If you start rapidly losing (or gaining) weight with no obvious explanation that is a cause for concern. If you start eating much less, and exercising much more, you probably don't need to ask a doctor why you're losing weight.
http://forum.bodybuilding.com/showthread.php?t=121373121&pag...
Higher Metabolism, more water weight to shed.
That said, he (and anyone doing it) should probably check in with the doc just to make sure something internal isn't getting out of whack.