You are what you eat, and yet, we know little about how what you eat influences your health as a unique individual. Understanding this relationship is however paramount as diet and gut function are broadly and strongly linked to health and disease, including: development, aging, cancer, diabetes, and even neuropsychiatric disorders. If we could faithfully monitor the composition and function of the gut throughout time in individual people we could build a mechanistic understanding of this linkage and guide healthy and diseased individuals to their desired health outcomes. We recently created a radically new technology to fill this gap – an engineered bacterium that traverses the gastrointestinal tract while recording rich and quantitative information describing gut composition and function along the length of the intestine, which is revealed through the non-invasive sampling and sequencing of the engineered bacteria in feces. Leveraging results from preclinical animal studies, we believe that we can operationalize our engineered bacteria as non-invasive diagnostics.