Introduction — Making the Connection
The real reason you feel so good when you get your blood pumping has nothing to do with your waistline. Exercise is crucial to the way we think and feel — it cues the building blocks of learning, affects mood, anxiety, and attention, guards against stress, and reverses some of the effects of aging in the brain. Most people have no idea that toxic levels of stress erode the connections between billions of nerve cells, or that chronic depression shrinks certain brain areas. Conversely, exercise unleashes a cascade of neurochemicals and growth factors that can reverse this process, physically bolstering the brain’s infrastructure.
In October 2000, Duke University researchers showed that exercise is better than sertraline — Zoloft — at treating depression. If exercise came in pill form, it would be hailed as the blockbuster drug of the century. It is simply one of the best treatments we have for most psychiatric problems.
Chapter 1 — Welcome to the Revolution
Students in the Zero Hour PE program at Naperville Central High School work out intensely before their first academic class. At the semester’s end, they show a seventeen percent improvement in reading and comprehension, compared to ten-point-seven percent among students who slept in. The administration incorporates the program into the curriculum. Literacy students are then split between a second-period class — still feeling the effects of exercise — and an eighth-period class. As expected, the second-period group performs best. Phil Zientarski, the teacher behind it: “My job is to make them understand exercise, show them the benefits. That’s a radical transformation.”
Chapter 2 — Learning
Exercise influences learning directly, at the cellular level, improving the brain’s potential to log in and process new information. Darwin taught us that learning is the survival mechanism we use to adapt to changing environments — inside the brain, that means forging new connections between cells. The brain is not hardwired, as scientists once believed; it is plastic, constantly being rewired by thoughts, behavior, and environment.
Understanding the mechanics starts with the synapse. An electrical signal shoots down the axon until it reaches the synapse, where a neurotransmitter carries the message across in chemical form. About eighty percent of signaling is carried by two neurotransmitters that balance each other: glutamate stirs up activity, and GABA clamps it down. Neurons that fire together wire together — this makes glutamate a crucial ingredient in learning. Going for a run is like taking a little bit of Prozac and a little bit of Ritalin, because exercise elevates these same neurotransmitters and balances the entire neurochemical system.
Beyond neurotransmitters, there is another class of master molecules: brain-derived neurotrophic factor, or BDNF. Neurotransmitters carry out the signaling; BDNF builds and maintains the cell circuitry — the infrastructure itself. When researchers sprinkled BDNF onto neurons in a petri dish, the cells automatically sprouted new branches, producing the structural growth required for learning. That is why BDNF is best thought of as Miracle-Gro for the brain. It is a crucial biological link between thought, emotions, and movement.
Carl Cotman, the neuroscientist who discovered that exercise spikes BDNF, had found that exercise, education, and self-efficacy were the factors with the least cognitive decline in a four-year aging study. A 2007 German study confirmed the practical stakes: people learn vocabulary words twenty percent faster following exercise, and the rate of learning correlated directly with BDNF levels. Exercise also spawns new neurons in the hippocampus from stem cells — environmental enrichment and social contact help those cells survive by giving them a reason to fire. The body was designed to be pushed, and in pushing our bodies we push our brains too.
Exercise improves learning on three levels: it optimizes your mind-set for alertness and motivation; it prepares nerve cells to bind to one another, which is the cellular basis for logging in new information; and it spurs the development of new nerve cells from stem cells in the hippocampus. You cannot learn difficult material during high-intensity exercise because blood is shunted away from the prefrontal cortex — but blood flow shifts back almost immediately after you finish. The post-exercise window is the perfect time for sharp thinking and complex analysis. For sustained improvement in executive function, jogging thirty minutes two or three times a week for twelve weeks was enough in one scientifically sound study.
Mix aerobic exercise with complex motor skills for the biggest brain benefit. Rats trained on balance beams and rope ladders showed a thirty-five percent increase of BDNF in the cerebellum, whereas running rats had none there. The ideal prescription is either a sport that taxes both the cardiovascular system and the brain — tennis is a good example — or a ten-minute aerobic warm-up before something skill-based, such as rock climbing or tango, which also demands attention, judgment, and social reaction.
Chapter 3 — Stress
Neurons get broken down and built up just like muscles — stressing them makes them more resilient. The fight-or-flight response calls into action powerful hormones and scores of neurochemicals. The amygdala, the brain’s panic button, assigns intensity to incoming information. Two neurotransmitters put the brain on alert: norepinephrine arouses attention, then dopamine sharpens it. An imbalance is why some people with ADHD become stress junkies — they have to get stressed to focus, which explains the common habit of waiting until a deadline looms to start working.
Once the alarm fires, cortisol signals the liver to release more glucose and blocks insulin receptors at nonessential tissues. If this continues unabated, as in chronic stress, it amasses a surplus fuel supply around the abdomen as belly fat. One of the central problems with chronic stress is that if the HPA axis is guzzling all the fuel to keep the system on alert, the thinking parts of the brain are robbed of energy. Neurons in acute stress cement the survival memory and shut out less important stimuli — which likely explains why people with chronically high cortisol from depression have so much difficulty learning new material. Excess cortisol can also block access to existing memories, which explains how people forget where the fire exit is when there is actually a fire. The neurological point of fire drills is to burn in the memory.
Cellular stress takes two distinct forms. Metabolic stress happens when cells cannot produce adequate ATP, either because glucose cannot get in or because there is not enough to go around. Excitotoxic stress occurs when there is so much glutamate activity that there is not enough ATP to keep up with demand — if this continues without recovery, dendrites shrink and the cell eventually dies, which is the mechanism underlying Alzheimer’s, Parkinson’s, and aging itself. Mark Mattson at the National Institute on Aging found that mice given a third of their normal calories live up to forty percent longer. The advantage of using exercise to inoculate the brain is that it ramps up growth factors more than other stimuli do — building up cellular defenses against degeneration. The best way to build these defenses is by bringing mild stress on yourself: using the brain to learn, restricting calories, exercising, and eating your vegetables.
Exercise kick-starts the cellular recovery process and raises the fight-or-flight threshold. It increases intercellular energy efficiency, allowing neurons to meet fuel demands without increasing toxic oxidative stress. You do get waste buildup, but you also get the enzymes that chew it up, plus a janitorial service that disposes of broken bits of DNA — both of which help prevent cancer and neurodegeneration. The stress of exercise is predictable and controllable because you are initiating the action — these two variables are key. With exercise you gain a sense of mastery and self-confidence. The mechanisms by which exercise changes how we think and feel are far more effective than donuts, medicines, and wine. When you say you feel less stressed after a swim, you are. A 2004 Leeds Metropolitan University study of two hundred and ten workers found that sixty-five percent fared better on days they exercised — interacting with colleagues, managing time, and meeting deadlines — while feeling less fatigued despite the energy spent. Active people have fifty percent less chance of developing colon cancer, and active men over sixty-five have a seventy percent lower chance of developing the advanced, typically fatal form of prostate cancer. The more stress you have, the more your body needs to move.
Chapter 4 — Anxiety
Anxiety is fear — specifically, the memory of danger on infinite replay, with the all-clear signal broken. The amygdala overwhelms the hippocampus’s attempts to put fear in context. Brain scans of people with generalized anxiety disorder show the area of the prefrontal cortex responsible for sending cease-and-desist signals to the amygdala as being smaller than it should be. As fear memories form connections with each other, the anxiety snowballs — the amygdala tags too many situations as survival threats and burns them into memory. Your world shrinks. A huge part of the problem with social anxiety is that the more you withdraw, the less practice you get interacting, and the scarier the prospect becomes.
The majority of studies show that aerobic exercise significantly alleviates symptoms of any anxiety disorder. Moving the body triggers the release of GABA — the brain’s major inhibitory neurotransmitter and the primary target for most antianxiety medicines. Over time, exercise teaches the brain that the physical symptoms of anxiety — elevated heart rate, quickened breathing — are also inherent to aerobic exercise, and beginning to associate those symptoms with something positive you can control causes the fear memory to fade. Scientists call this reattribution; psychologists call it building self-mastery — a powerful prophylactic against anxiety sensitivity.
Exercise works on anxiety through multiple pathways. It provides distraction — the antianxiety effects of exercise last longer than other directed distractions. It reduces muscle tension: in 1982, researcher Herbert de Vries showed that people with anxiety have overactive electrical patterns in their muscle spindles and that exercise reduced that tension, which he called the tranquilizing effects of exercise. It raises serotonin and norepinephrine both in the moment and over the long term — serotonin works at nearly every junction of the anxiety circuitry, improving the prefrontal cortex’s ability to inhibit fear and calming the amygdala itself — while also increasing GABA and BDNF. It teaches a different outcome by associating elevated heart rate and quickened breathing with something positive you initiated and can control — your mind is expecting a panic attack but ends up with a positive association instead: a biological bait and switch. It sets you free — the opposite of anxiety’s paralysis is taking action and moving through the environment. For panic disorder, combining medication with exercise is powerful: medicine provides immediate safety, exercise gets at the fundamentals, and the relearning process is needed for long-term relief. Number-one recommendation for anxious people: exercise with somebody.
Chapter 5 — Depression
Depression is best thought of as an erosion of connections — in your life as well as between your brain cells. When elevated endorphin levels were detected in the blood of runners, it gave us the expression runner’s high. But the story of how exercise defeats depression goes far deeper. In the landmark 1999 SMILE study — Standard Medical Intervention and Long-term Exercise — James Blumenthal at Duke randomly divided one hundred and fifty-six patients into three groups: Zoloft, exercise, or a combination. The exercise group walked or jogged at seventy to eighty-five percent of aerobic capacity for thirty minutes, three times a week. All three groups showed a significant drop in depression; about half of each group was completely in remission. Blumenthal concluded that exercise was as effective as medication. Subsequent research by Madhukar Trivedi showed that patients not responding to antidepressants lowered their depression scores by 10.4 points on a seventeen-point scale after twelve weeks of exercise — a huge drop.
To understand why, look at what depression does to the brain. In 1996, Yvette Sheline of Washington University found that the hippocampus of depressed patients was up to fifteen percent smaller than controls, and the degree of shrinkage was directly related to the length of depression. High cortisol kills neurons in the hippocampus — flood a neuron with cortisol and its connections retract, dendrites wither, and the communication breakdown could partly explain why a depressed brain gets locked into negative thoughts: it is recycling a negative memory because it cannot branch out to form alternatives.
Exercise boosts BDNF at least as much as antidepressants, and sometimes more. One study showed that combining exercise with antidepressants spiked BDNF by two hundred and fifty percent. Trivedi quantified exercise as a dose: high-intensity groups burning fourteen hundred calories per week — about eight calories per pound — cut their depression scores in half. Low-intensity groups burning five hundred and sixty calories lowered scores by only a third, roughly equal to a placebo stretching control. Some exercise is good, more is better. The ideal is thirty minutes of moderate aerobic activity on most days — roughly three hours per week for a hundred-and-fifty-pound person.
Part of the initial recovery from depression is simply getting moving. The best kind of behavioral therapy is to go outside and take a walk. If your prefrontal cortex has been offline for a while, you need to reprogram it, and exercise is the perfect tool — you begin to look at the world differently, and seeing yourself moving is itself proof that you can help yourself. Because human beings are social animals, the ideal is exercise that encourages making connections outside. Asking someone to join you gives newly hatched neurons a powerful reason for being.
Exercise is even more important for prevention than treatment. The key is to get moving before a mood drops and not stop. Set up a daily schedule. Get a friend or family member to come by at the same time each day to escort you outside. At its core, depression is defined by an absence of moving toward anything, and exercise is the way to divert those negative signals and trick the brain into coming out of hibernation.
Chapter 6 — Attention Deficit
Motivation is biological. Functional MRI scans reveal distinct differences in activity at the reward center — a cluster of dopamine neurons called the nucleus accumbens — in people with ADHD. A more helpful way to think of ADHD is as an attention variability disorder: the deficit is one of consistency, not an absence of ability to focus. The amount of data in the world is doubling every few years, but our attention system was built for an environment ten thousand years ago.
The attention circuits are jointly regulated by norepinephrine and dopamine — the chemicals targeted by ADHD medications. People with ADHD favor immediate gratification and are prisoners of the present; they cannot maintain focus on a long-term goal, and a failure of working memory means they literally forget to worry about passing time — hence procrastination. The cerebellum, which takes up ten percent of the brain’s volume but contains half of all neurons, keeps rhythm not just for motor movements but for broader brain systems, regulating the flow of information. Dopamine works like transmission fluid — if there is not enough of it, as in ADHD, attention cannot easily be shifted. This parallel matters for Parkinson’s disease as well: scientists induced Parkinson’s in rats by killing dopamine cells in their basal ganglia, then forced half of them to run twice a day in the ten days following onset — the runners’ dopamine levels stayed within normal ranges and their motor skills did not deteriorate.
Countless parents report the same thing: Johnny is so much better when he is doing tae kwon do. Any highly structured or complex physical activity — martial arts, ballet, rock climbing, skateboarding — combines challenging the brain and the body in ways that aerobic exercise alone cannot match. One Hofstra University study found that ADHD boys ages eight to eleven who practiced martial arts twice a week improved behavior and performance compared to those on a typical aerobic program — though both kinds of exercise dramatically outperformed nonactive controls.
A controversial treatment for dyslexia — which occurs in about thirty percent of ADHD patients — is DDAT: dyslexia, dyspraxia, and attention treatment, based on the theory that disruption in the brain’s ability to coordinate movement may cause eye-tracking problems and difficulty learning to read. In 2003, British researchers tested DDAT on thirty-five children with dyslexia over six months: students showed significant improvement in reading and writing fluency, eye movement, cognitive skills, dexterity, and balance. Notably, more than eighty percent of a large prisoner population had had serious learning problems as children.
With regular exercise you can raise baseline levels of dopamine and norepinephrine by spurring the growth of new receptors. The best strategy for managing ADHD is to exercise in the morning, then take medication about an hour later — when the immediate focusing effects of exercise begin to wear off. Many patients find that daily exercise allows them to take a lower dose of stimulant.
Chapter 7 — Addiction
Scientists now characterize behaviors such as gambling, compulsive shopping, and overeating in the same biological terms as substance abuse. The common denominator is an out-of-control reward system. Behavioral neuroscientist Terry Robinson of the University of Michigan draws a crucial distinction between liking something — the actual experience of pleasure — and wanting it — the motivational state. Dopamine is involved in wanting, not in liking.
Typically, when you learn something, dopamine levels tail off over time. With addiction, especially drug addiction, dopamine floods the system with each use, reinforcing the memory and pushing other stimuli further into the background. Drug use makes dendrites in the nucleus accumbens bloom, increasing synaptic connections — changes that can remain months or years after stopping, which is why relapse is so easy. If you continually subject your brain to a dopamine overload, the number of receptors dwindles — so the more drugs you take, the more you need to feel the same rush. The same is true of overeating.
Exercise was a conduit for shifting focus to a more productive life for many patients: it reprograms the basal ganglia to wire in an alternative reflexive behavior, offsetting the hopelessness many drug users feel. Exercise fights the urge to smoke by increasing dopamine smoothly while lowering anxiety and tension, fending off cravings for fifty minutes and doubling or tripling the interval to the next cigarette. Self-regulation is a resource that can be depleted but recharged like a muscle — the more you use this faculty, the stronger it gets. Thirty minutes of vigorous aerobic exercise five days a week is the bare minimum to root out an addiction.
Chapter 8 — Hormonal Changes
Exercise is not optional for hormonal health — it is central to it. German researchers brought stationary bicycles into the labor suite and found that eighty-four percent of women reported contractions were less painful during exercise than at rest, with pain ratings inversely proportional to endorphin levels. For ten to fifteen percent of new mothers, postpartum depression strikes after delivery and can last a year or more — exercise may be even more effective for them than for the general population because it normalizes neurotransmitter levels.
The American College of Obstetricians and Gynecologists now endorses thirty minutes of moderate-intensity aerobic activity every day during pregnancy for healthy women. Picking up the routine as soon as possible after birth reduces fatigue and melts away anxiety and depression. Women over sixty-five who reported higher levels of physical activity were fifty percent less likely to develop any form of dementia. Whatever your age, exercise keeps the mind firm and taut — well equipped to handle the hormonal fluctuations every woman experiences throughout her life.
Chapter 9 — Aging
The average seventy-five-year-old suffers from three chronic medical conditions and takes five prescription medicines. Many of the same factors that reduce risk for cardiovascular disease and diabetes also reduce risk for age-related neurodegenerative disorders — running to lower blood pressure also keeps capillaries in the brain from collapsing; lifting weights to prevent osteoporosis releases growth factors that make dendrites bloom. The mental and physical diseases of old age are tied together through the cardiovascular and metabolic systems, which is why obese people are twice as likely to suffer from dementia and why those with heart disease are at far greater risk of Alzheimer’s.
Exercise keeps you going as you age through nine specific pathways. It strengthens the cardiovascular system, reducing resting blood pressure and strain on the vessels of the brain. It regulates fuel by increasing IGF-1, which regulates insulin and improves synaptic plasticity, and bolsters BDNF. It reduces obesity — body fat damages cardiovascular and metabolic systems, and exercise counteracts it by burning calories and reducing appetite. It elevates the stress threshold, making proteins that fix damage and delay neurodegeneration. It lifts mood, shoring up the hippocampus against atrophy from depression and anxiety. It boosts the immune system, combating chronic inflammation — lack of activity is the most consistent risk factor for cancer. It fortifies bones — women reach peak bone mass around thirty and lose about one percent per year until menopause, when the pace doubles; weight-bearing exercise counteracts this. It boosts motivation by counteracting the natural decline of dopamine, guarding against Parkinson’s. And it fosters neuroplasticity — aerobic exercise creates more synapses, expands the web of connections, and spurs newly born stem cells to become functional neurons in the hippocampus.
The one proven way to live longer in lab rodents is to consume fewer calories: mice and rats given thirty percent fewer calories live up to forty percent longer than animals allowed to eat as much as they want. For the rest of us, the practical program looks like this. Walk outside with a friend for your aerobic base — consistency is probably more important than intensity. Add two days per week at seventy to seventy-five percent of maximum heart rate for twenty to thirty minutes. Hit the weights twice a week, doing three sets at weights that allow ten to fifteen repetitions — a Tufts University study of women fifty to seventy years old showed that one year of strength training added one percent of bone density in their hips and spine, while the sedentary group lost two-and-a-half percent. Focus on balance and flexibility twice a week through yoga, Pilates, tai chi, or dance. And volunteer — social contact challenges the brain. Statistics show a tight inverse relationship between sociability and mortality. Novel experiences demand more from your brain and build its ability to compensate. You get more Miracle-Gro, more connections, more neurons, and more possibilities.
Chapter 10 — The Regimen
One of the key differences between moderate and high-intensity exercise is that near your maximum — especially in the anaerobic range — the pituitary gland unleashes human growth hormone, or HGH. HGH levels dwindle to a tenth of childhood levels by middle age, and a sedentary lifestyle exacerbates this decline. HGH burns belly fat, layers on muscle fiber, and pumps up brain volume — researchers believe it can reverse the loss of brain volume that naturally occurs with age. A single thirty-second sprint on a stationary bike generated a sixfold increase in HGH, peaking two hours after the sprint. During a forty-minute treadmill run, volunteers who did two three-minute sprints showed significantly higher increases in BDNF and norepinephrine compared to low-intensity subjects.
The most important thing is to do something, and to start. Finding the right social context is the key to sticking with it — a city basketball program, Masters swimming, walking with someone you love, tae kwon do, rock climbing. The point is cooperation over competition, and finding the activity that makes you want to come back. What the teachers at Naperville built — a culture of movement and understanding rather than performance and fear — is the template. Exercise is not the punishment at the end of a bad day. It is the most powerful thing you can do for the organ running your life. Lace up your shoes.