To this day astronauts need to spend good amounts of time exercising during long stints in zero-g aboard ISS to avoid significant bone loss. Of course, maybe if you never leave Mars, you would be just fine with the bone density/strength you have.
Citing specific problems can be instructive, and I would agree that bone and muscle mass loss could prove very bad. My issue is with nebulous claims that 'anything could happen'.
I would add that the bone and muscle problems are very well known to just about everyone who has done as much as read a science fiction book. Heinlein went into the subject in detail in his 1966 novel "The Moon is a Harsh Mistress", and more recent writers such as Kim Stanley Robinson have written about it too.
It is worth remembering that Heinlein and KSR are writers of fiction, which tends to gloss-over difficult problems in favour of pleasing stories. Neither were/are scientists, although Heinlein had an engineering degree, and it would be inadvisable to rely on their work.
In 1966 the space endurance record was just under 14 days (Gemini 7) and very little was known about the long-term effects of low gravity environments at that time.
That's a good point. I always associate the astronauts in the ISS with having very severe health effects from the zero-g, but if they never came back, perhaps they would be okay!
