In a recent study published in the American Journal of Gastroenterology, the author proposed and discussed the hypothesis that irritable bowel syndrome (IBS) may result from gravity.
IBS is a chronic debilitating disorder of the gastrointestinal (GI) tract. Since the actual pathogenesis of IBS is uncertain, numerous evidence-based theories have been postulated to explain clinical characteristics. In the present study, the author hypothesized that IBS might stem from ineffective gravity management systems meant to optimize GI functions, protect visceral and somatic integrity, and maximize survival.
Despite being the weakest fundamental force, gravitational force (g-force) profoundly impacts the function and form of the visible world and all life forms. Humans evolved to manage gravity in many ways that optimize structural support, upright stability, neuropsychological integration, locomotion, and fluid dynamics.
The author posited that IBS susceptibility is determined by three factors of the g-force: resistance, detection, and vigilance. G-force resistance governs the ability of the GI tract to resist gravity, detection defines the ability of the peripheral nervous system (PNS) to detect gravity strain on somatic and visceral structures, and vigilance determines the readiness of the central nervous system (CNS) to predict and prevent dangerous g-force events.
The GI tract forms a functional stack accommodating and resisting downward traction in a way that aligns, bolsters, and suspends the abdominal viscera to optimize function and form. The author discussed four mechanisms of g-force resistance (suspension system, chassis, ceiling mount, and bolster) in humans, which collectively support the abdominal viscera.
The suspension system
The mesentery originates at the lumbar spine along the posterior abdominal wall and functions like a suspension system preventing intestines from collapsing. The intestinal transit would be undermined severely or cease without the mesentery. It is suggested that mesenteric attachments are critical to support the upright posture of humans to optimize GI function.
Moreover, the large intestine is supported by three strips of smooth muscle traversing throughout the colon, namely the taenia coli. Together, the taenia coli and mesentery suspend, bolster, strengthen, and protect the GI tract. If these resistance mechanisms become ineffective, GI distress and IBS may result.
The chassis and ceiling mount
Musculoskeletal support of the GI tract starts with the spinal column. As per Newton’s third law, the downward force exerted by the mesentery must be balanced with an equal force of the lumbar spine to preclude the sagging of intraperitoneal organs. As such, the paraspinal extensor muscles are engaged to strengthen the backbone, which allows it to function as an anti-gravity support chassis.
The diaphragm forms the ceiling mount for the abdominal cavity. The ribcage, spine, support ligaments, and diaphragm act like an abdominal crane to suspend and stabilize the peritoneal contents. Acute pain can result at each point along this crane due to excess tension between the gravity-bound load and the support scaffold.
Acute pain can become chronic over time through central sensitization. Back pain is reported by up to 80% of IBS patients. Anti-gravity extensor muscles are constantly strained to support the abdominal load under the pull of gravity. If the spine fails, then the crane will sag, leading to angulation of the mesenteric root and disruption of visceral function.
The anterior abdominal wall bolsters the peritoneal cavity, preventing its contents from spilling, and contributes to gravity-dependent distension and bloating through abdominophrenic dyssynergia. Intraluminal gas causes paradoxical diaphragm contraction and dyssynergic abdominal wall relaxation in many individuals with distension and bloating. This chain of events leads to abdominal distention, commonly observed in IBS patients.
When anatomical structures deflect excessively due to gravity, indicating a mismatch between actual and expected g-force strain, the PNS senses it and transmits signals to the CNS. Sensory neurons begin frequent discharging at a lower threshold and larger magnitude through peripheral sensitization if the effects of gravity strain are beyond the safe range for body integrity. This might cause more discomfort and pain.
There is substantial evidence suggesting that IBS patients develop peripheral sensitization. The g-force strain on support structures of the GI tract causes the overproduction of nociceptive mediators, triggers afferent neurons, and leads to PNS sensitization. Abnormal serotonin levels have been long implicated in IBS, and variations in serotonin biology can have profound effects throughout the body.
Serotonin evolved to manage gravity, as humans cannot stand up, maintain balance, pump intestinal contents, or circulate blood without it. Around 95% of serotonin is synthesized in the GI tract, where it regulates intestinal peristalsis. Moreover, intestinal bacteria are involved in serotonin synthesis, suggesting the co-evolution of anti-gravity pump functions and the gut microbiome.
Since serotonin is a pain sensitizer and excess levels promote peripheral pain sensitization, gravity mismanagement can result in an altered gut microbiome, elevated serotonin levels, and pain sensitization, alerting the brain of the g-force strain that triggered the feedback loop. As such, the microbiome plays a crucial role in g-force detection, supporting the communication (of gravity intolerance) between the gut and the brain.
Some individuals show greater resilience to g-forces than others. For instance, some people may experience amusement while dropping from the rollercoaster, whereas others may be threatened and vulnerable. These represent the two extremes of the g-force vigilance factor. Gut feelings can be pathologic when the g-force perilously deviates from the normal limits, and the chances of its occurrence are improbable or impossible.
Pathologic gut feelings might be visceral allodynia due to the overprediction of g-force events that may rarely or never occur. Functional imaging of Alex Honnold’s brain, the man who climbed the El Capitan in Yosemite National Park without ropes, revealed a quiescent amygdala in the face of threats.
In contrast, IBS patients have elevated amygdala reactivity and threat sensitivity. The amygdala forms a part of emotional and salience arousal circuits that appraise and respond to threats, which are dysregulated in IBS, partly due to serotonin-associated gene polymorphisms, leading to overestimating threat severity and fight-or-flight arousal.
In summary, the author reviewed how humans live with gravity, focusing on the GI tract, and explored the gravity hypothesis of IBS pathogenesis, suggesting that IBS may result from gravity. Of note, this hypothesis was intended to accommodate the various theories of IBS but not to supplant the existing explanations.