The Regimen
Build Your Brain
I’VE TRUMPETED THE astounding impact of aerobic activity on the brain in the hope that if you understand what’s actually going on up there when you go for a run, you’ll develop a genuine motivation to lace up your sneakers every day. Or you’ll go for a swim or hop on your bike or do whatever you enjoy doing to work up a sweat. I want nothing less than to get you hooked.
The point I’ve tried to make — that exercise is the single most powerful tool you have to optimize your brain function — is based on evidence I’ve gathered from hundreds and hundreds of research papers, most of them published only within the past decade. Our understanding of how the brain works has absolutely exploded in this relatively short period, and it’s been an incredibly exciting time for anyone interested in the human condition. For me, personally, researching this book has redoubled my enthusiasm for the benefits of exercise and replaced intuition with hard, scientific fact.
As an illustration of just how new this territory is, I’ll go back to the story of neurogenesis, the once-heretical theory that the brain grows new nerve cells throughout life. “Ten years ago people weren’t even convinced that it happened,” says neurologist Scott Small. It was at his Columbia University lab, in 2007, where they witnessed telltale signs of neurogenesis for the first time in live humans. “Five years ago people said, OK, it might happen, but is it really meaningful? Now there isn’t a week that goes by where there’s not another study that shows neurogenesis has some kind of effect on the brain.”
In his study, Small put a group of volunteers on a three-month exercise regimen and then took pictures of their brains. By manipulating a standard MRI machine’s processing — essentially zooming in and cocking the shutter open — he captured images of the newly formed capillaries required for nascent neurons to survive. What he saw was that the capillary volume in the memory area of the hippocampus increased by 30 percent, a truly remarkable change. The real breakthrough here might turn out to be the ability to map neurogenesis without slicing into the brain, which could shift the research focus from lab rats to people. The new technology should allow scientists to test the impact of any given variable on neurogenesis, such as how much exercise is necessary. “Is it one hour a week? Is it every morning? Is it only a grueling marathon regimen that maximizes neurogenesis?” Small asks. “We simply don’t know that. No one knows that. Now by having this tool that can measure neurogenesis indirectly, we can actually try to optimize an exercise regimen.”
Several years down the line, that is. At this point, he and his peers view exercise primarily as a surefire trigger to increase the growth of new cells. It’s something they use to observe another process; most haven’t gotten around to studying exercise per se.
It’s a similar story with many of the other positive effects of exercise I’ve discussed, from increasing neurotransmitters and neurotrophic factors to releasing factors from the muscles that build new capillaries in the brain and encourage synaptic plasticity. William Greenough, the neuroscientist who saw that exercise caused neurons to sprout new branches with an electron microscope in the early 1970s, will tell you there’s no question that aerobic exercise is great for your brain. And he’s pretty confident that it’s important to include complex motor movements (such as aerobic dance or martial arts) in your routine. But he can’t give specific recommendations just yet.
And that’s OK. We don’t need to rely exclusively on neuroscientists to get started. First of all, there are certain conclusions we can draw from the work they’ve already done. Second, there is instructive evidence from other fields. Research from kinesiologists to epidemiologists shows again and again that the better your fitness level, the better your brain works. Charles Hillman proved that fit children score better than unfit children on cognitive tests of executive function; Arthur Kramer showed that getting in shape increases brain volume of older adults; and population studies including tens of thousands of people of every age show that higher fitness levels relate directly to positive mood and lower levels of anxiety and stress.
When people ask me how much exercise they should do for their brain, I tell them the best advice is to get fit and then continue challenging themselves. The prescription for how to do that will vary from person to person, but the research consistently shows that the more fit you are, the more resilient your brain becomes and the better it functions both cognitively and psychologically. If you get your body in shape, your mind will follow.
Does that mean you have to look like an underwear model to enjoy the brain benefits of exercise? Not at all. In fact, many of the most convincing studies use walking as the mode of exercise. But I focus on getting fit because we know with certainty that having a normal body mass index and a robust cardiovascular system optimizes your brain. Any level of activity will help, certainly, but from a practical standpoint, if you’re going to bother doing something for your brain, you might as well do enough to protect your body against heart disease, diabetes, cancer, and the like. Body and brain are connected. Why not take care of both?
In his book Racing the Antelope: What Animals Can Teach Us about Running and Life, biologist Bernd Heinrich describes the human species as an endurance predator. The genes that govern our bodies today evolved hundreds of thousands of years ago, when we were in constant motion, either foraging for food or chasing antelope for hours and days across the plains. Heinrich describes how, even though antelope are among the fastest mammals, our ancestors were able to hunt them down by driving them to exhaustion — keeping on their tails until they had no energy left to escape. Antelope are sprinters, but their metabolism doesn’t allow them to go and go and go. Ours does. And we have a fairly balanced distribution of fast-twitch and slow-twitch muscle fibers, so even after ranging miles over the landscape we retain the metabolic capacity to sprint in short bursts to make the kill.
Today, of course, there’s no need to forage and hunt to survive. Yet our genes are coded for this activity, and our brains are meant to direct it. Take that activity away, and you’re disrupting a delicate biological balance that has been fine-tuned over half a million years. Quite simply, we need to engage our endurance metabolism to keep our bodies and brains in optimum condition. The ancient rhythms of activity ingrained in our DNA translate roughly to the varied intensity of walking, jogging, running, and sprinting. In broad strokes, then, I think the best advice is to follow our ancestors’ routine: walk or jog every day, run a couple of times a week, and then go for the kill every now and then by sprinting.
Your choices aren’t limited to these modes of aerobic activity, naturally, but I think they’re helpful categories to distinguish between low-intensity (walking), moderate-intensity (jogging), and high-intensity (running) exercise. If you want to make the most of your time and effort, you’ll need a way to accurately judge your level of exertion along these divisions. When I talk about walking, or low-intensity exercise, I’m referring specifically to exercising at 55 to 65 percent of your maximum heart rate. By my definition, moderate intensity falls in the range of 65 to 75 percent, while high intensity is 75 to 90 percent. The upper end of high-intensity exercise is sometimes painful but always powerful territory that has gained a lot of scientific interest recently.
Unless you’re content to toil away on an exercise machine with built-in pulse sensors, the only way to accurately gauge your level of intensity is to use a heart rate monitor. These gadgets are the cornerstone of the revolutionary physical education program in Naperville, and they’re simple enough that elementary school kids there know how to use them. They consist of a chest-strap sensor that picks up your heartbeat, and a digital watch that receives the signal and displays the number of beats per minute on its screen. Let’s say your regimen calls for a high-intensity run. If you’re forty-five years old, your theoretical maximum heart rate would be about 175, based on the rough formula of 220 minus your age. If you calculate 75 percent and 90 percent of your maximum, the lower and upper limits for a high-intensity workout are 131 and 158. This is your target heart rate zone for the workout. All you have to do is punch in these limits on the watch, which is no more difficult than setting the time, and then adjust your pace according to what the monitor tells you. The watch will beep if your heart rate strays from the desired zone. It’s a reasonably accurate way to listen to your body.
Heart rate monitors are cheap, easy to use, and indispensable for anyone who’s seriously interested in tapping the potential of exercise. It’s also just nice to know that you’re doing enough and not too much. But again, the question is how much? Public health recommendations, from the Centers for Disease Control to the American College of Sports Medicine, suggest doing some form of moderate aerobic exercise for thirty minutes at least five days a week. But I think they’re hedging their bets. Americans are so inactive that the experts are wary of providing guidelines that are too stiff, for fear that the whole country will give up. “Everyone wants to know the minimum amount they can do for the bang,” says Duke University exercise physiologist Brian Duscha, who was inundated with media requests after he published a study showing that walking as little as three hours a week has cardiovascular benefits. “I try not to overwhelm people because they quit.” He also points out to anyone who’ll listen that increasing the duration or the intensity carries even greater fitness gains.
Duscha is an expert in cardiovascular health, but he says the same thing almost every neuroscientist cited in these pages has said: “A little is good, and more is better.” The best, however, based on everything I’ve read and seen, would be to do some form of aerobic activity six days a week, for forty-five minutes to an hour. Four of those days should be on the longer side, at moderate intensity, and two on the shorter side, at high intensity. And while there’s conflicting evidence about whether high-intensity activity, which can force your body into anaerobic metabolism, impacts thinking and mood, it clearly releases some of the important growth factors from the body that build up the brain. So, on the shorter, high-intensity days, include some form of strength or resistance training. These days should not be back to back; your body and brain need recovery time to grow after high-intensity days. In total, I’m talking about committing six hours a week to your brain. That works out to 5 percent of your waking hours.
That said, I do agree with experts like Duscha that the most important thing is to do something. And to start. This last bit may sound obvious, but for the sedentary — especially if inactivity is due to depression — taking that first step may seem impossible. For some people it’s a catch-22: they can’t start because they don’t have the energy, and they don’t have the energy because they’re not exercising. I’ve seen this happen with some of my patients, and it’s a very real problem, not simply an issue of willpower. The key is to attack the business of starting as a challenge in itself.
It’s well established that it’s easier to work out with someone else, whether you’re running with a friend, cycling with a group, or walking with a neighbor. Beyond that, there are several new studies showing that the neurological benefits I have described are greater when exercising with someone else. I tell patients who are really stuck to consider getting a personal trainer for a while, because then they’re less likely to cancel a session (you pay regardless of whether you show up as scheduled, and money is a great external motivator). Write exercise into your schedule just like a dentist appointment. After a while, your brain will absorb it into your routine, just like brushing your teeth.
If you haven’t been active, I think the best way to begin is to start walking. Take the stairs instead of the elevator, park at the back of the lot, and go for a stroll around the block at lunchtime. There’s a decades-old health initiative called ten thousand steps that encourages people to use a simple pedometer to calculate how much they walk every day, as a way to work exercise into their routine without having to think much about it. Based on the average stride of 2.5 feet, ten thousand steps is close to five miles. It’s a clever way to begin to get in shape without even setting aside much extra time. And it works. Counting your steps, like weighing yourself or using a heart rate monitor to guide your efforts, helps keep you focused and motivated, especially if you understand what your body and brain are doing at various intensity levels.
The process of getting fit is all about building up your aerobic base. The more you work your heart and lungs, the more efficient they become at delivering oxygen to your body and brain. With the increased blood flow, of course, comes the chemical cascades that produce serotonin, brain-derived neurotrophic factor (BDNF), and other nourishing molecules.
If you start walking an hour a day at between 55 and 65 percent of your maximum heart rate, you’ll naturally increase the distance you walk in that time period and gradually get in shape. At this level, you’re burning fat as fuel, and this begins to gear up your metabolism. When a body carries too much fat, its muscles build up resistance to insulin, which exacerbates the fat buildup and curtails the production of insulin-like growth factor (IGF-1). A study published in 2007 from the University of Michigan reported that a single session of aerobic activity completely reversed insulin resistance the next day. The researchers compared muscle biopsies from before and after the session, and they also saw that the exercised fibers produced proteins important for fat synthesis. They don’t know how long the effect lasts, but the findings underscore how even a minor amount of activity sparks a positive domino effect.
When you place a demand on your body and your muscles sense the need for more fuel, all sorts of good things happen. Low-intensity, fat-burning exercise also pumps up free tryptophan in the bloodstream, which, you’ll recall, is a necessary ingredient for the production of mood-stabilizing serotonin. This level of activity also changes the distribution of norepinephrine and dopamine. When you look at this in the evolutionary context of Heinrich’s endurance predator, it makes elegant sense: while tracking their prey, our ancestors needed to have the patience, optimism, focus, and motivation to keep at it. All these traits are influenced by serotonin, dopamine, and norepinephrine.
Walking will make you feel more invested in the world around you. Before long, you’ll want to get out there even more. One simple way doctors gauge the fitness of their patients is to see how far they can walk in six minutes. But researchers at the University of Alabama School of Medicine found that people improve so rapidly that the best way to get an accurate reading is to have them do two practice walks first. Which is to say, you might be pleasantly surprised at how quickly you begin to cover more ground.
Once you work up to an hour at a pace just above where you can carry on a conversation, you’re ready to add in moderate-intensity exercise. When you challenge yourself at that level, you’ll be able to do more not only while you’re working out but also in every domain of your life. You’ll have more vigor and energy, less negativity, and you’ll develop a greater sense of control. Above all, if you’re in an active mode, you won’t be sitting at home, isolated and stagnant.
Once you move into moderate-intensity activity, between 65 and 75 percent of your maximum heart rate, your body shifts from burning fat alone to also burning glucose, and the muscle tissue develops microtears as a result of the stress. All the cells in your body and brain are in a constant state of damage and repair, but the metabolic demands at this level ratchet up the response. Your body knows it needs a more robust oxygen-delivery system, so the muscles release vascular endothelial growth factor (VEGF) and fibroblast growth factor (FGF-2), and they start cells dividing to make more tissue for more blood vessels — the new capillaries that Scott Small captured on film. In lab cultures, researchers have found that VEGF and FGF-2 activate cells to make more blood vessels within just two hours of exposure. In the brain, in addition to building new blood vessels, these two factors also encourage cell binding and neurogenesis.
Inside your brain cells, the higher activity level triggers the release of metabolic cleanup crews, producing proteins and enzymes that dispose of free radicals, broken bits of DNA, and inflammation factors that can cause the cells to rupture if left unchecked. More and more, research suggests that taking antioxidants in pill form may not be helpful — and may actually be harmful — but most people don’t realize that aerobic exercise is a way of creating your own supply of antioxidants inside the cells themselves. And antioxidants are only part of the story. Assuming there’s an adequate recovery period, the repair response to exercise leaves your neurons stronger.
Moderate-intensity exercise also releases adrenaline into the bloodstream. In the untrained person, the hypothalamus-pituitary-adrenal (HPA) axis is activated. This is the fight-or-flight stress response I described in chapter 3, in which your body is on high alert, and cortisol begins coursing through your brain. At moderate levels, cortisol cues up the cellular machinery of learning in the interest of recording a situation that your body assumes is important to survival. But if cortisol remains chronically elevated, it’s toxic to nerve cells. BDNF is the best defense for your neurons. By elevating restorative chemicals with a moderate workout, you’re strengthening the circuits in your brain and tuning up the HPA axis so that it isn’t as trigger-happy to future incidents of stress. Likewise, the immune system becomes stronger and better prepared to handle a genuine assault on the body — fighting off everything from colds to cancer.
Another factor from the body that comes into play here is atrial natriuretic peptide (ANP). Produced by the muscles of the heart itself when it’s really pumping, ANP travels through the bloodstream and into the brain, where it helps to further moderate the stress response and reduce noise in the brain. It’s a potent part of a cascade of chemicals that relieve emotional stress and reduce anxiety. Along with pain-blunting endorphins and endocannabinoids, the increase in ANP helps explain why you feel relaxed and calm after a moderate aerobic workout. When you talk about burning off stress, these are the elements at work.
At this level, you’re tearing things down and building them back up again, stronger than they were before. It’s crucial to build in recovery time, so your body and brain have the opportunity to bounce back.
At high intensity, between 75 and 90 percent of your maximum heart rate, your body enters into a full-fledged state of emergency, and its response is appropriately powerful. It’s also within this range, usually at the upper end, that metabolism shifts from aerobic to anaerobic, in which your muscles go into a state of hypoxia because they can’t pull enough oxygen from the bloodstream. Since oxygen is necessary for the efficient burning of glycogen, without it your muscles begin to burn creatine and glycogen stored directly in the muscle tissue, a messy mechanism that creates a buildup of lactic acid (the burn you feel in your thighs and chest). The so-called anaerobic threshold occurs at different intensities for different people, but during a high-intensity workout, you want to be flirting with that thigh burn (after warming up) but staying just below it throughout your run. And while physiologists can’t give a magic heart rate at which your body crosses over from aerobic to anaerobic, one recent study by kinesiologist Panteleimon Ekkekakis of Iowa State University showed that the most reliable marker of this metabolic change was when subjects reported that their level of exertion became “somewhat hard.” It sounds vague, but Ekkekakis has found the correlation to be remarkably consistent. Another way to judge this level is to keep in mind that even though going at just below your anaerobic threshold is “somewhat hard,” it shouldn’t be so hard that you couldn’t keep up the same pace for thirty minutes to an hour.
If you want to really challenge yourself by mixing in intervals, you’ll sprint above that threshold for short bursts in the middle of your high-intensity session.
One of the key differences between moderate and high-intensity exercise is that once you get closer to your maximum, and especially when you get into the anaerobic range, the pituitary gland in your brain unleashes human growth hormone (HGH). This is what life-extension groups call the fountain of youth. The levels of HGH naturally secreted into the bloodstream decrease over your life span, so that by middle age they dwindle to a tenth of what they were during childhood, for both women and men. And a sedentary lifestyle exacerbates this decline: high levels of cortisol, insulin resistance, and excess fatty acids in the bloodstream all clamp down further on the hormone’s release.
HGH is the body’s master craftsman, burning belly fat, layering on muscle fiber, and pumping up brain volume. Researchers believe it can reverse the loss of brain volume that naturally occurs as you age. Athletes such as Olympic sprinters and football players are essentially spiking their levels of HGH when they do interval training, doping the natural way. The result is a buildup of fast-twitch muscle fibers, which adds power to their movements. Also, recruiting new muscle fibers enhances your metabolism overall, and your body retains the improved ability to burn fat and carbohydrates after the interval training.
Normally HGH stays in the bloodstream only a few minutes, but a session of sprinting can keep the level elevated for up to four hours. In the brain, HGH balances neurotransmitter levels and boosts the production of all the growth factors I’ve mentioned. But it seems to have the most dramatic impact on IGF-1, the evolutionary linchpin tying together activity, fuel, and learning. It gets into the very cell nucleus and switches on genes that crank up the mechanisms of neuronal growth.
Psychologically, this is where you “confront the self,” in the words of my colleague Robert Pyles, the psychiatrist and marathoner I wrote about in chapter 3. By going beyond where you thought you could, straining and stressing and lingering in that pain for even just a minute or two, you sometimes transcend into a rarefied state of mind, in which you feel like you can conquer any challenge. If you’ve ever experienced the phenomenon of runner’s high, it probably came in response to a near maximal effort on your part. The euphoric feeling is likely due to the mixture of extremely high levels of endorphins, ANP, endocannabinoids, and neurotransmitters pumping through your system at this intensity. It’s the brain’s way of blocking everything else out so you can push through the pain and make the kill.
High-intensity exercise toughens you up, both physiologically and psychologically. It’s the reason why we climb mountains and sign up for fitness boot camps and go on Outward Bound trips. Yet you don’t need to go to such extremes to reap the rewards I’m talking about. One study from the University of Bath, in England, found that adding a single spurt of sprinting for thirty seconds — in this case while pedaling on a stationary bike — generated a sixfold increase in HGH, which peaked two hours after the sprint.
And a recent study by neurologists at the University of Muenster, in Germany, reported that interval training improves learning ability. During the course of a forty-minute treadmill run, volunteers did two three-minute sprints (separated by two minutes at a lower intensity). Compared with subjects who stayed at low intensity, the sprinters had significantly higher increases in BDNF as well as norepinephrine. Accordingly, in cognitive tests immediately following the run, the sprinters learned vocabulary words 20 percent faster. So even a small dose of pushing yourself to the limit has profound effects on your brain.
As good as all this sounds, interval training isn’t something you can just jump off the couch and do. You really need to have a rock-solid aerobic base and talk to your doctor about what you’re planning. It’s not a good idea to put that kind of strain on your heart if it’s not used to it. Depending on what sort of shape you’re in, I’d say you should have at least six months of six-days-a-week aerobic activity before mixing in interval training. But again, your doctor needs to clear you for takeoff.
I haven’t devoted much space to the discussion of nonaerobic exercise because, frankly, there is very little research into how it affects the brain in terms of learning, mood, anxiety, attention, and the other issues I’ve covered. It’s difficult to get rats to pump iron or do yoga, so scientists are restricted to studying humans, which means they can’t biopsy brain tissue after the experiments. They have to rely on blood samples and behavioral tests, which leave much more room for interpretation. The results we do have on nonaerobic exercise aren’t as robust as those for aerobic exercise.
That said, strength training is obviously important for building muscles and protecting joints, and practices such as yoga and tai chi improve balance and flexibility — all of which contribute to your body’s ability to remain active throughout life. A very recent study among older adults found that lifting weights twice a week for six months made participants stronger and actually reversed aspects of the aging process at the genetic level. The genes responsible for producing some of the key factors for brain growth (VEGF, FGF-2, and IGF-1) acted as if they were thirty years old, not sixty-five.
Most of the brain research on resistance training has focused not on learning and memory, but on mood and anxiety. In one study from Boston University dating back ten years, researchers put a group of older adults on a twelve-week strength-training program (three sessions a week) and measured various aspects of psychological and cognitive function. They concluded that in addition to improving muscle strength by about 40 percent, the regimen lowered anxiety and improved mood and confidence levels, but had no significant effect on thinking ability. Around the same time, another study from the Institute of Psychology at the University of Bern, in Switzerland, tested the effects of eight weeks of strength training. Doing a ten minute warm-up followed by eight weight-machine exercises once a week improved psychological well-being and had a slight effect on memory. And according to follow-up tests, the results remained for a year regardless of continued exercise levels. But there were too many variables for the researchers to conclude that strength training had a measurable effect on memory.
The intensity level of the strength training seems to affect the results, in that moderate weights have been shown to have a more positive impact than heavy weights, at least in a small group of older women. Other research has shown that high-intensity strength training actually increases anxiety levels in both men and women. In this case, high intensity was defined as lifting 85 percent of the maximum weight possible, but many studies haven’t defined this crucial variable. A study published a few years ago in the American Journal of Sports Medicine showed that cross training — combining thirty minutes of weights with thirty minutes of stationary cycling — improved anxiety levels, but the study was designed in such a way that it was impossible to know what caused the change. And almost all of the studies on this topic have used populations of older adults, who are prone to pronounced improvements because their muscles are naturally diminished when they start.
One factor clearly affected by strength training is HGH. A recent study looked at the hormone levels during weight training versus aerobic activity in well-trained men. Doing squats doubled HGH levels compared with running at high intensity for thirty minutes, and I think this will turn out to have important implications for exercise recommendations.
There is even less research into the effect of rhythm, balance, and skill-based activites on the brain. Small studies have shown that yogic breathing reduces stress and anxiety levels, and tai chi reduces the activity of the sympathetic nervous system (judging from heart rate and blood pressure). One recent study used MRI scans on eight yoga practitioners and found that their levels of the neurotransmitter gamma-aminobutyric acid (GABA) increased 27 percent after a sixty-minute session. GABA is the target of Xanax-like drugs and is very involved with anxiety, so this may be part of the reason why yoga helps some people relax. Much of the evidence from this realm is anecdotal, but I’m sure that as neuroscientists delve deeper into the brain they’ll discover pathways that connect the dots.
Statistics show that about half of those who start up a new exercise routine drop out within six months to a year. Not surprisingly, one of the biggest reasons seems to be that people often jump in at high intensity, which makes them feel bad physically and emotionally, and then they simply quit. The kinesiologist Ekkekakis has focused much of his research on the relationship between exercise intensity and discomfort. People vary as to how they feel leading up to the shift from aerobic to anaerobic metabolism, but he has found that once they cross the line almost everyone reports negative feelings on psychological tests and high ratings on scales of perceived exertion. It’s your brain putting you on alert that there’s an emergency. The point is, if you feel lousy even at lower intensity levels, don’t take on interval training in the early stages of your new routine. (Again, it’s more important to do something than nothing.)
And don’t get down on yourself if you don’t love exercise — you may be genetically predisposed to dislike it. In 2006 European researchers compared physical activity levels of 13,670 pairs of identical twins and 23,375 pairs of fraternal twins, who share only half the same genes. They found that 62 percent of the variation in whether the twins tended to exercise was explained by different genes. Other research has found that gene variations have an impact on whether you enjoy the feeling of exercise, whether you stick with it once you begin, and even whether you notice a dramatic improvement in mood. Among the many genes involved, researchers have focused on one related to dopamine, the reward and motivation neurotransmitter, and another that controls BDNF expression. People with the dopamine variation may have reward deficiency syndrome, which robs them of that rush of pleasure they imagine everyone else at the gym is experiencing. And if your BDNF signaling is off, the mood-improving mechanism of exercise may be sluggish. I offer this information not as an excuse, but as a reminder — all of us can rewire our brains by taking action. It’s not as easy as when we were kids, but it’s clearly possible.
Exercise immediately increases levels of dopamine, and if you stay on some sort of schedule, the brain cells in your motivation center will sprout new dopamine receptors, giving you newfound initiative. You’re wearing in new neural pathways or perhaps refurbishing ones that are rusty from disuse, and it only takes a few weeks to solidify a habit. Exercise can become a self-reinforcing behavior that helps you trump your genes. The truth is, your genes are only one part of a very complex equation, and you have control over many of the other variables.
It’s a similar story with BDNF: it may take longer for you to get over the hump of forming a routine and to feel good while you’re exercising, but once you do your brain becomes more and more efficient at producing Miracle-Gro. Carl Cotman, the neuroscientist who runs the aging center at the University of California, Irvine, discovered that the hippocampus has what he calls a “molecular memory” for producing BDNF. In a three-month trial, he measured BDNF levels of lab rats on various exercise routines, comparing daily versus alternate-day wheel running, and looking at the effect of quitting for several weeks. The impetus for the study was based on his wry observation that although most lab experiments use daily exercise, “In humans, exercise patterns are generally less rigorous, and rarely follow a daily consistency.”
He came to a number of telling conclusions. First of all, daily exercise ramps up BDNF more rapidly than alternate days — at a rate of 150 percent versus 124 percent after two weeks. Curiously, after a month, the alternate-day exercisers had caught up with the daily group. When the rats stopped exercising, regardless of the routine, it took just two weeks for levels of BDNF to return to baseline. But the most interesting finding was that when the exercisers were allowed access to their running wheels again, BDNF levels shot back up in just two days (137 percent above normal for the daily exercisers, and 129 percent for the alternate-day group). This is what he means by molecular memory: if you’ve had the experience of exercising regularly, your hippocampus can get back up to speed very quickly.
Cotman concluded that every day is best but that even intermittent exercise works wonders. And I think it’s important for people to recognize that exercise isn’t an all-or-nothing proposition. If you miss a few days, or even a week or two, picture your hippocampus cranking out high levels of BDNF after just your second day back at it.
One of the best ways to get on a roll is to get in a group. The stimulus of social interaction starts your neurons firing like nothing else — it’s complicated, challenging, rewarding, and fun. And when you combine this sort of mental activity with the priming effect of exercise, you’re maximizing the growth potential of your brain. Exercise cues up the building blocks of learning, and social interaction cements them in place.
Princeton neuroscientist Elizabeth Gould, a pioneer in the field of neurogenesis whose research focuses on how experience and environment change the brain, has studied the different effects of exercise on animals living alone versus those living in a group. She has found that social interaction has a powerful impact on neurogenesis. In one experiment, after twelve days of running, rodents housed in social groups showed a significant increase in neurogenesis over others that exercised just as much but were kept in isolation. In fact, the isolated runners had the same low level of cell proliferation as group-housed controls that didn’t exercise. The reason has to do with the stress hormone cortisol. In her study, published in Nature Neuroscience in 2006, Gould found that while all the runners had elevated cortisol during exercise, levels for the isolated group were also high at other times of the day. In other words, cortisol won out over neurogenesis in the isolated condition, but social support “blunted the reactivity” of the HPA axis and kept the stress hormone from interfering with growth. Does that mean that going for a run by yourself is bad news? Not at all.
Remember that exercise itself is a stressor, which activates the HPA axis and may elevate cortisol. So is isolation. It seems that the cumulative stress of running and being alone elevated cortisol to such an extent that it prevented neurogenesis from happening — perhaps because the rats didn’t have an adequate recovery period. Making matters worse, the rodents were sedentary to begin with, and going from no activity to running several kilometers a day presents a major new stress on the system.
When Gould extended the experiment beyond the initial twelve days, it was an entirely different story. She found that if the rats were kept in the same conditions, over the long term the isolated runners’ systems caught up. Somewhere between twenty-four and forty-eight days of running, the rate of neurogenesis leveled out between the isolated and social groups. She speculates that one explanation might have to do with serotonin, which is increased by social interaction and in turn enhances neurogenesis. Both isolation and prolonged exposure to cortisol reduce the number of serotonin receptors in the hippocampus. It might be the case that even though running increases levels of serotonin, without enough receptors for it to plug into the neurons, it can’t do its job.
Gould is attempting to elucidate extremely complex relationships between stress, environment, and exercise, and there are several crucial points to take away from her study: First, it’s important to start slow if you haven’t been exercising and have a lot of other stress in your life. Second, social support has a powerful effect on the brain and can both prevent the negative impacts of stress and clear the way for exercise to ramp up the machinery of growth, so keep connected to keep your connections. And third, if you keep at your routine, your system will adjust itself to take advantage of the activity.
Of course, Gould emphasizes that there are limitations to drawing conclusions from animal studies. “Rodents are really different from humans,” she says. “If you give a rat or a mouse access to a running wheel, every single one of them will run. That’s not true of humans. A lot of them buy treadmills that become coatracks.”
While it’s true that we’re born to run, we’re also programmed to take advantage of bountiful periods and to conserve our energy for the long stretches of foraging and hunting that surely lurk around corner. It’s not that the instinct to plop down on the couch suddenly appeared in our DNA in the past hundred years; it’s that our modern environment is incongruous with our genes. Food is never far from hand — foraging requires ten steps to the fridge, not ten miles across the savannah — so it’s important to replace the need to work for it with the demands of aerobic exercise.
But don’t become a lab rat. Reserve your treadmill running for rainy days or for times when you can’t arrange to do something with other living, breathing human beings. Joining a team or setting a goal of doing a 10K charity run and training with a group of friends adds a sense of obligation, which can be a powerful motivator. Back in Naperville, Zientarski teaches cooperation, not competition, but for some adults being part of a team can get them hooked, whether it’s a three-on-three city basketball program, an adult soccer league, or Masters swimming.
Maybe walking with the one you love will turn out to be the key, or maybe you’ve always wanted to learn tae kwon do, or maybe, like Naperville Central graduate Jessie Wolfrum, you’ll discover a passion for the infinite challenge of rock climbing (which requires a partner). She was lucky, as a high school student, to have eighteen different activities to choose from. You, on the other hand, are lucky to be able to choose from any form of activity you can imagine. The beauty of exercise is that the more you do, the more you’ll imagine yourself doing.
Naturally, it’s important to stretch, but it’s just as important to keep your mind flexible. The problem with any routine, of course, is that it goes against nature. The world around us is constantly changing, and it’s difficult to keep doing the same thing over and over. I wouldn’t ask you to. The best strategy is to do something almost every day, yet stay flexible within that framework, allowing it to bend, so it doesn’t break. By mixing up your program and trying new activities, you’ll continue to adapt and challenge yourself. My own experience with exercise is a perfect example of what can go wrong and also what can go right.
I grew up in western Pennsylvania, in the era during which that area produced football stars such as Joe Namath, Mike Ditka, and Tony Dorsett. I played the major sports — football, basketball, and baseball — but was more of a hardworking scrub than a game maker. I found my athletic calling on the tennis courts and played constantly with my best friend and doubles partner all through high school. I was supposed to play at Colgate, but I broke my arm and leg in a car accident just before heading off to college. My arm required two surgeries that kept me out of commission for several years. I quit competing in tennis and didn’t do much else athletic for at least a decade.
I got back into being active during my residency, which was during the running craze surrounding Bill Rogers’s success and the popularity of the Boston Marathon. Running made me hungry to play tennis again, and then I got into squash with a couple of colleagues, including my good friend and longtime collaborator Ned Hallowell. We played three times a week for almost twenty-five years, competing, cajoling, and encouraging one other. We were all extremely busy, but our squash appointments were sacrosanct. It was a magical stretch.
About seven years ago, I tore the rotator cuff of my right arm beyond repair, and I can no longer swing a racket. I started lifting weights for rehabilitation, and it was the first time that I consistently went to the gym. I started going three or four times a week, doing forty minutes or so on the Stairmaster or elliptical trainer, and continuing with the weights two days a week. Then I kicked it up a notch and blocked out an hour a day, but I missed the camaraderie of our squash games. Ned coerced me into signing on with his personal trainer, Simon Zaltzman. He is a relic: a former boxing coach with a thick Russian accent and what seems to be an endless imagination for challenging me.
I got into the habit of doing weights plus crunches and balance exercises two times a week, three times when I’m really focused. The other days I do forty minutes on the elliptical trainer, or on the treadmill when I want to add in some intervals.
While researching this book, I learned about the magic of HGH and how sprinting might get me where I really wanted to be. Two days a week, I started including a handful of sprints during my treadmill runs, and let me tell you, they hurt. Just writing about it makes me cringe a little, but it has been well worth the extra effort. After one month of this business, I lost the final ten pounds I’d been after for years — it peeled right off my midsection. Not that I was overweight. It’s just that nothing I tried seemed to budge my spare tire. Now, on two of my aerobic days (and no more than two) I do just twenty minutes of jogging, interspersed with five sprints of twenty to thirty seconds each in which I run as fast as I can. When people want to know what to do if they’re short on time, I tell them this story.
Although I’m almost sixty, I feel much younger, and if I could get Art Kramer to scan my brain, I’m sure it would look younger too. I’m doing everything I can to keep my prefrontal cortex, and everything it’s connected to, pumped up. I miss days, certainly, but I try not to miss two in a row. When I just can’t make it to the gym, my wife and I take the dogs for a thirty-minute speed march instead of a ten-minute walk. For Jack and Sam, our spunky and inexhaustible Jack Russells, my dalliance is their great good fortune. Little do they know.