It has something to do with the lack of gravity.
Astronauts also have bone loss. Which is why they have to work out every day.
He explained to me that they(NASA) know how to build a rocket that would take people to Mars. The problem is that between the lack of gravity and radiation that the astronauts may not survive the trip.
We exchanged business cards. We became friends on Linkdin. He retired from NASA a few years ago.
Interesting.
Something sci-fi writers never foresaw.