The body has an incredible ability to repair itself, and IMO, your increased activity level has contributed to the effectiveness and efficiency of your body as a whole. The sudden appearance of hair pigment may simply be an easily identifiable physical manifestation of improved health as a result of your increased activity level.Hair by hair?'Cause that's what's happening. It's not that my entire beard is changing color all at once, but rather that a few of the hairs have changed color, and new ones are growing in darker.....but not all of them.
Fitness and regular exercise have countless benefits, and IMO, you have improved the conditions within your body that has facilitated the ability of those hair follicles to once again produce the pigment that they once did.