Have you ever wondered why some people eat exactly the same food as their friends but seem to put on more weight? Researchers at Washington University in Missouri, US, think they may have found the answer - it's not pies, but something a great deal smaller. The team have been studying a type of bacteria in the gut called Methanobrevibacter smithii that live in the gut. The bugs help to clear up the waste products of other gut bacteria, such as Bacteriodes theta, that break down components in food that our gut enzymes cannot digest. The team found that mice carrying both types of bacteria in their guts were on average 15% fatter than mice that just had the theta strain of bugs. The researchers think that the smithii bacteria help the theta type to thrive and grow, so they can produce more fatty acids from undigested food. These fatty acids are taken up by the mice and turned into fat. Around 85% of humans have the smithii bacteria in their guts, and we get about 10% of our calories from fatty acids made by microbes such as B. theta. However, we don't yet know if overweight people have more smithii bacteria than underweight people, but the researchers are working on finding out.