Bad things happen to a human body in zero gravity. Just look at what happens to astronauts who spend time in orbit: Bones disintegrate. Muscles weaken. So does immunity. “When you go up into space,” says Saïd Mekari, who studies exercise physiology at the University of Sherbrooke, in Canada, “it’s an accelerated model of aging.” Earthbound experiments mimicking weightlessness have revealed similar effects. In the 1970s, Russian scientists immersed volunteers in bathtubs covered in a large sheet of waterproof fabric, enabling them to float without being wet. In some of these studies, which lasted up to 56 days, subjects developed serious heart problems and struggled to control their posture and leg movements.
Weightlessness hurts us because our bodies are fine-tuned to gravity as we experience it here on Earth. It tugs at us from birth to death, and still our intestines stay firmly coiled in their stack, blood flows upward, and our spine is capable of holding up our head. Unnatural contortions can throw things off: People have died from hanging upside down for too long. But as a general rule, the constant push of g-force on our body is a part of life that we rarely notice.
Or at least, that’s what scientists have always thought. But there is another possibility: that gravity itself is making some people sick. A new, peer-reviewed theory suggests that the body’s relationship with gravity can go haywire, causing a disorder that has long been a troubling mystery: irritable bowel syndrome.
This is a rogue idea that is far from widely accepted, though one that at least some experts say can’t be dismissed outright. IBS is a very common ailment, affecting up to an estimated 15 percent of people in the United States, and the symptoms can be brutal. People who have IBS experience abdominal pain and gas, feel bloated, and often have diarrhea, constipation, or both. But no exact cause of IBS has been pinned down. There’s evidence behind many competing theories, such as early-life stress, diet, and even gut infections, but none has emerged as the sole explanation. That is a problem for patients—it’s difficult to treat a condition when you don’t know what to target.
Brennan Spiegel, a gastroenterologist at Cedars-Sinai Medical Center, in Los Angeles, has a different idea: People with IBS are hypersensitive to gravity as a result of any number of factors—stress, weight gain, a change in the gut microbiome, bad sleep patterns, or another behavior or injury. The idea came to him after watching a relative confined to a nursing-home bed develop classic symptoms of IBS. “We’re upright organisms,” he told me. “We’re not really supposed to be lying flat for that long.” The hypothesis, published late last year in The American Journal of Gastroenterology, is just that, a hypothesis. Spiegel hasn’t conducted any experiments or patient surveys that point to a “mismatch” in our body’s reaction to gravity as the cause of IBS, though the mechanics are all based in firm science. But part of what makes the theory so alluring is that it might encompass all of the other conventional explanations for the disease. “It’s meant to be a new way of thinking about old ideas,” he said.
So exactly how would someone’s relationship with gravity get off-kilter? Consider serotonin, a chemical that carries messages from the brain to the body. Spiegel sees serotonin as an “anti-gravity substance” because of the role it plays in so many important bodily functions influenced by g-force, such as blood flow. Serotonin can cause blood vessels to narrow, slowing circulation. It can make certain muscles contract or relax. It’s also crucial to digestion, helping with bowel function, getting rid of irritating foods, and regulating how much we eat. Without serotonin, gravity would turn our intestines into a “flaccid sac,” Spiegel writes. Because 95 percent of the body’s serotonin is produced in the gut, if levels spike or plummet from factors such as stress, then the chemical’s possible handling of gravity would be thrown into chaos, affecting digestion. The result, he theorizes, is IBS.
Other parts of our body that respond to gravity can also be in on the problem. We are hardwired to react negatively to situations in which the pull of gravity might harm us; walk to the edge of a cliff and your body will tell you something. The amygdala in our brain is key to fear responses, and stress of various kinds can cause it to go into overdrive. Spiegel thinks that when stress taxes the amygdala, a person begins overreacting to potential threats, including gravity. The digestive issues that make up IBS are a manifestation of that overreaction. Sure enough, people with IBS have been shown to have a hyperactive amygdala.
That is hardly anything close to proof. The thought that this painful and prolonged condition could be a gravity disorder is a major stretch, relying on a renegade interpretation of basic biology. “People just think I’m crazy,” Spiegel said. Many of his fellow doctors are not sold on the idea. The gravity hypothesis is another in a long parade of unconvincing theories about IBS, Emeran Mayer, a gastroenterologist at UCLA, told me. He’s heard them all: “It doesn’t exist; it’s a hysterical trait of neurotic housewives; it’s abnormal electrical activity in the colon.” He added, “I don’t think there’s any other disease that has gone through these peaks of attention-grabbing new theories.”
Spiegel’s idea has clear holes. If a faulty reaction to gravity triggers IBS, says David C. Kunkel, a gastroenterologist at UC San Diego, then you would expect to see higher rates of IBS among populations living at sea level versus at high altitudes, where g-force is slightly weaker. But that doesn’t seem to be the case: About a quarter of Peruvians live high in the mountains and most Icelanders live at sea level, yet both countries have high rates of IBS. Likewise, IBS rates appear to decrease with age, “which would not be expected if the disease was caused by a constant gravitational force,” Kunkel told me.
Spiegel is aware that the gravity hypothesis has little support in the field and no proof. But the gravity hypothesis has some logic behind it. The fact that the weightlessness of space travel can drastically change the body lends credence to the idea that other shifts in our relationship to gravity could do the same, says Declan McCole, a biomedical scientist at UC Riverside.
And the gut may be particularly sensitive to gravity changes. McCole has found that weightlessness made epithelial cells—which line the gut and stop invaders from entering the body—easier to evade. So if our internal chemistry can change in a way that makes us hypersensitive to gravity, then, to McCole, it stands to reason that such a shift could hit the gut hard. He’s less sure of whether that hypersensitivity exists. If it does, then why haven’t we identified any chemicals that help handle gravity, as we have for fear or sex drive or hunger? That molecule may indeed turn out to be serotonin, but right now there’s no proof.
The gravity hypothesis really matters only if it is meaningful for people with IBS. And that’s not guaranteed. Tying the very real pain of IBS to such a fantastical idea may seem closer to mythology than medicine, leaving patients feeling dismissed or belittled. Or they may throw up their hands in despair and prepare for a lifetime of pain: If the immovable force of gravity is the enemy, then why bother fighting?
But if there is some truth to it, then the hypothesis could also provide a possible starting place for treatments. Some of Spiegel’s suggestions are already common, such as weight loss and medications that decrease serotonin, but he also advocates for some gravity-specific therapies. “I do talk about it with my patients,” Spiegel said. “I recommend certain yoga poses; I recommend tilt tables.” People who have IBS may balk at his more radical ideas, such as moving to a higher altitude or farther from the equator.
The gravity hypothesis may never be anything more than a hypothesis. We have a long way to go before truly knowing whether the human body can develop a hypersensitivity to gravity that can make us ill, or whether some of us are better equipped to handle gravity than others. But the weight of evidence is enough to make us think twice before ignoring the idea that our body’s relationship to gravity can go awry—including those of us not coping with IBS. If gravity might contribute to IBS, why not other ailments too? And then, why can’t it also be harnessed for good? Mekari and his colleagues recently found that lying at a six-degree downward angle sped up response times to cognition tests—pointing to a possible link between gravity and executive functioning. Antigravity treadmills, which help astronauts prepare for weightlessness, are being studied for the treatment of cerebral palsy, Parkinson’s disease, and sports injuries.
All of these unknowns about gravity can feel haunting. Life on Earth has changed a lot since its first forms appeared about 4 billion years ago, but through it all, gravity has seemingly remained constant—perhaps the single thing that connects every organism that has ever lived. What if there’s still much we have to learn about what it’s doing to us? After all, right now your body is coping with gravity, just as it has been for every other second of your life. Perhaps it would be weirder if gravity wasn’t doing anything to us over time. “Every fiber in our body is straining to manage this force,” Spiegel said. You don’t need to spend 56 days in a bathtub to figure that out.
