The carbon (13C/12C) and nitrogen (15N/14N) stable isotope ratios of human hair can be used for the interpretation of dietary habits and nutritional status in contemporary or past populations. Although the results of bulk or segmental isotope ratio analysis of human hair have been used for the reconstruction of an individual's diet for years, only limited data of controlled dietary changes on the carbon and nitrogen isotopic composition of human hair are available. Hair of four individuals, two males and two females, who participated in a dietary change experiment for 28 days was segmentally analysed for δ13C and δ15N. The dietary change included a change from C3 to C4 plant enriched diets and a simultaneous replacement of terrestrial animal products by marine products. This resulted in an increase in δ13Cdiet of +8.5 to +9.9‰ and in δ15Ndiet of +1.5 to +2.2‰. All subjects showed significant increases in δ13Chair and δ15Nhair during the dietary change period, although no subject reached a new steady state for either carbon or nitrogen. The change in δ15Nhair was faster than the change in δ13Chair for all individuals. The magnitude of change of the isotopic composition during the dietary change period could be attributed to the degree of physical activity of the individuals, with a higher physical activity resulting in a faster change. Copyright © 2009 John Wiley & Sons, Ltd.