Brain and Neural
The phrases "gut feeling" or "going with your gut" have some practical science behind them. It's because of the Enteric Nervous System in each of us, and its control over our bodies and brains. So powerful is this neural control center, it's been nicknamed "the second brain."
The Second Brain
Most of us think of our nervous system as just the brain, spinal cord and our nerves. Which is right, but not the complete story. Connected to the brain, but operating independently, the enteric nervous system or ENS is comprised of about 100 million neurons. While substantial, that's still only about same number of neurons in the spinal cord and 1/1000 the neurons in the brain.
The ENS controls the gastronintestinal system and is part of the autonomic nervous system responsible for digestive contractions called peristalsis, secretion of enzymes and communication via neurotransmitters. The science of this region is called neurogastroenterology. Your gut is where about 90% of your serotonin and 50% of your dopamine reside, so the ENS plays an important part not just in digestive control, but in your general health, sleep, mood, aggression and emotions.
Gut / Brain / Illness Connection
While science continues new discoveries, some relationships between the ENS, the brain and instances of related illnesses have been reported. Connections are noted between the gut bacteria Clostridium bolteae and autism, perhaps due to the toxins and/or metabolites produced by this bacteria. Consumption of added fiber in the diet of adults may help them avoid stroke by reducing "bad" cholesterol, lowering blood pressure and fighting obesity.
Other recent studies have shown a potential relationship between the use of probiotics and an improvement in mood and stress levels, perhaps due to gut-brain signaling through neurotransmitters. And the challenges of diet-induced type 2 diabetes may extend to potential development of Alzheimer's and cognitive decline, in part due to how our bodies handle sugar. It's why Alzheimer's is sometimes called "diabetes of the brain."