The rebuttal is itself technically true but I think misses the point of the original statement: The gut is a complex system which has direct effects on many other systems in the body, including the brain. The resultant feedback loops are not well understood at the current time.
If you treat the gut as a simple "food processor" that does nothing more than take in food, break it down and transfer the resluting proteins & sugars into the bloodstream then you're vastly over simplifying things. Yes, it's perfectly true that gut bacteria do not make up a huge percentage of your body mass, but you could say the same thing about your kidneys & anyone who claimed that the kidney "just takes waste products from the bloodstream and excretes them as urine" would be over simplifying things enormously. That statement is as true as the "gut bacteria are only 1% of your body mass" one, but it also misses out on a lot of complexity - the kidneys are involved in all sorts of hormonal feedback loops & can go wrong in many weird and wonderful ways. The same is true of the gut, and that's the point of the original statement about the vast number of gut bacteria IMO - it's reminding us that the digestive system is in fact very complex and an active participant in hormonal feedback loops that affect the rest of the body, including those that affect how much we want to eat.
If you treat the gut as a simple "food processor" that does nothing more than take in food, break it down and transfer the resluting proteins & sugars into the bloodstream then you're vastly over simplifying things. Yes, it's perfectly true that gut bacteria do not make up a huge percentage of your body mass, but you could say the same thing about your kidneys & anyone who claimed that the kidney "just takes waste products from the bloodstream and excretes them as urine" would be over simplifying things enormously. That statement is as true as the "gut bacteria are only 1% of your body mass" one, but it also misses out on a lot of complexity - the kidneys are involved in all sorts of hormonal feedback loops & can go wrong in many weird and wonderful ways. The same is true of the gut, and that's the point of the original statement about the vast number of gut bacteria IMO - it's reminding us that the digestive system is in fact very complex and an active participant in hormonal feedback loops that affect the rest of the body, including those that affect how much we want to eat.