Tryptophan in human hair: correlation with pigmentation

The distribution of tryptophan content in human hair of various colours was evaluated, in order to study the accumulation of this amino acid, precursor of serotonin, melatonin and niacin, in hair and the influence on hair pigmentation. Pigmentation is an important factor in determining drug incorporation into hair. Results from 1211 samples of hair from healthy subjects (577 men and 634 women) show that tryptophan levels are significantly higher in males (37.83 +/- 3.45 microg/g dry hair) than in females (26.62 +/- 2.40 microg/g hair). Besides sex, age also influences the distribution of tryptophan in human hair, the highest levels being found in both sexes in the first few years of life, probably due to the influence of milk, and in aging subjects in the groups of 61-80 and > 80 years. In order to investigate the influence of hair colour, hair samples were subdivided according to colour into blond, dark blond, red, light brown, brown, black, grey and white. The hair contents of tryptophan in both sexes was higher in brown and black hair than in blond hair, but in grey and white hair concentrations were the highest, demonstrating that tryptophan accumulates among hair fibres with age. Grouping subjects by age in relation to hair colour, we observed that at ages 1-5 and 6-12 years, colour did not influence tryptophan contents, but at ages 13-19 and 20-40 years tryptophan content increased significantly from blond to brown at 13-19 years and from blond to black at 20-40 years in both sexes. Therefore, variations in tryptophan levels of human hair appear to be correlated with differences in hair colour in both sexes. Tryptophan also accumulates in hair during keratinization, as shown by the presence of high levels of this amino acid in grey and white hair.