One of the runners-up for the biobricks lifeform category of our Mad Science Contest was Jonathan Cline, who described what he calls a "ketonalizer." This would be a bacteria-based system for analyzing the metabolic state of "ketosis" in a person's body — this is the state where the body starts burning fat and turning it into energy. Ketosis is induced by the Atkins diet as well as caloric restriction. Cline proposes to make a system that you could breathe on, and measure your current state of ketosis. Here is his paper.