"What diet should you be eating to achieve an optimal, healthy microbiome in your gut? We don't know yet but finding out could be the key to helping people overcome many chronic diseases," said Jeff Leach, co-founder of the American Gut project.

(Read about the secret world of microbes in the January 2013 issue of National Geographic magazine.)

The concept of the crowd-funded project is simple: Pay $99, get a sample collection kit, and mail back a test tube containing "a little bit of brown" swabbed from your used toilet paper. Participants will also be asked to log their food intake for three days and answer a detailed questionnaire about how and where they live.

"Are you a vegetarian? Were you