The Body, page 21
At the very least—and it really is the very least—you should get up and move around a little. According to one study, being a committed couch potato (defined as someone who sits for six hours or more per day) increases the mortality risk for men by nearly 20 percent and for women by almost double that. (Why sitting too much is so much more dangerous for women is unclear.) People who sit a lot are twice as likely to contract diabetes, twice as likely to have a fatal heart attack, and two and a half times as likely to suffer cardiovascular disease.
Amazingly, and alarmingly, it doesn’t seem to matter how much you exercise the rest of the time. If you spend an evening on the seductive padding of your gluteus maximus, you may nullify any benefits you gained during an active day. As James Hamblin put it in The Atlantic, “You can’t undo sitting.” In fact, people with sedentary occupations and sedentary lifestyles—which is to say, most of us—can easily sit for fourteen or fifteen hours a day, and thus be completely and unhealthily immobile for all but a tiny part of their existence.
James Levine, an obesity expert from the Mayo Clinic and Arizona State University, coined the term “non-exercise activity thermogenesis,” or NEAT, to describe the energy we expend from normal daily living. We actually burn a fair amount of calories just existing. The heart, brain, and kidneys burn about 400 calories a day each, the liver about 200. The process of eating and digesting food alone accounts for about one-tenth of the body’s daily energy requirements. But we can do much more by simply getting up off our backsides. Even just standing burns an extra 107 calories an hour. Walking around burns 180. In one study, volunteers were instructed to watch television as normal through an evening, but to get up and walk around the room during every commercial break. That alone burned 65 extra calories an hour, about 240 calories over an evening.
Levine found that lean people tend to spend two and a half hours more a day on their feet than fat people, not consciously exercising, but just moving about, and it was this that kept them from accumulating fat. Then again, another study found that people in Japan and Norway are just as inactive as Americans, yet only half as likely to be obese, so exercise can only partly account for slimness.
In any case, a bit of extra weight may not be such a bad thing. A few years ago, The Journal of the American Medical Association caused a stir by reporting that people who are slightly overweight, particularly if middle-aged or older, may survive some serious illnesses better than those who are either lean or obese. The idea has become known as the obesity paradox, and it is hotly disputed by many scientists. Walter Willett, a researcher at Harvard, called it “a pile of rubbish” and said that “no one should waste their time reading it.”
There’s no doubt that exercise improves health, but it is hard to say by how much. A study of eighteen thousand runners in Denmark concluded that people who jog regularly can expect to live five to six years longer on average than non-joggers. But is that because jogging truly is that beneficial, or is it because people who jog tend to lead healthy, moderate lives anyway and are bound to have improved outcomes over us more slothful types, with or without sweatpants?
What is certain is that in a few tens of years at most you will close your eyes forever and cease to move at all. So it might not be a bad idea to take advantage of movement, for health and pleasure, while you still can.
11 EQUILIBRIUM
Life is an endless chemical reaction.
—STEVE JONES
THE SURFACE LAW is not something most of us ever have to think about, but it explains a lot about you. The law states simply that as the volume of an object grows, its relative surface area decreases. Think of a balloon. When a balloon is empty, it is mostly rubber with a trivial amount of air inside. But blow it up and it becomes mostly air with a comparatively small amount of rubber on the outside. The more you inflate it, the more its interior dominates the whole.
Heat is lost at the surface, so the more surface area you have relative to volume, the harder you must work to stay warm. That means that little creatures have to produce heat more rapidly than large creatures. They must therefore lead completely different lifestyles. An elephant’s heart beats just thirty times a minute, a human’s sixty, a cow’s between fifty and eighty, but a mouse’s beats six hundred times a minute—ten times a second. Every day, just to survive, the mouse must eat about 50 percent of its own body weight. We humans, by contrast, need to consume only about 2 percent of our body weight to supply our energy requirements. One area where animals are curiously—almost eerily—uniform is with the number of heartbeats they have in a lifetime. Despite the vast differences in heart rates, nearly all animals have about 800 million heartbeats in them if they live an average life. The exception is humans. We pass 800 million heartbeats after twenty-five years, and just keep on going for another fifty years and 1.6 billion heartbeats or so. It is tempting to attribute this exceptional vigor to some innate superiority on our part, but in fact it is only over the last ten or twelve generations that we have deviated from the standard mammalian pattern thanks to improvements in our life expectancy. For most of our history, 800 million beats per lifetime was about the human average, too.
We could reduce our energy needs considerably if we elected to be cold-blooded. A typical mammal uses about thirty times as much energy in a day as a typical reptile, which means that we must eat every day what a crocodile needs in a month. What we get from this is an ability to leap out of bed in the morning, rather than having to bask on a rock until the sun warms us, and to move about at night or in cold weather, and just to be generally more energetic and responsive than our reptilian counterparts.
We exist within extraordinarily fine tolerances. Although our body temperature varies slightly through the day (it is lowest in the morning, highest in the late afternoon or evening), it normally doesn’t stray more than a degree or so from 98.6 degrees Fahrenheit. (That’s in adults. Children tend to run about one degree higher.) To move more than a very few degrees in either direction is to invite a lot of trouble. A fall of just two degrees below normal, or a rise of four degrees above, can tip the brain into a crisis that can swiftly lead to irreversible damage or death. To avoid catastrophe, the brain has its trusty control center, the hypothalamus, which tells the body to cool itself by sweating or to warm itself by shivering and diverting blood flow away from the skin and into the more vulnerable organs.
That may not seem a terribly sophisticated way of dealing with such a critical matter, but the body does it remarkably well. In one well-known experiment cited by the British academic Steve Jones, a test subject ran a marathon on a treadmill while the room temperature was gradually raised from minus 49 degrees Fahrenheit to 131 degrees Fahrenheit—roughly the limits of human tolerance at both extremes. Despite the subject’s exertions and the great range of temperatures, his core body temperature deviated by less than one degree over the course of the exercise.
That experiment largely recalled a series of experiments conducted more than two hundred years earlier for the Royal Society in London by Charles Blagden, a physician. Blagden built a heated chamber—essentially a walk-in oven—in which he and willing associates would stand for as long as they could bear it. Blagden managed ten minutes at a temperature of 198 degrees Fahrenheit. His friend the botanist Joseph Banks, freshly returned from circling the world with Captain James Cook and soon to become president of the Royal Society, managed 210 degrees Fahrenheit, but only for three minutes. “To prove that there was no fallacy in the degree of heat shewn by the thermometer,” Blagden recorded, “we put some eggs and a beef-steak upon a tin frame, placed near the standard thermometer….In about twenty minutes the eggs were taken out, roasted quite hard; and in forty-seven minutes the steak was not only dressed, but almost dry.” The experimenters also measured the temperature of their urine immediately before and after the test and found that it was unchanged despite the heat. Blagden additionally deduced that perspiration had a central role in cooling the body—his most important insight, and indeed his only lasting contribution to scientific knowledge.
Occasionally, as we all know, our body temperature is elevated beyond normal in the condition known as a fever. Curiously, no one knows quite why this happens—whether fevers are an innate defense mechanism aimed at killing invading pathogens or simply a by-product of the body working hard to fight off infection. The question is important because if fever is a defense mechanism, then any effort to suppress or eliminate it may be counterproductive. Allowing a fever to run its course (within limits, needless to say) could be the wisest thing. An increase of only a degree or so in body temperature has been shown to slow the replication rate of viruses by a factor of two hundred—an astonishing increase in self-defense from only a very modest rise in warmth. The trouble is, we don’t entirely understand what is going on with fevers. As Professor Mark S. Blumberg of the University of Iowa has put it, “If fever is such an ancient response to infection, one would think that the mechanism by which it benefits the host would be easy to determine. In fact, it has been difficult.”
If elevating our temperature a degree or two is so helpful at fending off invading microbes, then why not raise it permanently? The answer is that it is just too costly. If we were to raise our body temperature permanently by only 3–4 degrees Fahrenheit, our energy requirements would shoot up by about 20 percent. The temperature we have is a reasonable compromise between utility and cost, as with most things, and actually even normal temperature is pretty good at keeping microbes in check. Just look at how swiftly they swarm in and devour you when you die. That’s because your lifeless body falls to a delicious come-and-get-it temperature, like a pie left to cool on a windowsill.
The idea, incidentally, that we lose most of our heat through the top of our heads is, it seems, a myth. The top of your head accounts for no more than about 2 percent of your body surface area, and is, on most of us, pretty well insulated by hair, so the top of your head will never be a good radiator. On the other hand, it you are outdoors in cold weather and your head is the only part of you that is exposed, then it will play a disproportionate part in any heat loss, so listen to your mother when she tells you to put a hat on.
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Maintaining equilibrium within the body is called homeostasis. The man who coined the term and is often referred to as the father of the discipline was the Harvard physiologist Walter Bradford Cannon (1871–1945). A stocky man whose grim and stiff gaze in photographs belied an apparently warm and genial manner in person, Cannon was undoubtedly a genius, and part of that genius seems to have been an ability to persuade others to do rash and uncomfortable things in the name of science. Curious to understand why our stomachs gurgle when we are hungry, he persuaded a student named Arthur L. Washburn to train himself to overcome the gag reflex in order to push a rubber tube down his throat and into his stomach, where a balloon on its end could be inflated to measure the contractions when he was deprived of food. Washburn would then spend the day going about his normal business—attending classes, working in the lab, running errands—while the balloon uncomfortably expanded and collapsed and people stared at him for being the source of strange noises and having a tube coming out of his mouth.
Cannon persuaded other of his students to consume food while being X-rayed so that he could watch as it proceeded from mouth to esophagus and onward into the digestive system. In so doing, he became the first person to observe the actions of peristalsis—that is, the muscular pushing of food through the digestive tract. These and other novel experiments became the basis of Cannon’s classic text, Bodily Changes in Pain, Hunger, Fear, and Rage, which was the last word on physiology for years.
Cannon’s interests seemed to know no bounds. He became the world authority on the autonomic nervous system—that is, all those things the body does automatically, like breathe, pump blood, and digest food—and on blood plasma. He did groundbreaking research on the amygdala and hypothalamus, deduced the role of adrenaline in survival response (he coined the term “fight or flight”), developed the first effective treatments for shock, and even found time to write an authoritative and respectful paper on the practice of voodoo. In his spare time, he was an enthusiastic outdoorsman. A mountain peak in Montana, in what is now Glacier National Park, was named Mount Cannon in honor of him and his wife after they were the first to scale it, on their honeymoon in 1901. At the outbreak of World War I, he enlisted as a volunteer for the Harvard Hospital Unit, even though he was forty-five years old and the father of five children. He spent two years in Europe as a field doctor. In 1932, Cannon distilled practically all of his knowledge and years of research into a popular book, The Wisdom of the Body, outlining the body’s extraordinary ability to regulate itself. A Swede named Ulf von Euler followed up on Cannon’s studies into the fight-or-flight impulse in humans and won the Nobel Prize in Physiology or Medicine in 1970; Cannon himself was long dead by the time the importance of his work was fully appreciated, though he is now widely venerated retroactively.
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One thing Cannon didn’t understand—no one did yet—was what a staggering amount of energy the body requires at the cellular level in order to maintain itself. It took a very long time to figure that out, and when the answer came, it was provided not by some mighty research institute but by an eccentric Englishman working pretty much on his own in a pleasant country house in the west of England.
We now know that inside and outside the cell are charged particles called ions. Between them in the cell membrane is a kind of tiny air lock known as an ion channel. When the air lock is opened, the ions flow through, and that generates a little buzz of electricity—though “little” here is entirely a matter of perspective. Although each electrical twitch at the cellular level produces just one hundred millivolts of energy, that translates as thirty million volts per meter—about the same as in a bolt of lightning. Put another way, the amount of electricity going on within your cells is a thousand times greater than the electricity within your house. You are, in a very small way, exceedingly energetic.
It’s all a matter of scale. Imagine, for purposes of demonstration, firing a bullet into my abdomen. It really hurts and it does a lot of damage. Now imagine firing the same bullet into a giant fifty miles tall. It doesn’t even penetrate his skin. It’s the same bullet and gun, just a different scale. That’s more or less the situation with the electricity in your cells.
The stuff responsible for the energy in our cells is a chemical called adenosine triphosphate, or ATP, which may be the most important thing in your body you have never heard of. Every molecule of ATP is like a tiny battery in that it stores up energy and then releases it to power all the activities required by your cells—and indeed by all cells, in plants as well as animals. The chemistry involved is magnificently complex. Here is one sentence from a chemistry textbook explaining a little of what it does: “Being polyanionic and featuring a potentially chelatable polyphosphate group, ATP binds metal cations with high affinity.” For our purposes here it is enough to know that we are powerfully dependent on ATP to keep our cells humming. Every day you produce and consume your own body weight in ATP—some 200 trillion trillion molecules of it. From ATP’s point of view, you are really just a machine for producing ATP. Everything else about you is by-product. Because ATP is consumed more or less instantaneously, you have only sixty grams—that is a little over two ounces—of it within you at any given moment.
It took a long time to figure any of this out, and when it came, almost no one at first believed it. The person who discovered the answer was a retiring, self-funded scientist named Peter Mitchell who in the early 1960s inherited a fortune from the Wimpey house-building company and used it to set up a research center in a stately home in Cornwall. Mitchell was something of an eccentric. He wore shoulder-length hair and an earring at a time when that was especially unusual among serious scientists. He was also famously forgetful. At his daughter’s wedding, he approached another guest and confessed that she looked familiar, though he couldn’t quite place her.
“I was your first wife,” she answered.
Mitchell’s ideas were universally dismissed, not altogether surprisingly. As one chronicler has noted, “At the time that Mitchell proposed his hypothesis there was not a shred of evidence in support of it.” But he was eventually vindicated and in 1978 was awarded the Nobel Prize in Chemistry—an extraordinary accomplishment for someone who worked from a home lab. The eminent British biochemist Nick Lane has suggested that Mitchell should be as famous as Watson and Crick.
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The surface law also dictates how big we can get. As the British scientist and writer J. B. S. Haldane observed almost a century ago in a famous essay, “On Being the Right Size,” a human scaled up to the hundred-foot height of the giants of Brobdingnag in Gulliver’s Travels would weigh 280 tons. That would make him forty-six hundred times heavier than a normal-sized human, but his bones would be just three hundred times thicker, not nearly robust enough to support such a load. In a word, we are the size we are because that is about the only size we can be.
Body size has a great deal to do with how we are affected by gravity. It will not have escaped your notice that a small bug that falls off a tabletop will land unharmed and continue on its way unperturbed. That is because its small size (strictly, its surface area-to-volume ratio) means that it is scarcely affected by gravity. What is less well known is that the same thing applies, albeit on a different scale, to small humans. A child half your height who falls and strikes her head will experience only one thirty-second the force of impact that a grown person would feel, which is part of the reason that children so often seem to be mercifully indestructible.
