&gt; Wrong. Calorie surplus is neither necessary for nor a guarantee of fat gain.<p>I was excluding exercise from my (admittedly) very simplistic rule of thumb of weight gain.  You seem to have a general understanding of nutrition, so I would say to you that it is absolutely disingenuous to those who don&#x27;t have that understanding to say that calorie surplus != fat gain.  It&#x27;s a general rule, and it applies to the majority of people.<p>&gt; Why is that disconcerting?<p>It&#x27;s disconcerting in the same way that liposuction is disconcerting.  You could make very similar arguments about liposuction as you did about gut engineering: if it improves health, then it&#x27;s a good thing.  But at the end of the day, it doesn&#x27;t address the issue of being able to control what you do with your body in the interest of your health.  Getting too fat?  Just reset your gut bacteria, regardless of what other unknown hormone changes it will have.  It&#x27;s ridiculous.