No, it's not "true". It's "unknown". To date, there have been no studies of how human (or any other) physiology holds up in low-gravity environments.
There have been a lot of studies showing that zero gravity[1] is reasonably unhealthy, but there is a lot of room between earth gravity and zero gravity. At what point does lower gravity become unhealthy? 90% earth gravity? 10% earth gravity? 1% earth gravity? Nobody knows; no research has been done. We need to get some experience with it.
It doesn't seem reasonable to speculate that anything less than 100% earth gravity will be unhealthy. Gravity itself can be a significant and damaging stress on the body; a bit less gravity may actually improve health, not hurt it. For this reason, it's also not reasonable to speculate a linear slope between "healthy at 100% earth gravity" and "unhealthy at zero gravity". The slope is very likely non-linear, but nobody knows what it looks like.
Some space medicine people I've talked to speculate that lower gravity is probably harmless, up to the point where fluids stop flowing downhill. Your body relies on gravity to redistribute fluids throughout your extremities; in zero-gravity, this no longer happens, and a lot of weird things start happening as a result. If this is the case, then lunar gravity (1/6th earth gravity) should be just fine to live in. But, again, we don't know, and all the zero-gravity experiments in the world won't tell us. Until we get some proper low-gravity experience, it's a big question-mark.
1: Technically, "micro-" gravity, since "zero-" doesn't exist in reality. But it's very, very, very micro -- like, 1/1000th earth gravity or less.
There have been a lot of studies showing that zero gravity[1] is reasonably unhealthy, but there is a lot of room between earth gravity and zero gravity. At what point does lower gravity become unhealthy? 90% earth gravity? 10% earth gravity? 1% earth gravity? Nobody knows; no research has been done. We need to get some experience with it.
It doesn't seem reasonable to speculate that anything less than 100% earth gravity will be unhealthy. Gravity itself can be a significant and damaging stress on the body; a bit less gravity may actually improve health, not hurt it. For this reason, it's also not reasonable to speculate a linear slope between "healthy at 100% earth gravity" and "unhealthy at zero gravity". The slope is very likely non-linear, but nobody knows what it looks like.
Some space medicine people I've talked to speculate that lower gravity is probably harmless, up to the point where fluids stop flowing downhill. Your body relies on gravity to redistribute fluids throughout your extremities; in zero-gravity, this no longer happens, and a lot of weird things start happening as a result. If this is the case, then lunar gravity (1/6th earth gravity) should be just fine to live in. But, again, we don't know, and all the zero-gravity experiments in the world won't tell us. Until we get some proper low-gravity experience, it's a big question-mark.
1: Technically, "micro-" gravity, since "zero-" doesn't exist in reality. But it's very, very, very micro -- like, 1/1000th earth gravity or less.