In animals, brain sizes increase in proportion to body size with larger animals having bigger brains. However on a per weight basis, humans pack in more neurons than any other species. This density is what makes us so super smart, but it takes a whole lot of fuel to power those complex brains and there’s a trade-off between body size and sustainable number of neurons. While the adult brain makes up about only 2% of the body’s mass, it uses about a whopping 20% of the energy intake.
According to the Ted-Ed Animation, What percentage of your brain do you use?, “A 25 kilogram ape has to eat 8 hours a day to sustain a brain with 53 billion neurons.” The human brain is generally believed to have around 100 billion neurons.
So, how come we aren’t having to eat around the clock to support our humongous brains?
One theory is that the invention of cooking food gave humans the means to power their growing brains because their guts could more easily digest and absorb energy from food. Cooking could have then enabled our ancestors’ gut size and teeth to shrink and sparked the evolution of our bigger-brained, larger-bodied, humanlike forebears.
The Brain In Your Belly
From the time food hits your mouth and as it moves through your gastrointestinal tract, it’s causing a cascade of changes in your body and brain. In addition to that big brain in your head, you also have a brain in your gut, called the enteric nervous system. This second brain consists of a network of some 500 million nerve cells and 100 million neurons – equivalent to about the size of a cat brain – lining your gut, esophagus to anus. That’s more neurons than are in your spinal cord or peripheral nervous system. The enteric nervous system can function without any input from your central nervous system and transmits information to it.
Just like the brain in your head, the brain below uses over 30 neurotransmitters including dopamine and serotonin. In fact, 95% of the body’s serotonin is found in the bowels. It’s no wonder that SSRI (Selective Serotonin Reuptake Inhibitors) antidepressants, meant to alter the mind, often have gastrointestinal side effects.
Scientists are learning that a big part of our emotions and mental health is influenced by this gut brain through information carried from it to your head via the vagus nerve. I’ll bet you can remember a time when you had one of those insistent gut feelings, the unmistakable flutter of butterflies or felt nauseous thanks to your enteric nervous system.
Living in your gut are tens of trillions of micro-organisms, including bacteria, parasites, protozoans, and fungi, making up part of your unique microbiome. We’ve always known these little guys play a major role in digestion, allergies, and metabolism, but now we know that the bacteria in your gut aids in the production of vitamins and neurotransmitters and greatly influences your immune function. Science has also uncovered connections between intestinal bacteria and anxiety, depression, autism, Alzheimer’s disease, and other mental disorders.
In One billion reasons probiotics protect your brain, Dr. Sarah McKay, neuroscientist, explains the gut/brain connection:
- a healthy balance of gut bacteria contributes to normal behavior, cognition, emotion and a well-functioning immune system.
- poor balance of the bacteria (perhaps brought on by stress, disease or antibiotics) disrupts the gut-brain signaling pathways.
- disruptions in gut-brain signaling may lead to abnormal brain function, changes in our behaviour, thoughts, emotions, our perception of pain, and may also impact our immune system.
Your Gut Bacteria And Your Brain
Your gut bacteria affect your health in many ways, from helping to build your immune system and influencing your weight and risk of certain diseases, like diabetes, obesity, and autoimmune, heart, and colon diseases, to impacting your brain, and in turn, your mental health and behavior.
You are what you eat, right? Same goes for the microorganisms in your gut.
The trillions of bacteria inside you eat what you eat, and turn those meals into molecules which influence your brain through a relationship called the “gut-brain axis.” In other words, the choices that you make about what goes into your mouth directly affects the bacteria communities inside you, greatly impacting your health in a surprisingly short period of time. One study found that the bacteria in peoples’ guts shifted within three to four days of a major diet change.
There’s early evidence suggesting that your gut flora influences your mood, and emotions. One study showed that people who had more fermented foods, which are full of healthy bacteria known as probiotics, in their diet exhibited less neuroticism and social anxiety. The article, Sauerkraut Could Be The Secret To Curing Social Anxiety, explains why:
There are several possible mechanisms by which gut bacteria can influence mood. For starters, increasing the balance of good bacteria in the gut may decrease intestinal permeability, also known “leaky gut” — a condition that has been linked with depression. More good bacteria also generally means less inflammation, which may be connected to a decrease in anxiety, stress and depression. Probiotics may also reduce anxiety by increasing GABA, a neurotransmitter that tempers the brain’s fear response.
It’s not yet clear how the microbiome alters the brain exactly, but it’s probably via multiple mechanisms, including neural, hormonal, and immune pathways. Scientists have found that gut bacteria produce neurotransmitters which play a key role in determining moods. Gut bugs may generate other neuroactive chemicals, that have been linked to reduced anxiety and depression. It’s also been shown that some microbes activate the vagus nerve, the main line of communication between the gut and the brain, which has a calming effect. Additionally, your microbiome is intertwined with your immune system, which itself influences your mood and behavior.
Managing Your Microbiome
Your body’s unique collection of microbes is partly inherited from your mother at birth and partly determined by your lifestyle. The modern lifestyle can be incredibly toxic to your gut bacteria.
Antibiotics, prescription and over-the-counter medications, pesticides and other chemicals, douches and colon cleanses, colonoscopies and other medical procedures, chemotherapy and radiation, antidepressants and sleeping pills, altered fat in food, sugar and carbohydrate intake, and many other things drastically change the diversity and number of bacteria in your gut.
According to the article, What Is Your Gut Telling You?, “There’s a good chance your microbiome is associated with every disease you can think of.” It’s possible that one day in the future, altering gut bacteria could be a treatment for neurodevelopmental disorders and mental illnesses. Anxiety, depression and several pediatric disorders, including autism and hyperactivity, have been linked with gastrointestinal abnormalities.