Dr Brennan Spiegel (physician, gastroenterologist, and author of Pull) and Dr Lachlan Kent (cognitive scientist and founder of Mental Gravity) explore serotonin as a gravity-management molecule, reframing it not simply as a “happiness chemical,” but as a fundamental biological system that allows life to stand up, move fluids, regulate balance, and remain psychologically buoyant in a gravitational field.
Together, Brennan and Lachlan unpack new research on serotonin’s role across the gut, cardiovascular system, lymphatics, vestibular system, reproduction, and mood—showing how 95% of serotonin is produced in the gut, in symbiosis with the microbiome. They trace serotonin’s evolutionary origins from ocean life to land, altitude, and even space, and discuss practical implications for diet, exercise, sunlight, microbiome care, and mental health.
1. What Is Serotonin?
A whole-body gravity resilience molecule, not just a brainneurotransmitter
Exists in two largely separate compartments: brain and gut
Central to movement, circulation, balance, mood, and vitality
Evolutionarily conserved across plants, animals, and humans
Serotonin as a Gravity-Management System
Gut & DigestionMotility against gravity in any body orientation
Pumps & Tubes
Blood pressure regulation, cardiac contractility, lymphatic return
Balance& OrientationVestibular function; dizziness during SSRI withdrawal
Reproduction
Retrograde uterine contractions that move sperm upward against gravity
Mood &Energy
Mental elevation, vitality, resilience, and time perception
The Microbiome–Serotonin Alliance
Trillionsof microbes are required to trigger serotonin production
The gut as a “dark, sulfurous, hydrothermal” evolutionary niche
A symbiotic bargain: microbes get mobility; humans get serotonin
Changes in altitude, microgravity, and spaceflight rapidly alter serotoninbiology
STACK TEN: The Anti-Gravity Diet (introduced)
A practical framework from Pull for supporting serotonin production via tryptophan-rich foods.
Core principles:Tryptophan as substrateMicrobiome diversityDietary balance rather than supplementation alone
Therapeutic & Self-Care Implications
Dietary modification (tryptophan-rich foods, microbiome-supportive diets)Exercise and movementSunlight and time outdoorsWorking with dietitians for GI and mood conditions (e.g., low-FODMAP when indicated)Understanding limits and side effects of SSRIsNaturally supporting serotonin across body and mind
Metaphor & Meaning
The gutmicrobiome as the “spice factory” of human biologySerotonin as the true “spice of life”—powerful, regulating, and double-edgedGravity as the organizing principle linking biology, psychology, and medicine
Timecodes
00:00 —Welcome & why serotonin matters
01:10 — Serotonin beyond the “happiness chemical”
02:20 — Gut vs brain serotonin compartments
03:20 — Pumps, tubes & whole-body effects
04:25 — Lymphatics, reproduction & balance
06:20 — Evolutionary origins & gravitation
08:10 — Microbiome symbiosis metaphor
10:15 — Altitude, spaceflight & serotonin disruption
11:30 — STACK TEN diet introduced
12:45 — Diet, microbiome & mood disorders
13:50 — Dune, spice & serotonin metaphor
15:45 — SSRIs: benefits, limits & side effects
17:00 — Natural serotonin support & integration
17:45 — Closing reflections & next steps
Resources
The Gravity Doctors website: https://thegravitydoctors.com
Dr Brennan Spiegel: https://brennanspiegelmd.com
Dr Lachlan Kent: https://lachlankent.au
Book — Pull: How Gravity Shapes Your Body, Steadies the Mind, and Guides Our Health
Dr Brennan Spiegel (physician, gastroenterologist, and author of Pull) and Dr Lachlan Kent (cognitive scientist and founder of Mental Gravity) explore how diet, microbiome diversity, and serotonin production shape your capacity to stand up to gravity—physically and mentally.
They introduce the Anti-Gravity Diet and Brennan’s STACK TEN framework, a simple, evidence-informed way to support the gut’s serotonin “factories” and improve whole-body gravity resilience. Along the way, they unpack how food, fibre, omega-3s, polyphenols, vagus-nerve activation, and mindful rest all participate in the body–mind system that keeps us upright and energised.
Together, Brennan and Lachlan discuss babies as “gravity novices,” leaky gut, the gut–brain axis, evolutionary embodiment, time dilation in depression, and why serotonin—serum tone—is essential for managing the constant downward pull of Earth’s gravity.
A memory device for foods that supply the raw material (tryptophan) for serotonin production:
Salmon
Turkey
Avocado
Chickpeas
Kidney beans
Tofu/Tempeh
Eggs
Nuts
Paired with:
Fibre → feeds microbes that create short-chain fatty acids
Omega-3s → shift the microbiome toward serotonin-supporting species
Polyphenols → found in berries, cocoa, turmeric, and green tea
00:00 — Welcome & overview of the Anti-Gravity Diet
01:20 — Serotonin as the body’s anti-gravity molecule
04:20 — Microbiome diversity & the gut barrier
06:30 — Babies, serotonin & early gravity resilience
09:40 — Tryptophan, fibre & short-chain fatty acids
11:00 — Leaky gut, inflammation & decoherence
13:00 — Food–mood science & depression
16:40 — What to eat: introducing STACK TEN
17:00 — The ten tryptophan-rich foods
18:50 — Polyphenols, omega-3s & microbiome tuning
19:50 — Beyond diet: exercise, nature, sunlight
21:20 — Balancing sympathetic vs parasympathetic tone
23:00 — Vagus-nerve stimulation & rest-and-digest
24:30 — Gravity as the integrator across biological levels
26:00 — Life as a system for managing gravity
27:20 — Closing reflections & future topics
The Gravity Doctors website: https://thegravitydoctors.com
Gravitype Quiz: virtualmedicine.org/pull
Dr Brennan Spiegel: brennanspiegelmd.com
Dr Lachlan Kent: lachlankent.au
Book — Pull: How Gravity Shapes Your Body, Steadies the Mind, and Guides Our Health
In this episode, Dr Brennan Spiegel (physician, gastroenterologist, and author of Pull) and Dr Lachlan Kent (cognitive scientist and founder of Mental Gravity) dive deep into gravity resilience and introduce listeners to the new Gravitype Quiz—a wellness tool that blends biomechanics, neuroscience, and psychology to help people understand their personal relationship with gravity.
Together, Brennan and Lachlan take the quiz live on the show and compare results, unpacking the three pillars of biogravitational health: gravity fortitude, gravity sensing, and mental gravity. Along the way, they explore back pain, dizziness, cold exposure, resilience, interoception, emotional balance, and how experiences like roller coasters or elevators reveal something about your gravitational tendencies.
You can try the full interactive quiz here:
brennanspiegelmd.com
virtualmedicine.org/pull
thegravitydoctors.com
1. What Is a Gravitype?
A personalised profile of gravity resilience
Based on the framework in Pull and grounded in emerging concepts in biogravitational medicine
Not a diagnostic tool — currently evidence-informed and intended for education/wellness
Gravity Fortitude
Muscles, bones, pumps, and tubes
Cardiovascular and postural strength
Back pain, leg swelling, stair climbing, grip strength
Gravity Sensing
Inner ear, proprioception, vestibular function
Balance tests (like one-leg standing)
Cold sensitivity, interoception, dizziness/vertigo
Mental Gravity
Weighted-down vs uplifted
Stress tolerance, emotional recovery, “flow” states
Dual-continuum model of mental health
Grip strength as a predictor of longevity
BMI and physical load on the spine
Orthostatic light-headedness and blood pressure regulation
Inner-ear disorders (e.g., BPPV) and sensory hypersensitivity
The anterior insula as the “gravity conductor” of the body–mind symphony
They answer questions on:
Opening a jar
Climbing stairs
Back pain
Light-headedness
Balance
Cold tolerance
Emotional reactivity and resilience
Roller coaster sensitivity
Positive emotional absorption (flow)
Each reflects personally on past injuries, thyroid health, yoga, meditation, diet, travel, and evolving gravitational resilience across their lives.
Durable fortitude
Normosensitive sensing
Lifted mental gravity
Notes how training (cold exposure, yoga, flying) reshaped Lachlan's sensitivity over time.
Reflections on vertigo, cold intolerance, and emotional uplift as part of Brennan's profile.
Both discuss how gravitypes can change with training, awareness, and targeted practices.
Examples include:
Down-regulating sensory overload (slow nasal breathing, longer exhales, box breathing)
Strengthening the gravity fortitude system (movement, posture, reducing sitting time)
Noticing “watch-outs” such as back pain, dizziness, or emotional rumination
Links included in the show notes.
00:00 — Welcome, origins of the gravitype
01:30 — Gravity resilience basics
03:00 — Gravity fortitude (muscles, tubes, pumps)
06:45 — Mental gravity explained
08:00 — Starting the quiz
09:00 — Grip strength & longevity
11:00 — Stair climbing & cardiovascular load
12:00 — Back pain & evolution
15:00 — Orthostatic dizziness & fainting
18:00 — BMI, weight, and gravitational load
20:00 — Leg swelling, pumps & tubes
23:30 — Gravity sensing & vertigo
25:00 — Balance tests (one-leg stance)
26:30 — Cold sensitivity & nervous-system tuning
29:30 — Interoception & the anterior insula
33:00 — Stress, heaviness & mental gravity
36:00 — Roller coasters, turbulence & fear responses
38:00 — Emotional resilience and bouncing back
41:00 — Flow states and uplift
43:00 — Revealing their gravitypes
46:30 — What their results mean
48:00 — Final reflections & where to take the quiz
The Gravity Doctors website: https://thegravitydoctors.com
Gravitype Quiz: available on BrennanSpeigelMD.com & virtualmedicine.org
Dr Brennan Spiegel: brennanspiegelmd.com
Dr Lachlan Kent: lachlankent.au
Book — Pull: How Gravity Shapes Your Body, Steadies the Mind, and Guides Our Health
Dr. Brennan Spiegel (Cedars-Sinai) and Dr. Lachlan Kent explore how gravity shapes not only our bodies but our minds. Through the lens of Cathy Freeman’s gold-medal run, they discuss mental gravity — the feeling of emotional “weight” and lightness that defines human experience.
From Einstein’s relativity to embodied cognition and Cedars-Sinai’s virtual-reality therapy, they connect physics, psychology, and physiology to show how alignment with gravity may be the key to wellbeing. Topics include depression as a “mental gravity well,” glucose and perception, VR and levity, and how mindfulness, diet, and physical balance can help us stay buoyant in heavy times.
Key themes:
Mental Gravity: The experience of emotional and cognitive “weight,” shaped by our alignment with the physical world’s gravitational pull.
Embodied Cognition: How bodily states and forces (like gravity, glucose, or posture) shape perception and thought.Gravity Intolerance: A unifying framework for disorders where physical and mental balance are disrupted.
Mind over Matter: Using imagery, mindfulness, and movement to generate upward momentum and emotional lightness.Timecodes:
00:00 – 01:30 | Welcome to The Gravity DoctorsIntroducing Brennan and Lachlan; overview of physical and mental gravity.
01:30 – 03:50 | The Cathy Freeman StoryHow Cathy transcended the “weight of expectation” to run with joy and lightness — an example of mental gravity at its best.
03:50 – 06:00 | The Weight of EmotionExploring why positive emotions feel light and negative emotions feel heavy — from embodied metaphors to physical postures of depression.
06:00 – 08:10 | Embodied Cognition and Glucose StudiesBrennan cites research showing blood-glucose levels alter perception of hill steepness — a literal bridge between physiology and perception.
08:10 – 11:00 | The Birth of the Theory of Mental GravityLachlan recalls his “aha” moment linking Einstein’s general relativity lecture on black holes to psychological depression — both described using the same language of heaviness, collapse, and isolation.
11:00 – 12:40 | Gravity Wells, Black Holes, and RecoveryComparing mental “gravity wells” to depression — but unlike black holes, people can pull themselves out.
12:40 – 16:20 | Champagne, Joy, and the Physics of EmotionWhy celebration rituals mirror gravitational metaphors — bubbles rising, buoyancy, and the inevitable “come-down.”
16:20 – 19:30 | Virtual Reality as a Gravity-Hacking ToolCedars-Sinai research: VR experiences that simulate floating or flying reduce pain, anxiety, and inflammation — measurable physiological effects of simulated levity.
19:30 – 22:00 | Mindfulness and Mental ImageryUsing mental imagery and meditation to reshape the brain-body landscape and foster feelings of lightness and alignment.
22:00 – 25:50 | The Continuum of Mind and BodyWhy mind and body aren’t separate but part of one continuous system — like a cross-country landscape shifting seamlessly from desert to sea.
25:50 – 28:30 | Gravity Intolerance and Health ResilienceReframing conditions like IBS, POTS, and anxiety as variations of gravity intolerance; maintaining physical and mental balance as the essence of wellbeing.
28:30 – 30:00 | Everyday Applications & Closing ReflectionsFrom mindfulness to serotonin-rich diets and time in nature — practical tools to stay buoyant.
Final reflections on finding balance between empowerment and medical treatment.
Want to discover your 'Gravitype'? Visit The Gravity Doctors website:https://thegravitydoctors.com/
Order Brennan's book "Pull" to learn more about how gravity shapes your health in both body and mind via his personal personal website: https://www.brennanspiegelmd.com/pull
Visit Lachlan's website to learn more about his work in mental gravity: https://lachlankent.au/
Music by ALLIRA and Visceral Sound - https://www.youtube.com/@ALLIRAmusic
Recording at JTB Studio by David Iskandaryan.
In this uplifting and deeply human episode of The Gravity Doctors, Dr. Brennan Spiegel (Cedars-Sinai gastroenterologist and author of PULL) and Dr. Lachlan Kent (cognitive scientist and theorist of Mental Gravity) sit down with Harmon Clarke, a patient-turned-researcher whose remarkable recovery story illuminates the future of biogravitational medicine.
From septic shock and 200 days in hospital to yoga teacher, breathwork practitioner, and health researcher, Harmon’s journey embodies resilience, creativity, and the healing power of re-engaging with gravity. Together, they explore how movement, inversions, virtual reality, and breathwork can transform pain and depression into buoyancy and balance — culminating in a spontaneous “Gravity Rap” that captures the spirit of the show.
Core Themes
Gravity as a unifying metaphor for physical and mental health
Movement and inversion therapy for resilience and recovery
Virtual reality and embodied cognition
Tensegrity, balance, and the biomechanics of wellbeing
Patient empowerment and the hero’s journey in healing
Biogravitational medicine as a synthesis of East–West paradigms
00:00 – Welcome and Introductions
01:00 – Facing the Grave
03:00 – Virtual Reality and the Mind–Body Connection
06:00 – Yoga, Breathwork, and Natural Recovery
08:00 – The Science of Inversion and Tensegrity
14:00 – Hospitals and Gravity Intolerance
18:00 – Depression as a Gravitational Condition
24:00 – Cathy Freeman and Gravitational Flow
25:45 – East Meets West
30:00 – Rebalancing Medicine
33:00 – The Simple Tools of Healing
35:00 – Designing the Future of Biogravitational Medicine
38:00 – Generational Health and Modern Comfort
41:00 – The Gravity Rap Finale
Order Brennan's book "Pull" to learn more about how gravity shapes your health in both body and mind via his personal personal website.
Visit Lachlan's website to learn more about his work in mental gravity.
Music by ALLIRA and Visceral Sound
Recording at JTB Studio by David Iskandaryan.
Editing by Dr Karisma Suchak.
Biogravitational Medicine is a holistic approach to maintaining health in both body and mind. It's like a toolkit where you can find what works for you and your particular situation. Here are our top ten that work for most people most of the time. (As always, consult a doctor for personalised medical advice before attempting anything that might pose a risk to your physical or mental health, or for treatment of acute or chronic conditions. The information here is for educational and information purposes only.)
Order Brennan's book "Pull" to learn more about how gravity shapes your health in both body and mind via his personal personal website.
Visit Lachlan's website to learn more about his work in mental gravity.
Music by ALLIRA and Visceral Sound
This episode was recorded at Myst Music.
Audio engineering by Jeremy Roberts.
Audio and video editing by Dr Karisma Suchak.
Dr Spiegel has written a book about gravitational health called 'Pull' on the wide ranging themes underpinning this podcast - gut health, mental gravity, serotonin, the inner ear, the body's pumps and tubes, the gravitostat, and many more. The hosts take a tour through the world of biogravitational medicine by telling the story of what inspired the book, where the science is heading, and how people can apply the principles in their daily lives.
Order Brennan's book "Pull" to learn more about how gravity shapes your health in both body and mind via his personal personal website.
Visit Lachlan's website to learn more about his work in mental gravity.
This episode was recorded at Myst Music.
Thanks to Jeremy Roberts for audio engineering and Dr Karisma Suchak for editing.
You know the feeling you get in your belly when falling on a rollercoaster? Ever wondered why we get similar gut feelings when we're "falling" in love? In this episode, Brennan (a gastroenterologist) and Lachlan (a cognitive scientist) talk about how gravity shapes gastrointestinal function and what relation gut feelings have to physical and mental health. They discuss Brennan's viral scientific paper, "Gravity and the Gut: A Hypothesis of Irritable Bowel Syndrome" and Lachlan's breakthrough theory of the mind called "Mental Gravity", and the connections between their shared ideas. Conditions like IBS often lead to anxiety and depression, which are the mental equivalent of "falling" and having "fallen", and so they draw a connection between gut health, gut feelings, and mental health. Strap yourself in for a rollercoaster ride through the topsy-turvy world of gut feelings in both body and mind.
Order Brennan's book "Pull" to learn more about how gravity shapes your health in both body and mind via his personal personal website.
Visit Lachlan's website to learn more about his work in mental gravity.
Music by ALLIRA and Visceral Sound.
This episode was recorded at Myst Music.
Thanks to Jeremy Roberts for audio engineering.