Lots of things that people don’t think about, like depression or anxiety, are very clearly modified by your gut microbes. Appetite and ability to digest food are modified by gut microbes. The key finding recently is the link with the immune system. Basically, the gut microbiome is controlling it, sending signals, because most of your immune system is in your gut, helping you fight infections, such as Covid and early cancers, that the immune system is picking off.
Your gut microbiome weighs about 2kg and is bigger than the average human brain. It’s a bustling community of trillions of bacteria, archaea, fungi and viruses, containing at least 150 times more genes than the human genome.