Sudden overstimulation of the vagus nerve can be a bad thing—it can basically shut you down, slowing your heart so much that it stops, causing a psychogenic death or a drowning-like event. Yet milder increases in vagal activity are quite good for you, keeping your body and mind relaxed and in top condition. Some research actually suggests that we could even treat chronic pain and depression by applying electrical stimulation to the vagus nerve. A common measure used by scientists that reflects the workings of the vagus nerve is heart rate variability. If you haven’t come across it yet reading fitness and health magazines, you soon may. Heart rate variability, or HRV for short, is emerging as a particularly valuable indicator of the health of the vagus nerve, the proper functioning of our fight-and-flight response, the mind-body connection, and even potential longevity.
It may seem like a good thing to have a heart that beats as evenly as the most coordinated marching band, but it is not. In fact, the more varied your heart rate the better. In a normal heart, the time that lapses between two successive beats is always slightly different—even breathing in and out changes the rate. These tiny differences between heartbeats are what is measured as heart rate variability. If the variability is high, it means that your heart is capable of quick adjustments to the changing environment. It can mobilize resources fast and relax fast. In the evolutionary past, if you could control your fear, suppress unwanted thoughts, and mobilize your inner resources, you were certainly better off when faced with a large mouth full of sharp teeth that was trying to eat you. It meant that your vagus nerve, the link between your brain and your heart, was working well.
But when we are chronically stressed, our fight-and-flight response can get stuck in high gear and prevent the vagus from initiating the relaxation response. The system becomes dysregulated, with the HRV permanently low. Lack of resilience and proper emotional regulation, worrying a lot, anxiety, loneliness—these have all been linked to low HRV. Poor physical health may follow. More and more research is now showing that low HRV can lead to diabetes, cardiovascular disease, and even an earlier death.
The good news is that you can monitor your HRV at home and evaluate whether your health interventions are working. The easiest way is to buy a chest-strap heart monitor from a company like Wahoo or Polar and download a free app that will analyze the data. When I first turned on my newly acquired HRV monitor, I hoped for a readout indicating that I had a body of a twentysomething. The reality was less rosy. After precisely two minutes of sitting still, I discovered that I had a body of someone…exactly my age. Oh, well. Luckily for me studies show that lifestyle changes, such as yoga, can improve HRV quickly—the difference can be seen in as little as eight weeks (more on that in chapter 10).
Besides sending messages from your brain to your heart and back, the vagus nerve, which reaches deep into your body, also functions as an information highway between your central computer and your gut—or, to be more precise, between your central computer and the trillions of bacteria residing in your intestines. To learn more about how such tiny creatures can connect minds and bodies, I went mouse-catching in Oxfordshire, UK.
Of Mice and Guts
At 7:30 a.m. one sunny June morning, I crossed through a timber gate into Wytham Great Wood, a forested area just seven miles away from the University of Oxford. After a half-hour walk alone through a dark forest, during which I grew increasingly anxious, I finally reached a Swiss-style chalet, which, had I not known better, I would have taken for abandoned. The place was overgrown with weeds and surrounded by piles of discarded animal cages. Gingerly I stepped inside. There, in a tiny room that smelled of disinfectant, Aura Raulo was sweeping mouse poop out of a metal container. Noticing me, the young researcher smiled and took off her latex gloves to shake my hand.
I had contacted Raulo a few weeks before to talk about her research on how sociality influences the gut microbiome, and vice versa. That’s when she suggested I join her and her team in Wytham Great Wood, where they catch wild British wood mice, run behavioural tests on them, and analyze their poop.
Very early in the morning of my arrival, rectangular, metal traps were set in the forest surrounding the chalet. Each carefully numbered trap contained something few mice can resist: peanuts. “Some of them like peanuts so much, they can be caught in the traps again and again,” Raulo told me. As she showed me around, stepping carefully over thorny shrubs and nettles, she pointed to little yellow flags marking barely visible holes in the undergrowth: mouse burrows. Beside the burrows, wireless loggers equipped with motion sensors were ready to turn on whenever something warm moved past them. The equipment would also read the tiny tags that each mouse carried. “This way we can see who goes in and out,” Raulo told me, then added with a laugh, “I think these are the most intensely monitored mice in the UK.”
Back at the chalet’s cramped lab, Raulo opened the trap to release mouse number 931 into her gloved hand. She measured and weighed him, took a fur sample to check cortisol levels, and collected the poop that he had conveniently left on the cotton wool bedding inside the container—it would be analyzed for microbial content later on. After that, mouse number 931 was sent off to a “personality assay chamber” in the next room.
The mouse was placed in the middle of an empty cage and left alone, his movements tracked by a hidden camera. He spent the next five minutes wandering shyly around, sniffing at corners. You can tell a lot about a mouse’s personality by the way it behaves when placed in a new environment, Raulo told me. The brave, outgoing ones will calmly check everything out. The anxious ones will end up running in circles. And some will be so terrified they will just freeze in one spot. If you compare this behavioural data with information gathered from the poop on the gut microbiome, and if you do this over and over, you can get some idea of the links between bacteria that reside in the intestines and the rodent personality.
What Raulo does as well—and what is her prime focus—is to investigate how social networks influence the gut microbiome. She analyzes the data from the logging devices outside the burrows, checking who hangs out with whom, and then runs it by the poop microbiome stats. She has already discovered that not only do mice share bacteria with their friends, but also that the more diverse friendships they have, the more diverse their microbiomes—which is generally an indicator of a healthy gut.
She believes a similar pattern may apply to humans, too. “Imagine you have just two friends. One of them is a famous disco queen and the other is a famous metal singer. Because they represent very distinct social networks they have very different microbiomes. If you are the link between the two of them you get the richness of all their bacteria combined. A similar thing would happen if you were from a multicultural family, with relatives in different countries,” Raulo said. Some early research on humans does confirm that we may transfer friendly bacteria between ourselves the way Raulo’s mice do. One such study showed, for example, that roller derby players exchange microbes with opposing teams during tournaments. Family members, meanwhile, share bacteria with each other and even their dogs.
Other experiments suggest that changes in the gut microbiome can influence personality. If you breed mice devoid of any beneficial gut bacteria, the rodents will be loners, preferring to sit away from all the others. And if you recolonize their intestines with microbes, their personality will change once again—back to social. So how do you breed germ-free mice? You make sure they are born by Caesarean section so that they don’t get any beneficial microbes from their mothers, have them live in sterile quarters, and feed them purified water and food. If that reminds you of people you may know, that’s bad news. Germ-free rodents have been shown to have overreactive HPA axes and to be less resilient to stress.
Again, human studies point in a similar direction. In the Philippines, for instance, young people in their twenties who had a lot of contact with animal feces in infancy have been proven to be more resistant to stress as adults. In case “cont
act with animal feces” does not sound particularly appealing to you as a mood-boosting intervention, the good news is that probiotics work, too. When a group of American volunteers drank probiotic-rich fermented milk for about a month, their brain activity patterns changed—suggesting that their processing of emotions had changed—in a way that could make them, for example, less prone to overreact to fear- or anxiety-inducing situations. A similar experiment showed that consumption of the bacteria Bifidobacterium longum, which you can find in kefir or miso soup, reduces cortisol release and dampens stress.
And in case miso soup doesn’t do the trick, there are always fecal transfers (back to poop, sorry). In one particularly revealing study scientists have taken stool from depressed patients and transplanted it into some unlucky rats. The rodents became instantly depressed themselves, losing interest in things they used to enjoy. Fecal transfers have even more striking results if done within the same species. Take poop from an anxious mouse and insert it into a second mouse, and your mouse number two will become anxious as well. Same thing happens with poop from inquisitive rodents—it can turn others inquisitive and curious, too. Which made me wonder: does having gloomy friends turn you gloomy, too, since you trade microbes through hugs and skin contact? Should I avoid touching people with particularly dark or unpleasant temperaments? Sounds a bit extreme, but maybe better safe than sorry? Dear scientists, please research.
Just the way the bacteria in your intestines can influence your mood, on the flip side, worry and anxiety can damage your gut microbiome, too. When experimental animals are stressed, the composition of germs in their poop changes in response—for the worse. Which means that every time I worry that my daughter’s runny nose will develop into a full-blown sickness, or when I stress over a deadline, or panic that climate change will soon kill us all, I’m hurting the beneficial microbes in my intestines, while giving a boost to the harmful ones. The reason for this is that stress hormones tamper with the physiology of the gut, modifying the habitat for the bacteria in a way that’s unfriendly to the good guys.
So how exactly can brain talk with the gut? One such communication channel is the vagus nerve; another is neurotransmitters such as serotonin and dopamine. Besides being produced by the brain, neurotransmitters can also be synthesized by the intestinal microbiota. If you lived in a completely sterile environment and had no gut bacteria whatsoever, like the germ-free lab rats, the serotonin system in your brain wouldn’t develop properly, playing havoc with your emotions. Gut bacteria can also work on your brain through the metabolites they produce, such as short-chain fatty acids. Butyrate, a short-chain fatty acid that’s a cousin chemical to the obnoxious butyric acid (responsible for the sour-bitter stench of rancid butter), may help the growth of new neurons in the brain and survival of the old ones. What’s more, intestinal microbes can also tweak your immune system and regulate your HPA axis.
Besides chatting with your central nervous system and having an influence on our moods and behaviours, friendly gut bacteria are essential for our physical health, too. Messed-up intestinal microflora have been implicated in many diseases, including diabetes, multiple sclerosis, rheumatoid arthritis, and allergies. On the flip side, transplants of poop from young donors can prolong life—at least in fish.
When researchers from the Max Planck Institute for Biology of Ageing in Germany made middle-aged turquoise killifish nibble on the feces of their younger companions, the animals lived 37 percent longer than those that were not provided with such unusual food. From a human perspective, that would be like extending the current American average lifespan of 78 years to 107. Imagine that. Fecal transplants are already used to treat diabetes and obesity in people—although for now, no one seems to be testing this therapy for human longevity.
Of course it’s far too early to recommend poop supplements from youthful donors to people who want to live long—assuming anyone would be up for trying these (I know I wouldn’t). And maybe we won’t have to. Maybe just thinking you are getting a miracle treatment can suffice. When it comes to mind-body connections, the placebo effect can truly work wonders.
Water Injections and Sham Surgery
Mr. Wright had cancer. An untreatable one that had spread all through his body. He couldn’t even breathe without extra oxygen. He really wanted to live, though, and asked his physician to administer him a new experimental drug called Krebiozen. But the doctor knew Krebiozen was worthless, so without Mr. Wright’s knowledge, gave him water injections instead, curious to see what would happen. To the physician’s surprise, the sick man improved dramatically. Within days, Mr. Wright was basically symptom-free—he was so well, in fact, that once he was discharged from the hospital, he flew himself home in his own airplane. His good health lasted until one day, two months later, the American Medical Association published a statement saying that Krebiozen did nothing for treating cancer. Once Mr. Wright got the depressing news, his health plummeted. He got readmitted to a hospital, and two days later, he was dead.
The use of placebos in medicine is probably as old as humanity itself. In ancient Egypt, the sick were treated with concoctions made from crocodile dung and swine teeth. In medieval Europe, you might have received such cures as ground-up “unicorn’s horn,” which was in fact just ivory, or something that was supposedly a “crystallized tear from the eye of a deer bitten by a snake” (usually a gallstone). These days, even though most of us would not fall for unicorn pills, many people happily spring for homeopathic drugs, which are placebos, too.
Which does not mean these things don’t work—research shows that placebos are actually quite effective. They help for conditions as varied as Parkinson’s disease, depression, chronic pain, and nausea. Even a sham arthroscopic knee surgery works as well as a real one. What’s more, placebos work even if you know you are getting nothing but a sugar pill. In one study of cancer survivors suffering from fatigue, taking a placebo that was clearly marked as being just that helped ease the symptoms as much as a standard treatment.
One reason placebos work is because of expectations (even if you know it’s a placebo). In a way, when faced with a sugar pill that promises to help us feel better we act like the famed Pavlov’s dogs, which would salivate at a mere sound of a bell. When we see a doctor in a white coat with a well-equipped medical office, and we smell the scent of hospital-grade disinfectant, our central nervous system recalls when similar things happened to us in the past and helped us get healthy, conditioning us to improve this time, too. That’s why the more medicalized the placebo, the better it works. A pill given by a doctor works better than a pill given by a friend. A drug administered in a hospital works better than one given at home. A fake injection works better than a pill—simply because it’s more invasive, which makes it appear more serious and real.
Yet, as renowned placebo researcher Fabrizio Benedetti told me, there is no single explanation for why placebos work because there isn’t a single placebo effect. “These are complex phenomena that involve many mechanisms,” he said. They may act through neurotransmitters such as endocannabinoids and dopamine or through the HPA axis. When scientists put people who have received placebos into functional magnetic resonance imaging scanners, they can see the effects of the sham treatments in the activation patterns of the brain, for instance in the amygdala.
One mechanism particularly stands out when it comes to placebos—I’m talking about the opioid system. Opioids are substances akin to heroin and morphine that are churned out by your own body so that you can deal with pain. Opioids are the reason you may not perceive pain when under huge stress, which makes sense from an evolutionary perspective: it was better for our ancestors not to fixate on unpleasant yet distracting sensations coming from, say, a mauled limb, while fighting wild animals. Today, when you take a placebo that’s supposed to ease your aches, you start to stimulate production of these natural painkillers. Your mind and your body act together to help you improve.
* * *
Our feelings, thoughts, and social behaviours can influence our health and longevity. It’s no magic. The connection between the mind and the body is a very ancient one and is based on the mechanisms that helped our ancestors fight or flee for their lives when faced with a threat. Your HPA axis, the sympathomedullary pathway, the vagus nerve, the immune system, the trillions of microbes in the gut—they all respond to the messages coming from your brain to keep you alive. Your emotions, from fear to anger to happiness, help inform these systems about the state of your body and the environment, preparing them for action. If the stress response is initiated, a cascade of hormones washes through your body, changing the way your muscles, heart, lungs, and digestive system work. Once the threat is gone, the relaxation response kicks in. The vagus nerve does its job. You feel calm, composed.
These days, however, that ancient stress response often malfunctions. We may be lonely, switching on the inflammatory response of someone stranded away from the tribe. We live fast, under constant mental pressure, bombarded by challenges. The HPA axis becomes dysregulated, and the vagus nerve has no chance of starting the relaxation response. The stress systems stay turned on, damaging different organs in the process, so we end up with permanently raised blood pressure, clogged arteries, or insulin resistance. With time, that can lead to diabetes, heart disease, or a shortened lifespan.
You can monitor your heart rate variability for the condition of your vagus nerve. You can check your blood for signs of chronic inflammation. You can even get your gut microbiota tested for its health. The practical tips about such services can be found on this book’s website. But you probably know in your gut (the proverbial one) whether your stress and relaxation response might be out of whack. Luckily science has revealed many different approaches and techniques that we can use to improve the mind-body connection, from the functioning of the HPA axis to the quality of the gut microbiota. Drinking kefir and eating miso soup certainly won’t hurt, but changing your mindset and the way you live in society is even more important: improving close relationships (which, according to studies, may lower mortality by about 45 percent), working on empathy and kindness (44 percent), adding mindfulness and volunteering (22 percent).
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