Those molecules released by contracting muscles target the brain, and blood flow delivers them. One of those myokines is called irisin, named after Iris, the Greek goddess of rainbows and Hera’s personal messenger. Alarmingly low levels of irisin have been found in humans with Alzheimer’s, a disease that impacts one in ten people over 65.
When the production of irisin is blocked in mice, they performed terribly trying to remember where the cheese was in a maze. When the irisin flowed again, the same mice recovered. The mice that performed the best were the ones that exercised. In mice, at least, irisin goes straight to the hippocampus where it protects neurons from degeneration.
Another myokine is called brain-derived neurotrophic factor (BDNF). It may be even more important than irisin. The walkers whose hippocampus increased by 2% also had higher levels of BDNF than the non-walking group. BDNF has been called “Miracle-Gro for your brain.”
Depression. There is some evidence that walking helps relieve symptoms of depression and anxiety. They may believe they weren’t walking because they were depressed and enervated, but perhaps they are depressed because they weren’t walking. Those who struggle with depression describe it as an exhausting abyss of hopelessness. When you are in it, you feel like you’ll never get out. One in twelve Americans knows this feeling. While many studies have shown that a regular walk can alleviate symptoms of depression and anxiety, it doesn’t work for everybody. Furthermore, the benefits appear to depend on where you walk.
When you examine the brain, the folds and fissures of the brain seem randomly distributed. To a neurologist, they are a map that reveals the workings of our most magnificent organ. The folds in the back of the brain are where visual cues are processed. A strip of nervous tissue across the top helps coordinate movement. The bulge in the front of the brain is where we make plans. 52 different regions of the brain were identified and named in the early 20th-century by German neurologist Kobinian Brodmann, and each one now bears his name. Brodmann area 22 processes sound. Brodmann 44 and 45 help you talk.
About three inches behind the bridge of your nose is Brodmann 25, or what modern neurologists call the subgenual prefrontal cortex (sgPFC). It plays an important role in regulating our moods, displaying increased activity during periods of sadness and rumination.
One study had 38 people fill out a survey that included questions about their mood and negative self-reflection. The survey was tallied into what was called a rumination score. MRI scans of the sgPFC accessed the blood flow to this region. Then they were sent out for a walk. Half took a 3 ½ mile walk through greenspace on a university. There was fresh air, the shade of coast live oak trees, and screeches from western scrub jays. The other half walked the same distance on a sidewalk along a busy, multilane street through the heart of the city. There, they had to be alert for cars coming and going from gas stations, hotels, parking lots, and fast-food restaurants. When the participants returned, they had another survey and another MRI scan.
Those that had walked along the busy road showed no change in their rumination scores or in the blood flow to the sgPFC. But those that had walked through the woods had lowered their rumination scores, and blood flow to the sgPFC had reduced significantly.
For our mental health, it seems, we should walk where there are trees, birds, and the soft whispers of the wind.
When treadmill walkers did better than non-walkers on creativity tests, a test group took its walk outside. The ones on footpaths performed even better than the treadmill walkers at coming up with original ideas.
Unfortunately, we are not only walking less but, with so many of us living in urban areas, we are doing it in places that erase some of the health benefits.
Perhaps Ray Bradbury was right about what the future may hold. In his 1951 short story “The Pedestrian,” set a hundred years in the future, a writer named Leonard Mead goes for his nightly walk. Bradbury writes: To enter out into that silence that was the city at eight o’clock of a misty evening in November, to put your feet upon that buckling concrete walk, to step over grassy seams and make your way, hands in pockets, through the silences, that was what Mr. Leonard Mead most dearly loved to do.
As usual, Mead walks alone as his city neighbors watch TV, their windows aglow. A robotic police officer stops him and asks what he is doing. “Just walking,” he responds.
“Walking where? For what?” the police officer wants to know.
“For air, and to see, and just to walk,” he responds.
“Have you done this often?”
“Every night for years,” Mead says.
“Get in,” orders the officer.
The story ends with Mead in the back of a police car that is taking him to the Psychiatric Centre for Research on Regressive Tendencies.
MYOKINES and the COST of IMMOBILITY
Recent headlines promote “10 Reasons to Go for a Walk Right Now,” “9 Surprising Health Benefits of Walking,” and “Benefits of Walking: 15 Reasons to Walk.” Walking is a superfood. In terms of evolution, walking is our default. Throughout Our History, if we wanted to eat, we had to walk. What’s new is not walking.
Consider the effect that immobility has had on our bones. Our skeleton consists of two different kinds of bone. One, called cortical or compact, is the thick outer shell of our bones. The other, called trabecular, located in the joints where our bones meet, is a network of thinner, spongy bone arranged like a honeycomb. Compared to our ape cousins, humans have less of both kinds. We rely more on our trabecular skeleton to absorb, like a sponge, the high impact forces of bipedalism. Why, then, would we have so little of it?
CT scans can be used to calculate trabecular bone density in the skeletons of humans, apes and fossil hominins. Chimpanzees, fossil Australopithecus, Neanderthals, and even Pleistocene Homo sapiens had the same density of trabecular bone in their joints: 30 to 40%. But humans today have less – 20 to 30%. This drop in bone density appeared to occur suddenly in the last 10,000 years. It has been suggested this happened because we don’t move around as much as our ancestors did.
Another study of four human populations – two nomadic groups and two farming communities – found that nomadic people have denser bones than farmers. While diet might have something to do with this, most scientists agree that bones are less dense in people that don’t move around as much. In fact, humans have lost as much bone density in the last 10,000 years as an astronaut loses in a trip through the low gravity conditions of space.
As we age, our bones naturally thin as levels of bone-stimulating estrogens fall, especially in postmenopausal women. But because we already have such low-density bone, this further reduction can lead to osteoporosis and broken bones in our aging population. But osteoporosis might be the least of our worries.
After age forty, what can we do to live longer and healthier? The answer could be as simple as a daily walk. A decade’s worth of data on 650,000 people found that those who did the exercise equivalent of a 25-minute daily walk – as long as they were not obese – lived close to four years longer than their more immobile counterparts. Even a 10-minute daily walk could make a two-year difference in life span.
In a study of 300,000 Europeans, inactivity caused twice as many deaths as obesity. A 20-minute walk daily cut the risk of dying by one-third. It is better to fit and fat than lean and lazy. To understand, one has to dive deeper into the science of physiology.
Walking affects the complex dance of molecules in our bodies in significant ways. A lot of the research has been done in breast cancer, especially estrogen-receptor-positive breast cancer, which occurs in two-thirds of all cases. Breast cancer is exceedingly complex.
Estrogen circulating in the bloodstream causes the cells of the breast tissue to grow and divide as part of the normal physiology of a woman. Every time a cell divides, it copies its DNA, and every time it does so, there is a chance of a mistake – a mutation. Usually, that is no big deal, but if a mutation occurs in a gene that limits how fast cells grow and divide, uncontrollable growth can produce a cluster of cells called a tumor. A mutation in a gene that keeps those cells in the breast where they belong could cause some to stray into the bloodstream and settle in the lungs, liver, bone, or brain. This process is called metastasis, and the result is stage IV breast cancer.
One in eight American women will be diagnosed with breast cancer in their lifetimes. Nearly 3,000 men are diagnosed annually as well. It kills 40,000 Americans and over half a million people globally every year.
But a daily walk reduces the chances of developing breast cancer. How? One possible explanation is that exercise lowers the levels of estrogen circulating in the blood. Exercise increases the body’s production of a molecule called sex hormone binding globulin. This molecule attaches itself to estrogen, reducing its concentration in the blood by 10 to 15%, thereby reducing the chance of a mutation in the DNA of breast cancer.
Even if a mutation occurs, exercise appears to help the damaged DNA repair itself. Exercising at least 20 minutes a day had a slight (1.6%) better ability to repair DNA copying mistakes, although it’s unclear how this works.
If the copying mistakes don’t get fixed and cancer results, walking still helps. Exercise – even just an hour of walking a week – decreased the chances of dying by about 40%. A Saudi Arabia study put the number at 50% for estrogen-positive breast cancer. Exercise reduced the chance of cancer recurring after remission by 24%. Similar reductions in recurrence have been found in men with prostrate cancer who routinely walk after diagnosis. A 2016 study of 1.5 million people found that moderate exercise lowers the risk of developing thirteen different cancers.
While cancer claims too many lives, the number one killer in industrialized nations is cardiovascular disease. In its various forms, it is responsible for one in four deaths, or 60,000 Americans a year. Walking can help stave that off, too. Frequent walkers have lower heart rates and lower blood pressure than sedentary individuals. A study of 40,000 American men found that a daily 30 minute walk lowered the risk for coronary heart disease by 18%.
Coronary heart disease is all but unheard of among hunter-gatherers. The Hadza of northern Tanzania are fourteen times more active than the average American. They also have lower blood pressure as they age, lower cholesterol, and not a hint of cardiovascular disease. Bolivia’s Tsimane people similarly have low levels of coronary heart disease and five times less blockage of their arteries than the average person in the industrialized world.
Diet has a lot to do with this, but there is evidence that physical activity plays a critical role. Perhaps, though, not in the way you think.
Everyone figured that the Hadza used more energy than the typical American as the adults walk between 6-9 miles a day while the average American spends six hours a day staring at screens. What was found was shocking and forces us to think about our bodies in a different way. The total daily energy used by active Hadza and couch-potato Americans is the same. How is this possible? A clue is hidden in the one thing that walking does not help use do: lose weight. Humans are so efficient at walking that a 150-pound person would have to walk at least 70 miles to lose a pound. So the extra steps the Hazda take compared to the typical Americans don’t burn much energy. But the Hazdas don’t just walk. They dig, climb, and run. Surely, they should be using more energy.
The currently accepted hypothesis for this mystery is that human bodies all over the world have the same daily energy allowance. How they spend that energy varies from culture to culture and from person to person. The Hazda use energy getting from one place to another, gathering food, fighting off illness, carrying children and growing new ones. Americans do many of the same things, but because we aren’t as active, our bodies spend the excess energy on something else, ramping up our body’s inflammation response. Here’s why that. Is a health problem.
The inflammation response refers to the way our bodies recruit large, vigilant, amoeba-looking cells called macrophages to ward off infections or repair injuries. These cells, whose name means “big eaters,” are key components of our immune system. They make an infection-fighting protein called tumor necrosis factor (TNF). Among a variety of roles in the body, TNF tells the hypothalamus to crank up the body temperature when we are overrun by a virus or bacteria – a fever.
But chronically high levels of TNF have been linked to heart disease. Walking can downshift the production of TNF by 5% after a brisk 20-minute walk. How? The answer appears to involve a whole class of proteins that weren’t even in textbooks a few years ago.
Interleukin-6 is a protein white blood cells use to communicate with one another. Interleukin-6 levels in marathon runners are a hundred times higher at the end of a race than at the start. Muscles make interleukin-6 and release it into the bloodstream. Many organs in our bodies make molecules and release them into our bloodstreams as a way to talk with other organs. These endocrine organs include the pancreas, the pituitary gland, the ovaries, and the testes. But few had thought of muscle as an endocrine organ. Over a hundred molecules that our muscles make are released into the blood as we walk. One of these, oncostatin M, shrinks breast tissue tumors in mice and could be yet another reason why exercise is beneficial to humans with breast cancer. This amazing family of molecules is called myokines.
As a myokine, interleukin-6 is an anti-inflammatory. Among other roles, it helps shut down the problematic tumor necrosis factor. It is the body’s natural ibuprofen. Interleukin-6 can mobilize cells called “natural killers” to attack and destroy cancerous tumors, at least in mice. For some reason, this myokine needs to be produced by muscles during exercise in order to work. But that does not require walking. Can the 3 million Americans in wheelchairs generate myokines? After wheelchair half-marathons and basketball games, interleukin-6 is elevated and TNF lowered.
Myokines, however, are not magic potions. They cannot be injected or swallowed. They are made only when the body is in motion, and in modern societies, if often is not. On average, Americans take 5,117 steps a day, which is a third of what the average Hazda takes. How much should we walk to ward off heart disease, certain cancers, and type 2 diabetes?
The answer is 10,000 steps a day. Pedometer smartphone apps use that to change colour from a disappointed red or orange to a happy green. Where does this magical 10,000 step threshold come from? In the 1964 Tokyo Summer Olympic Games, Abebe Bikila of Ethiopia defended his gold medal in the marathon, setting a new world record in 2:12:11:2. American sprinter and future NFL Hall of Famer Bob Hayes raced a hundred meters down a cinder track in only 10.06 seconds, tying the world record. And Soviet gymnast Larisa Latynina, competing in her last Olympics, took home six more medals, bringing her total to 18 and making her the most decorated of any Olympian until American Michael Phelps swam along.
The Olympics inspired the people of Japan. For the first time, the games were broadcast live on television, and by 1964, 90% of Americans had a TV. Researchers in Japan were concerned with how sedentary the Japanese public had become and with the rising prevalence of obesity in Japan. It was shown that Japanese were walking between 3,500 and 5,000 steps per day but that was not enough to be healthy.
The following year, a device that hooked onto people’s waists counted the steps they took. It was called Manpo-kei which means “10,000”, po is “step”, and keii is “meter”. Thus the 10,000-step meter.
Smartphone walking apps default to 10,000 steps as a daily goal. Most fitbits do the same. Though the 10,000-step goal was based on some research, it was mostly a marketing gimmick. Yet over a half-century later, it is still with us. How many daily steps should we take?
From 2011 to 2015, 17,000 women whose age averaged 72 wore an accelerometer for a week. As a group, they averaged 5,499 steps a day, a shade more than the number a typical American adult takes. In the slightly over four years that followed, 504 of these women died. The number of daily steps was a good predictor for those who were still alive and who were not. Women who averaged at least 4,400 steps fared much better than those who took only 2,700. With daily steps up to 7,500, women continued to be better off those walking fewer steps. But that’s where it plateaued – going beyond 7,500 steps did not make a difference.
For a younger population, the plateau may not occur at 7,500 steps. The amount of walking required to yield health benefits, depends on age and activity level. It is recommended that everyone should take 2,000 more steps a day than currently average.
One way to add that many steps to your routine is to get a dog. Dogs were the first animals domesticated by our species. Ancient DNA from a canine’s rib found in Siberia reveals that humans and the wolf ancestors of dogs started hanging out together 30,000 years ago. In comparison, pigs and cows were domesticated closer to 10,000 years ago. As humans migrated around the globe, our dogs walked alongside us.
Even today, dog owners average almost 3,000 more daily steps than non-dog owners and are more likely to reach the recommended 150 minutes of walking per week.
In addition to preventing some cancers and reducing the risk of dying from cardiovascular diseases, a daily walk can prevent autoimmune diseases and can help ward off type 2 diabetes by lowering blood sugar levels. It improves sleep and lowers blood pressure. It decreases circulating cortisol levels, which helps reduce stress. In 40,000 women over 45, a thirty-minute daily walk reduced the risk of stroke by 27%. Despite these health benefits and admirable attempts to get our sedentary population on its feet, it is an uphill battle. Many futurists predict that our walking days are behind us. Could it be that humans will stop doing the very thing that defined us from the beginning?
