Nobody knows. Whether or not we ever send humans to Mars, this is also of interest for ideas for habitats spinning for artificial gravity. If they only need Mars or lunar gravity, then they may be much easier to construct.
The thing is the human body is just too complex to simulate. And it's hard to know if bed rest analogies on Earth work. Resting with your feet up and head down, to simulate zero g levels of unhealthiness, then spinning in a centrifuge to try to simulate artificial gravity in zero g - it's just a few steps too far to be confident that this tells us about what the human body would be like in low levels of gravity - not without some ground truth from space.
We have two data points. Full g is healthy. Zero g is unhealthy.
Then we know that hyper gravity is unhealthy and can plot a graph there.
So if you have healthiness vertically and gravity level horizontally, it's something like this.
(The numbers there are arbitrary, just chose 500 for Earth and 100 for zero g, to make a nice graph, made with Line graph maker )
It could be any of those lines, or most likely none of them. Just drew a few at random there. As I suggested with the purple line, we don't even know for sure that low gravity is the worst of all for human health. And of course we don't know that Earth gravity is best.
It could have discontinuities too.
It could also be different lines for different people, or depending on your age or health.
Maybe if you have a heart condition, Mars is optimal, maybe if you have diabetes, the Moon is optimal, and if you want to give birth, Earth is optimal, maybe if recovering from a stroke, zero g is optimal. I mean those are all just random guesses there :). Mix and match as you like. Maybe some level of gravity is ideal for under 5s, but no good at all for adults. Or vice versa.
It might be that your circulation (heart rate, blood pressure etc) works best at one gravity level, the cells of your body are healthiest at another, your muscles keep their best muscle tone at another, your immune system works best at another, and it is easiest to lose heat at another while exercising. It may be that it is best to sleep at one level of gravity, to eat meals at another, to exercise at another, and to work at another, and to read a book at another, and another is optimal for using a toilet, another is best for ice skating, another is ideal if you want to be able to run as fast as you can unaided - might be that in future space Olympics, all the sports use different gravity levels :).
Maybe a space sprinter wouldn't think of running in anything except Mars gravity, but for the high jump you wouldn't dream of any other level except lunar g. Again just making up numbers there for fun :). While human flight, I think that is supposed to be possible in lunar g, so might be a new lunar sport, but who knows, maybe it is optimal in 1/100 g or maybe that is only used for newbies and kids trying to fly for the first time.
I think it's bound to be complex and multivarious, what g levels are best for what.
We have lots of data at full and zero g, not much data for lunar gravity, and zero data for Mars gravity and all the other levels including less than lunar - for humans that is. Plant cells change gene expression even at a hundredth of a g, so it would be worth investigating effects on humans right down to a hundredth of a g.
This could be discovered by doing experiments in artificial gravity in space, but there isn't much interest in the topic amongst the space agencies for some reason, despite some researchers saying it is a top priority to further research in artificial gravity.
See also my:
Can Spinning Habitats Solve Zero g Problem? And Answer Low g Questions?
Could Spinning Hammocks Keep Astronauts Healthy in Zero g?
Ingenious Idea: Soyuz Crew in Tether Spin On Way to ISS - For Artificial Gravity - Almost No Extra Fuel
Crew Tether Spin - With Final Stage - On Routine Mission To ISS - First Human Test Of Artificial Gravity?
Crew Tether Spin For Artificial Gravity On Way To ISS - Stunning New Videos - Space Show Webinar - Sunday
In any case, I'd say, why Mars, at this stage? The Moon has a lot going for it, see my
I’ve used some of the material in this answer in this new chapter in my Case for Moon First: Case For Moon First - WHAT ABOUT GRAVITY - ISN'T THAT A BIG ADVANTAGE FOR MARS OVER THE MOON?
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