Much research to this end is performed at space agencies and space-related institutions all over the world, such as at the Japan Aerospace Exploration Agency (JAXA). Here they have been studying the effects of long-term spaceflight on gene expression and mineral metabolism through analysis of hair samples from members of the ISS crew. Ten astronauts have been taking part in the "HAIR" experiment, running since 2009. An interesting early result, published in the PLOS One open access journal, indicates that gene expression in the human hair follicle is altered during spaceflight, with changes in genes related to hair growth suggesting that cell proliferation in hair follicles is inhibited - i.e., hair growth could slow down or maybe even stop. This would also seem to affect male astronauts more than female, although to varying degrees.
Clearly, this is a research with limitations, for example, the sample size is only 10, unsurprising given the limited numbers of people who enter space for any length of time. There are also other physiological factors that are not assessed in conjunction with these results, which could influence the results. However, importantly, it confirms that gene expression in the hair of different astronauts is similarly altered by spaceflight, giving rise to future research that could compare these results with gene expression changes in other organs of the human body in space.
For anyone who would like to read the JAXA scientific article - Effects of a Closed Space Environment on Gene Expression in Hair Follicles of Astronauts in the International Space Station - it can be reached through this link - Full Article