When your calories in is greater than your calories out, you gain fat.  The point that the article is making is that one collection of gut bacteria may be more efficient at extracting calories from the same food than another collection, resulting in more &quot;calories in.&quot;<p>What is disconcerting is the idea of engineering your gut bacteria to be less efficient at absorbing calories and nutrients in order to not require self control about what you give your body in the first place.