This science is relatively new, but is likely the most important. The Vagus nerve runs directly from the brain, the heart and then to your digestive system and microbiome.
For example, a leaky gut is very damaging to your brain.
Our gut is our second brain. We have 100 Trillion bateria in our intestines. Our gut makes more serotonin (the feel good harmone) than our brain. Our gut makes more dopamine than our brain.
Your emotional state effects your gut microbiome in a good or bad way. The foods we eat effect our gut health dramatically.
The enteric nervous system contains 100,000,000 neurons and it embeds itself within the gut wall. Nerves, harmones, proteins and the hypothalamus gland (an area of the brain that controls appetite) all live here.
It is the main link between your endocrine system and your nervous system. Your hypothalamus keeps your body balanced in a stable state called homeostasis.
As mentioned, your hypothalamus regulates body temperature, hunger, important aspects of parenting and maternal attachment behaviour, thirst, fatigue, sleep, circadian rythms, and is important in certain social behaviors, such as sexual and aggressive behaviors.