Answer:
Scientists don't actually know whether Mars 1/3 gravity will be a problem or not. It's never been tested. In fact, scientists really don't know much at all about how humans react to different gravity levels. The only DATA points are as following:
-We know 1G is good for people.
-We know 0G is bad for people, especially after a few months.
-We know 1/6G {the moon} is fine for a couple of days.