Are you listening?
(And more importantly — are you feeding it the right things?)
Here’s something that sounds like a joke but is completely true:
You have a second brain. It’s in your gut. And it’s been sending messages to your actual brain this entire time — about your mood, your energy, your motivation, and whether you feel like a functioning human being or a vaguely anxious bowl of cereal.
Scientists call it the gut-brain axis. We like to call it the most underrated system in your entire body.
What you eat doesn’t just fuel your muscles. It shapes your thoughts, your mood, and how motivated you feel to do literally anything.
Here’s how it all works — explained the way a smart middle schooler would understand it, backed by the science that convinced researchers at Harvard and Stanford to take it very seriously.
First: What Is the Gut-Brain Axis?
Imagine your brain and your gut are two best friends who text each other constantly. They’re not next to each other — one is in your head and one is in your belly — but they have a direct line, and they never stop communicating.
That direct line has a name: the vagus nerve. It’s one of the longest nerves in your body, running all the way from your brainstem down through your chest and into your digestive system. Think of it as a two-way highway — signals travel up from your gut to your brain, and back down from your brain to your gut, constantly.
The Gut’s Own Nervous System
Your gut has its own nervous system called the enteric nervous system, which contains over 100 million neurons. That’s more neurons than your entire spinal cord. It can actually function independently of your brain — which is why scientists sometimes call it “the second brain.” (Stanford Medicine, 2025)
And here’s the thing about this highway: the traffic mostly flows one way. About 80% of the signals on the vagus nerve travel from your gut up to your brain — not the other way around. Your gut is doing most of the talking.
This process — your brain building a picture of what’s happening inside your body — is called interoception. Scientists at Stanford describe it as the body’s version of a sixth sense: your internal GPS for hunger, fatigue, stress, and more.
※ Source: Stanford Medicine, “Gut-Brain Connection: What the Science Says,” 2025
The Mood Chemicals Living in Your Gut
You’ve probably heard of serotonin — the “feel-good chemical.” Most people assume it lives in the brain. They’re not wrong, but they’re missing most of the picture.
90% of your serotonin is made in your gut.
Serotonin helps regulate your mood, sleep, and anxiety levels. When Harvard Health talks about what influences how you feel on any given day, serotonin is always in the conversation. And the vast majority of it comes from your digestive tract — not your brain. (Harvard Health, 2022)
Here’s where it gets interesting. The serotonin your gut produces can’t directly cross into the brain — there’s a protective barrier called the blood-brain barrier that keeps it out. So how does gut-made serotonin affect how you feel?
It sends signals via the vagus nerve. Your gut serotonin stimulates the vagus nerve, which carries those signals up to the brain and influences your emotional state, your stress response, and your inflammation levels — all without actually entering the brain directly.
It’s a bit like your gut texting your brain: “hey, things are good down here” or “something’s off, you might want to know about this.”
The same goes for dopamine — the neurotransmitter linked to motivation, reward, and that feeling of satisfaction when you finish something hard. About 50% of the body’s dopamine is produced in the gut. Emerging research suggests it too may communicate with the brain via the vagus nerve.
※ Source: Harvard Health, “Nutritional Psychiatry: Your Brain on Food,” 2022 | Tiny Health, “How Your Gut Shapes Serotonin and Dopamine,” 2024
What Your Brain Tells Your Gut
This highway runs both ways. Your brain doesn’t just receive messages from your gut — it sends them too.
Ever felt your stomach drop when you got bad news? Ever had “butterflies” before something important? That’s your brain talking directly to your gut — in real time, no delay.
When your brain perceives stress, it activates what’s known as the HPA axis (hypothalamic-pituitary-adrenal axis) — your body’s stress response system. This triggers the release of cortisol, the stress hormone, which travels to your gut and changes how it functions.
Stress changes your gut bacteria.
Chronic stress doesn’t just make you feel bad — it literally alters the composition of your gut microbiome, increases intestinal permeability (sometimes called “leaky gut”), and can reduce the production of mood-regulating neurotransmitters. Your brain’s bad day becomes your gut’s bad day, which then becomes your brain’s worse day.
Harvard researchers discovered that the brain has highly specific control over the gut — different neurons in the brainstem connect to different parts of the digestive system, allowing the brain to issue very precise instructions. The stomach gets one set of signals. The intestines get another. The gallbladder gets its own.
This is how your mental state literally shapes your digestion — and why people with chronic anxiety often have chronic gut problems, and vice versa.
※ Source: Harvard Brain Science Initiative, “How the Brain Communicates with the Gut,” 2021
How the Two Together Shape Your Mood
By now you can see the loop taking shape. Your gut sends signals up. Your brain sends signals down. Those signals influence neurotransmitters. Those neurotransmitters shape how you feel. How you feel affects what you eat. What you eat affects your gut. And the cycle continues.
Scientists call this the gut-microbiome-brain axis — and it’s now considered one of the most important systems for mental health.
A healthy gut = a more stable mood.
Studies show that people with diverse, balanced gut microbiomes are less likely to experience depression and anxiety, and report more stable emotional states. An imbalanced microbiome (called dysbiosis) has been linked to mood swings, irritability, brain fog, and difficulty concentrating.
Here’s a simple way to think about it:
Your gut bacteria produce and influence neurotransmitters. Those neurotransmitters send signals to your brain. Your brain interprets those signals as mood, energy, and motivation. What you eat determines which bacteria thrive in your gut. Which bacteria thrive determines which signals get sent. And so your breakfast is, in a very real sense, a message to your brain about how the rest of the day is going to go.
Your breakfast is a message to your brain. What are you telling it?
※ Source: Journal of Behavioral and Brain Science, 2024 | Mayo Clinic, “Mood-Boosting Foods,” 2024
What Fuels Your Brain’s Focus
Your brain is the most energy-hungry organ in your body. It accounts for about 20% of your body’s total energy use — despite being only about 2% of your body weight. It runs on glucose, but how you deliver that glucose matters enormously.
Spike it fast (with refined sugar, cheap carbs, artificial sweeteners) and you get a brief surge followed by a crash. Your focus dips. Your mood dips with it. Your gut notices. Signals head north.
Feed it steadily (with complex carbs, healthy fats, complete protein, and real fiber) and your brain gets a consistent, clean energy supply. Neurotransmitter production stays stable. Inflammation stays low. The whole system runs more smoothly.
What your brain actually runs best on
According to Harvard Health, your brain functions best on what they describe as ‘premium fuel’ — high-quality foods rich in vitamins, minerals, antioxidants, and clean protein. These nourish brain tissue and protect against oxidative stress. Low-quality food doesn’t just fail to help — it can actively damage the system.
Complete protein is especially important here. Remember the amino acid tryptophan from our protein series? It’s the precursor to serotonin. Your brain literally uses tryptophan from the food you eat to build the neurotransmitters that regulate your mood. No tryptophan in your diet, no raw material for serotonin.
This is why protein source matters — not just how much, but what kind. A complete protein with all nine essential amino acids gives your brain the full toolkit. An incomplete one leaves gaps.
※ Source: Harvard Health, “Nutritional Psychiatry: Your Brain on Food,” 2022
Motivation, Energy, and the Gut Connection
Motivation isn’t just willpower. It’s chemistry.
Dopamine is the neurotransmitter at the centre of your motivation and reward system. When dopamine is flowing well, you feel driven, focused, and satisfied when you complete things. When it’s low, everything feels like more effort than it’s worth. You procrastinate. You feel flat. You reach for a sugar hit that spikes you briefly and then drops you further.
With roughly 50% of your dopamine produced in the gut, the health of your digestive system has a direct relationship with how motivated you feel on a given day.
Dopamine and what disrupts it
Poor gut health — from dysbiosis, inflammatory foods, or gut irritants — can impair dopamine production and regulation. This shows up not just as digestive discomfort, but as reduced motivation, difficulty concentrating, and lower mood. It’s not in your head. It’s in your gut.
Energy levels tell the same story. Chronic gut inflammation, even at low levels you might not notice consciously, diverts resources your body would otherwise use for clear thinking and sustained focus. When your gut is fighting something — an ingredient it doesn’t like, a microbiome out of balance — that’s energy that doesn’t go to your brain.
The result is what a lot of people describe as a kind of baseline fatigue. Not tired enough to stop, just never quite sharp enough to feel like yourself. Scientists have a term for this: low-grade chronic inflammation. And it’s more common than most people realise.
※ Source: We Don’t Waste, “Food & Your Mood: How Your Gut Tells Your Brain What to Feel,” 2024
The Most Interesting Thing Researchers Recently Discovered
In 2025, Stanford researchers studying long COVID found something remarkable. Patients with persistent brain fog and cognitive problems after COVID-19 showed reduced vagus nerve activity. Their guts and brains weren’t communicating properly.
When the scientists treated the condition by raising serotonin levels — targeting the gut-brain pathway — the cognitive deficits improved.
This finding matters beyond long COVID. It’s one of the clearest demonstrations yet that the vagus nerve isn’t just a passive cable — it’s an active system, and when it’s impaired, your brain feels it in ways that show up as mood problems, memory issues, and cognitive fog.
What this means for everyday health
You don’t need long COVID for your vagus nerve communication to be suboptimal. Chronic stress, poor diet, disrupted sleep, and gut dysbiosis can all dampen this signal. The good news: dietary changes, regular exercise, and gut-supporting nutrition have all been shown to improve vagal tone — the technical term for how well your vagus nerve is functioning.
※ Source: Stanford Medicine, “Gut-Brain Research: Long COVID, Parkinson’s, Anxiety,” 2025
So What Does This Mean for What You Eat?
If your gut and brain are in constant conversation, then every meal is part of that conversation.
The things that support the gut-brain axis are also the things that tend to feel intuitively right: plenty of fiber from whole foods, fermented foods that feed beneficial bacteria, clean protein sources that provide the amino acid building blocks for neurotransmitters, healthy fats that reduce inflammation, and minimal gut irritants that disrupt the whole system.
The things that work against it: ultra-processed foods, excessive sugar, gut-disrupting additives, low-quality protein sources that add gut load without sufficient nutrition, and chronic stress without recovery.
Here’s a practical way to think about it: before you eat something, you can ask yourself — is this going to support the conversation between my gut and my brain, or is it going to make it harder?
Every meal is part of the conversation between your gut and your brain. Feed it accordingly.
That’s not a diet rule. It’s just an understanding of how your body actually works.
A NOTE FROM MUJO
At Mujo, everything we build starts with this connection. Our coffee alternative was designed to fuel your brain without the crash. Our Lemna protein bars were built around ingredients your gut can actually work with — complete protein, allergen-free, no gut load. Because when your gut and brain are both supported, everything else gets easier.
We’re almost ready to show you what we built. If you haven’t joined the founding circle yet, the link is here.
SOURCES
※ Harvard Health, “Nutritional Psychiatry: Your Brain on Food” — health.harvard.edu, 2022
※ Harvard Health, “Serotonin: The Natural Mood Booster” — health.harvard.edu, 2023
※ Harvard Health, “Dopamine: The Pathway to Pleasure” — health.harvard.edu, 2024
※ Harvard Brain Science Initiative, “How the Brain Communicates with the Gut” — brain.harvard.edu, 2021
※ Stanford Medicine, “Gut-Brain Connection: What the Science Says” — med.stanford.edu, 2025
※ Stanford Medicine, “Gut-Brain Research: Long COVID, Parkinson’s, Anxiety” — news.stanford.edu, 2025
※ Mayo Clinic, “Mood-Boosting Foods” — communityhealth.mayoclinic.org
※ Journal of Behavioral and Brain Science, Vol. 14, 2024
※ Tiny Health, “How Your Gut Shapes Serotonin and Dopamine,” 2024
※ We Don’t Waste, “Food & Your Mood: How Your Gut Tells Your Brain What to Feel,” 2024