KERATIN REPLACEMENT AS AN AGING PARAMETER 54,5 o.q 0 0.85 0•0. z o.? s Figure 9. Yearly Nail Growth h]dex [or twenty years DISCUSSION A significant decline with age in the rate of nail growth was found in this study. Young males show faster growth rate than females this difference diminishes toward middle age and reverses itself after the seventh decade of life. This may be correlated with the "male meno- pause." The rate of growth is not noticeably affected by mild climatic changes, although severe changes in temperature have been shown to alter this rate markedly (11). Table II is a summary of various factors which are reported to alter nail growth rate (11-17). The table also lists those factors that have already been shown to have no effect. This technique of measuring the rate of nail growth can be used to monitor the potential effects of nail cosmetics and of products alleged to influence nail growth. The results of this study are in agreement with those of several previous experiments on the rate of linear nail growth. Bean (12, 13) performed a 20-year study of his own thumbnails, measuring the number of days which a scratch at the proximal edge of the nail plate took to grow out to the distal end. Converting his data into millimeter growth per week, his NGI decreased from 0.87 at age 32 to 0.82 at age 42 and to 0.73 at age 52 (Fig. 8). Figure 0 represents Bean's Nail Growth Index as measured each year from age 32 to age 52. Cross-sectional studies of linear nail growth have been performed by Hillman (17), who measured 300 individuals, and by Hamilton (16), who studied over a thousand subjects. Their average measurements•

546 o.qo, 0.•0 G70. •' GGO. O.qO JOURNAL OF THE SOCIETY OF COSMETIC CHEMISTS - 1500 5•IBJECT5 ß o -H RSE- YERR5 Figure 10. Average decline in nail growth with age, 1500 subjects L0 Rc-n Figure 11. Nail growth in ten dogs by decade, were converted into milliliter growth per week and, super- imposed onto the slope of the graph from this study, revealed similar values at all ages to those reported in this paper. The average slope of these figures from all three experiment, as shown in Fig. 10, represents over 1500 subjects whose nail growth rates decrease on the average of 4.5/•/week for every year of age, from age 20 to age 100. These figures represent a 40% diminution over a 70-year period and concur with the average changes which Shock found in his aging parameters. The decrease in rate of nail growth with aging was linear for the cross-sectional studies. However, the individual longitudinal study performed for 20 years by Bean showed that the rate of growth declined at varying rates. Many more such studies over longer periods of time