On an individual level the study shows why some people are more prone to weight gain than others.   But there&#x27;s been an epidemic of obesity over the past few decades, and there&#x27;s no reason to believe people&#x27;s gut flora changed over that period.   Even if it did, there must have been an underlying cause.<p>What has undoubtedly changed are people&#x27;s lifestyles:<p>(1) Less smoking.   (This is a good thing, but it has resulted in weight gain.)<p>(2) More driving and less walking&#x2F;bicycling.<p>(3) Children playing games on computers instead of outside.<p>(4) More sitting at desks, and less moving about, at work.<p>(5) More eating out.<p>and probably a few other factors.