Volunteers, mainly female, probably because female volunteers with an interest in health than males, and have a career as a maid, so join in the volunteer research than males.
Volunteers, mostly female volunteers, possibly because females are more interested in health than men and is a housewife became involved as a volunteer in the study than men.
Volunteers were mostly female, possibly due to female volunteers are interested in health than men and have professional housewife joined as a volunteer in research than male.