We just don't know. There are no facilities that can produce extended hypogravity to study those longer-term effects. Honestly I think that would be as good a reason as any to construct a spin-gravity space station capable of up to 1 g: Conduct a study to see how much gravity is required for humans to stay healthy. Because if we are going to make permanent settlements on the Moon or on Mars, I think we should probably figure out beforehand if a third or a sixth of Earth's gravity is enough for humans.