https://www.cnet.com/news/nasa-twins-study-reveals-the-startling-ways-that-space-travel-can-change-us/

The results of the "Twin Study," which featured researchers from 12 different universities working across 10 different projects, are set to be published in the April 12 issue of Science. The findings demonstrate how the human body responds to time spent in space, unraveling the complex changes that take place when it's exposed to the wholly alien experience of living in microgravity.

The research team studied a wide variety of human physiological processes, including gene expression, gut health, immunity and cognition, allowing them to pin down just how much space might change Scott while his brother stayed at home.