&gt; When your calories in is greater than your calories out, you gain fat.<p>Wrong. Calorie surplus is neither necessary for nor a guarantee of fat gain.<p>When your calories in are greater than your calories out, you gain net energy content of body mass. If you have very low body fat already, you aren&#x27;t going to be able to gain <i>lean</i> body mass without increasing net energy content, which means a calorie surplus; and a calorie surplus can sometimes, AFAIK, be beneficial in building lean body mass even if you <i>haven&#x27;t</i> already driven your body fat %age to a very low level.<p>Conversely, you can gain body fat while losing lean body mass with a calorie deficit.<p>&gt; 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.<p>Why is that disconcerting? If excessive efficiency in this one function is associated with adverse health outcomes, and people with less efficient (than the level raising concern) gut microbiomes tend toward healthier weights and better health outcomes, preference for maximum efficiency in this area is a harmful microoptimization, like optimizing a firm&#x27;s software development operations for maximum lines of code produced per day rather than business functionality delivered.