi et al. (1979). The Journal of Investigative Dermatology, 73: p. 109.
Sebum production is controlled hormonally by androgens that increase cell proliferation in sebaceous glands, and this cell proliferation causes the production of sebum. The androgen, (from andro Greek prefix meaning masculine) is also called androgenic hormone or testoid. There are variations in sebum production on a daily basis and also variations depending upon the season[i].
The production of sebum also varies with age. The scalps of the children do not produce as much sebum up to the age of puberty, as compared to the adults. Children’s scalps start to produce increased sebum from mid-to-late childhood (around 13 years of age) under the influence of androgens[ii]. Sebum production again starts to decrease after age 35 in men and age 45 in women[iii].
[i] M Gloor. In Cosmetic Sciences. (1978). Ed: M Breuer. Academic Press: New York. p. 218.
[ii] PE Pochi, JS Strauss, and DT Downing. (1979). Age-related Changes in Sebaceous Gland Activity. J Investigative Dermatology, vol 73 (1): 108-111.
[iii] J Strauss and P Pochi. (1963). The sebaceous glands. In Advances in Biology of Skin. Pergamon Press: New York. Vol 4, pp. 220-254.
[i] AM Kligman and WD Shelly. (1958). J Invest Dermatol. 30: 99.
[ii] L Kreplak, F Briki, Y Duvault, J Doucet, C Marigoux, F Leroy, JL Leveque, L Miller, GL Carr, GP Williams, and P Dumas. (2001). Profiling lipids across Caucasian and Afro-American hair ransverse cuts, using synchrotron infrared microspectrometry. Int J Cosmet Sci: 23: 369-374.