A simple breathalyzer might be able to tell you how susceptible you are to obesity. Doctors at Cedars-Sinai Medical Center in Los Angeles say the overgrowth of M. smithii, an organism that helps harvest nutrients from food, might make people more prone to weight gain. The way to measure the presence of M. smithii: By the amount of methane and hydrogen on the breath, gases the microbe emits during digestion. The higher the concentration of the gases, the researchers found, the more body fat. According to lead study author Dr. Ruchi Mather, M. smithii helped ancient humans get the max benefit from food when they'd go days between meals, but now that we eat every few hours, it's overkill, and fatty, high-calorie grub just gives it more to feast on. She says it's too early to know if zapping M. smithii with antibiotics would help people shed pounds or if eating fewer calories would disarm the microbe. For now, less calories, more veggies, and exercise will have to suffice.