It&#x27;s pretty much that simple.  Unless someone has some sort of lymphatic condition that causes them to retain fluids or an undiagnosed metabolic syndrome, you aren&#x27;t gaining weight unless you are consuming more than you are expending.<p>That isn&#x27;t to say that two identical people with two different gut floras would necessarily gain or lose weight identically while eating the same diet, but I&#x27;ve yet to see any evidence that an otherwise healthy person eating maintenance level calories is going to blow up like a balloon simply because his gut flora is different.  A person with a healthy gut flora might be able to consume more calories without gaining as much weight, and it&#x27;s important to understand WHY that is, but the fact remains that if you don&#x27;t consume any more than you expend, you aren&#x27;t going to gain weight (previously mentioned caveats notwithstanding).<p>That said, the western diet is horrible for you.  Go to any fast food restaurant and the only fiber you&#x27;ll get is in the lettuce on your burger.