DrRobKnight

I think the most promising (and understudied) area is linking the gut microbiome to mental health. Interestingly, this was one of the things Francis Collins suggested at the launch of the Human Microbiome Project -- at the time, a lot of people thought that perhaps his own mental health was in need of checking, but evidence of the roles microbes play in the gut-brain axis has accumulated remarkably rapidly since then (although human studies are still rare).

I don't think you need your poop to teach you that lesson...

It's a really good idea, and we had a grant from C2B2 (the Colorado Center for Biofuels and Biorefining) on this topic a few years ago. Many people are studying this in the context of different species' guts. Most focus is on herbivores that can digest wood, ranging from cows to termites to shipworms. The tricky part is not getting it to work in the lab but scaling up to commercially viable systems, though.

The Bristol Stool Scale (http://en.wikipedia.org/wiki/Bristol_stool_scale) provides some guidance on this. Alternatively, the National Folk Museum of Korea currently has a special exhibit in the children's wing dedicated to the topic of poop, including what it should look like: http://www.kidsnfm.go.kr/nfmkid/viewPage.do?screenId=SCREEN_ID_ENG_SPEC#NONE

Well, a lot of the evidence pre-dates studies of microbiome involvement but studies of the microbiome help us understand why things work. For example, there's a lot of evidence that fiber is good -- in part this is because bacteria ferment it into butyrate in the large intestine, and then the butyrate helps feed the cells that line the gut. Special considerations apply if you have specific conditions such as irritable bowel syndrome (and then it depends which subtype). One of the main things we're learning about the microbiome is that everyone is different, so it may be that the right advice on diet depends a lot on the individual (and that individual's microbes). But the general guidelines that are supported by evidence are things you'd expect: lots of leafy green vegetables, lots of brightly colored vegetables (these contain carotenoids, anthocyanins, lycopenoids, and other beneficial plant compounds), fermented foods, no fries or artificial sweeteners, or highly processed foods, etc.