Considering how much effort it takes just to go to the bathroom and the fact they're all living in what is basically a small box for months at a time I imagine they put an enormous amount of research into providing a diet that produces as little gas as possible whilst limiting the number of times they have to go take a shit.

I'm sure personal hygiene is also very high on the list, especially since you can't take a shower every day. Even so, I bet the ISS absolutely stinks most of the time.