Garland’s experiment had succeeded, but what biological predispositions had his team exploited? One possible explanation is that the anatomy of the super-runners differed in a way that made running easier. It is true that as the breeding experiment progressed, super-runners began to show distinct physical traits, including more symmetrical thighbones and muscle cells that use energy more efficiently. But the early super-runners didn’t have those attributes. Something else was motivating them to run. It turns out that the most notable distinction between normal mice and super-runners isn’t in their muscles or their bones; it’s in their brains. Specifically, the reward system. Super-runners have physically larger midbrains, including the structures of the reward system. They also show differences in gene expression and neurotransmitters throughout the reward circuit. This leads super-runners to get hooked on running more quickly than other mice. It takes less exposure to exercise to flip the brain’s molecular switch of addiction, and they develop signs of cravings and dependence sooner. These mice are born to run not because they have the right bodies, but because they have the right brains. The original super-runners hadn’t inherited an increased capacity to do the work of exercise; they had inherited an increased capacity to enjoy it. It was only through successive breeding that the super-runners’ anatomy caught up with their brains, and they began to develop physical traits that supported their desire to run.
Reading about Garland’s mice made me wonder: Are modern humans the equivalent of super-runners? Natural selection is a kind of selective breeding experiment. Some scientists argue that as the human species evolved, we developed a shared genome that reflects the survival advantage of being able, and willing, to exert ourselves. Maybe everything we know about how movement affects the human brain—from the runner’s high to our ability to get hooked on exercise and the psychological benefits of being active—is proof that, whether or not we have a closet full of sneakers, humans are all, at some level, genetic super-runners.
And yet, there are also clear individual differences in how active people are. Could some humans exercise more because they have brains that are more easily hooked? There is evidence that the tendency to be active is at least partly heritable. By comparing identical twins raised together and apart, scientists have estimated that about 50 percent of the variability in physical activity is due to genetics. When you look at not just how much people exercise, but how much they enjoy it, heritability estimates drop to between 12 and 37 percent. That puts it on par with other psychological traits, like self-esteem (22 percent), empathy (27 percent), and fear of the dentist (30 percent), but much lower than physical traits like height and curliness of hair (both about 80 percent). Still, these numbers suggest a genetic influence. And, in fact, recent large-scale genome-wide association studies have identified dozens of genetic variants in humans that predict being more physically active. Some of these strands of DNA are on genes that influence metabolism, others are on genes linked to brain function, and still others remain a mystery—scientists aren’t yet sure what roles they play in shaping human health or behavior.
I am utterly useless at team sports and was always one of the slowest runners in my class. But I fell hard for aerobics when I was eight, and I’m rarely as happy as I am when teaching group exercise. I was curious: Was this joy written in my genes? To find out, I ordered a DNA kit from 23andMe, drooled into a test tube, and shipped my saliva off to a processing center. The standard report didn’t tell me anything about the strands of DNA that scientists have linked to exercise habits. However, when I clicked on “Raw Data,” I discovered that I had access to every part of my genome that 23andMe had genotyped. As I entered each genetic identifier linked to physical activity into the search box, the results were mixed. I had many, but not all, of the variants associated with being more active. I wasn’t sure how to interpret this. I had been hoping to discover a genetic profile that screamed, “Born to Move,” but I realized I didn’t actually know what this would look like. The genetic variants linked to being physically active are common, not rare. Movement was so key to our ancestors’ survival that these mutations became broadly conserved across human populations.
Before I go further, I should acknowledge the profound limits of this investigation. Behavioral genetics is an emerging field, not settled science. Methods are constantly being improved and early findings discarded. Future work may uncover hundreds of genetic variants that shape how humans respond to exercise. Moreover, genes are not the whole story. There are many psychological, social, and environmental influences on human physical activity, far more than contribute to a rodent’s wheel-running. I know this. And yet I felt compelled to interrogate my genes. I was sure that my love of exercise was, somehow, inborn and innate. That’s because I have another source of data that has long convinced me that I was born to move: my identical twin sister, Jane, who not only shares my genes, but who also is just as committed to exercise as I am.
In early photographs, you cannot tell us apart unless one of us reveals the secret—I’m the one who usually looks like she’s about to cry, while my sister looks like she is crafting some diabolical plan to take over the world. As adults, my sister and I are alike in many ways. We both touch our throats squeamishly at the mention of blood, and have a sweet tooth so strong we could thrive on a cane sugar IV drip. We both live in the San Francisco Bay Area despite growing up in suburban New Jersey, and we both married the men we started dating at age twenty-two. Our careers as researchers and writers have followed strikingly similar paths, and it’s not uncommon for someone to tell me how much they love my work only to find out they are thinking of my sister. We are also both regular exercisers—it’s as much a part of our lives as sleeping and eating. Jane is a distance runner who puts in between twenty-five and forty miles a week. She competes in an average of twelve races a year, including half and full marathons. When she gets near one of her favorite running trails, “I feel like a horse that’s been in the stalls all week, antsy and stroking the ground with its front hooves, just raring to get out and gallop. I can’t even ride in a cab past Central Park without wanting to leap out and do the six-mile loop.”
I’ll admit, I am mystified by her devotion to running, which has never appealed to me. She is equally confused by my love of group exercise. “I don’t know how you get better,” she told me, unsure why anyone would commit so much time to something you can’t objectively improve at. “You don’t have to get better,” I countered. “You just enjoy it.” But we are equally devoted to our preferred activities. While traveling, she scouts out running routes and races, and I look for the best place to take a group exercise class. On holidays and other special occasions, you’ll find her with her husband and daughters at some themed 5K or half marathon, while I’ll be teaching a community dance class. Moving is how we celebrate life. It is one of the most “twinny” things about us. Armed with this anecdotal evidence, I went back to the scientific literature, determined to figure out if there is anything else a human being can inherit that inclines them to get hooked on exercise. As it turns out, the answer is yes, and my sister and I have it: a genetic predisposition to experience the mental health benefits of physical activity.
Scientists have identified several strands of DNA, on multiple genes, that are linked to the antidepressant and anxiety-reducing effects of exercise. Individuals with any of these genetic variations seem to be more sensitive to the psychological benefits of regular exercise. For example, they are especially likely to show a reduced risk of depression and suicidal thinking if they exercise at least twenty minutes a day. As I searched for these genetic markers in my 23andMe data file, I discovered that my sister and I have all of them. My heart leapt at these results. No matter how fledgling the science might be, I couldn’t help but be excited by what I was seeing. There is a possibility that our genes are peppered with nucleotides that make physical activity especially important for our mental well-being.
When I shared
this discovery with my sister, she texted back, “Holy smokes! That’s amazing!” And it is. Not just that exercise is good for mental health, but that our bodies and brains had independently guided us to something we both need. This has been especially true in recent years for my sister, who struggled with depression and suicidal thinking after a traumatic brain injury in our early thirties. Running has been one of the most effective and reliable ways she takes care of her mental health. “I am sure running does for me what antidepressants do for people for whom they work,” my sister says. “If I’ve been sick or injured and haven’t been able to run, as soon as I do my first run, I feel like the clouds have parted and the sun is shining, and I’m a human being again.”
I feel the same way, except the darkness exercise protects me from is anxiety. I have always had a tendency toward worry. My parents generously called my early temperament sensitive and shy, but a more accurate description of me as a child would have been scared. I was the kid begging not to go on amusement park rides, who got sick to my stomach before math tests and birthday parties, and who nervously calculated how long our family’s stockpile of Girl Scout cookies would last in a nuclear apocalypse (it was the early 1980s). I can dredge up no origin story, no childhood experience, that would explain this personality trait. All I’ve come up with is: I was born that way. I have a brain that expects calamity and is easily overwhelmed. If I had to guess my biologically determined psychological set point, I’d locate it somewhere between “on high alert” and “consumed by dread.”
One theory of chronic worry is that people like me have an overactive fear circuit in our brains, set off not by anything specific, but operating more like constant background noise: There’s something wrong. There’s something wrong. The hyperactivity of this circuit produces a vague feeling of anxiety, leaving our imaginations to figure out what exactly we should be worried about. I can’t say for sure that’s what’s happening in my brain, but it matches what I observe going on in my mind. And without a doubt, exercise has been the most powerful antidote I’ve discovered. Scientific research bears my experience out. A single dose of physical activity immediately decreases anxiety and rumination, and this effect becomes even more pronounced with regular exercise. A 2017 meta-analysis of exercise interventions found that physical activity can be an effective treatment for anxiety disorders.
When I was eight years old and first discovering the mood-altering effects of aerobics, none of today’s commonly prescribed antianxiety or antidepressant drugs existed. It was rare for a child to be in therapy or to have access to any mental health guidance. In fact, I don’t think it ever occurred to anyone that my mind was something that needed to be managed. Somehow I stumbled onto something that helped me handle the predispositions I was born with. There is something heartening about this revelation. I feel fortunate that my brain figured this out despite the fact that I lacked any natural athletic ability and grew up in a home where neither parent exercised nor played sports. I may not be a super-runner in the traditional sense, but this research helped me realize that there is more than one way to be born to move. My daily exercise habit doesn’t so much sedate me as it emboldens me, which is exactly the right remedy for my anxiety. Being active makes me a better version of myself, and for this reason, I am grateful to be hooked.
The summer after I finished college, I joined a health club as a graduation gift to myself. My first week there, the club held a fair. Along with a prize wheel where you could win water bottles and guest passes, a chiropractor provided postural analyses and a psychic offered readings in a corner of the weight room. Curious as to what kind of advice I would get next to a bench press, I sat for a reading. The psychic took one look at me and told me to try the cardio kickboxing class. Maybe she could see the anxiety that coursed through my veins; maybe I was wearing it on my face. At the time I lived alone, in a studio apartment, and for the previous year, I had been carrying pepper spray in my purse and walking home from work with keys splayed between my fingers. I don’t think the woman who guided me to kickboxing was psychic, but she gave me great advice. Those classes changed me. By the end of the summer, I learned how to land a hook, an uppercut, and a jab. I was used to clenching my hands into worried fists, but fists feel different when you’re throwing punches. To this day, no other form of exercise makes me feel so powerful.
Here’s something I’ve come to believe: Movement isn’t addictive only when it feels pleasurable. I think the brain can sense resilience being wired in. And in fact courage is another predictable side effect of how physical activity changes the brain. At the very same time that a new exercise habit is enhancing the reward system, it also targets regions of the brain that regulate anxiety. In laboratory studies with rats, twenty-one days of running altered their brain stems and prefrontal cortexes—areas of the brain that control the body’s response to fear and stress—in ways that made the rats braver and better able to handle stressful situations. In humans, exercising three times a week for six weeks increases neural connections among areas of the brain that calm anxiety. Regular physical activity also modifies the default state of the nervous system so that it becomes more balanced and less prone to fight, flight, or fright. The latest research even suggests that lactate, the metabolic by-product of exercise that is commonly, but erroneously, blamed for muscle soreness, has positive effects on mental health. After lactate is released by muscles, it travels through the bloodstream to the brain, where it alters your neurochemistry in a way that can reduce anxiety and protect against depression. I like to think that when I fell in love with kickboxing that summer twenty years ago, or even back when I slid my first aerobics tape into the VCR, my brain understood that a positive transformation was under way. Something deep in my DNA recognized a good thing, and said, Yes, thank you, keep going.
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IN A LABORATORY EXPERIMENT at the University of Wisconsin, Madison, researchers set out to capture what happens in the brains of mice who love wheel-running but are denied the opportunity. Just before their nightly run, the researchers blocked each mouse’s access to the running wheel. The mice were ready to exercise but couldn’t—as if you had shown up at the gym excited to work out, only to find the doors locked and the lights off. In that moment of thwarted desire, each mouse was sacrificed. The researchers scooped out its brain, slicing and staining the gray matter for examination. Under a microscope, they observed chemical evidence that the mouse had been in a state of heightened longing when it died. Areas of the brain associated with desire, motivation, frustration, and even the physical initiation of running were coactivated. This pattern is similar to what you’d see in a smoker craving a cigarette but not allowed to smoke. But you could just as easily liken it to a homesick child missing his mom or a widow staring at the now-empty side of the bed.
The word addict comes from the Latin addictus, meaning both “devoted” and “bound to.” Some neuroscientists have mused that all devotions—between lovers or between caregiver and child—are a kind of addiction. They point to similarities in the brain between adoration and dependence. When heartbroken young adults see a photo of their beloved, their brains instantly shift into a state resembling an addict’s cravings for cocaine. When a mother gazes at her baby, her brain’s reward system becomes activated in a way that neuroscientists compare to a drug high. The scent of her infant’s skin can trigger a neural response close to hunger. (One of my favorite headlines describing this research comes from the Daily Mail Australia, which assured readers, “Your compulsion as a mum to eat your baby is totally normal.”)
However, the tendency to view love through the language of substance abuse can lead to unsatisfying analogies. One research paper described missing a loved one as “withdrawal” and the desire to reunite a “relapse.” These analyses make it seem like the primary function of the reward system is to addict, and anything that taps into this system is merely exploiting that capacity. But why assume that addictio
n is the phenomenon that all other devotions mimic? From an evolutionary standpoint, this makes no sense. Cocaine isn’t why the reward system exists; the drug just happens to stimulate it exceptionally well. Moreover, the reward system is, evolutionarily speaking, ancient. In all manner of creatures, dopamine motivates behaviors that are key to survival: eating, mating, and caregiving. The reward system’s main job is not to make us dependent on things that are harmful, but to push us toward the things we actually need.
In humans, that includes other people. And when you fall in love or become a caregiver, the reward system helps you forge strong attachments so that you will stay together. Through the repeated pleasure you experience from contact with loved ones, you come to like, crave, and need these relationships. You become willing to sacrifice to maintain them. When you are separated, you long to be reunited. This isn’t a destructive dependence; it’s a neurobiological mechanism for commitment. The same brain responses that scientists compare to addiction are also signs of strong bonds. The burst of dopamine in a mother’s brain when she sees her infant predicts her ability to bond with and soothe her child. In long-term happily married couples, a dopamine surge upon seeing your spouse is linked to how much you view their well-being as integral to your own. And when a widow or widower in the throes of grief sees a photo of their spouse, the activity of the reward system correlates with their self-reported yearning for their loved one.
The Joy of Movement Page 6
