Survival of the Sickest: The Surprising Connections Between Disease and Longevity. Jonathan Prince

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СКАЧАТЬ same latitude as much of Siberia. But when the conveyor is disrupted – say, by a huge influx of warm fresh water melting off the Greenland ice sheet – it may have a significant impact on global climate and turn Europe into a very, very cold place.

      Just before the Younger Dryas, our European ancestors were doing pretty well. Tracing human migration through DNA, scientists have documented a population explosion in Northern Europe as populations that had once migrated north out of Africa now moved north again into areas of Europe that had been uninhabitable during the last ice age (before the Younger Dryas). The average temperature was nearly as warm as it is today, grasslands flourished where glaciers had once stood, and human beings thrived.

      And then the warming trend that had persisted since the end of the last ice age kicked rapidly into reverse. In just a decade or so, average yearly temperatures plunged nearly thirty degrees. Sea levels dropped by hundreds of feet as water froze and stayed in the ice caps. Forests and grasslands went into a steep decline. Coastlines were surrounded by hundreds of miles of ice. Icebergs were common as far south as Spain and Portugal. The great, mountainous glaciers marched south again. The Younger Dryas had arrived, and the world was changed.

      Though humanity would survive, the short-term impact, especially for those populations that had moved north, was devastating. In less than a generation, virtually every learned method of survival – from the shelters they built to the hunting they practiced – was inadequate. Many thousands of humans almost certainly froze or starved to death. Radiocarbon dating from archaeological sites provides clear evidence that the human population in Northern Europe went into a steep decline, showing a steep drop-off in settlements and other human activity.

      But humans clearly survived; the question is, how? Certainly some of our success was due to social adaptation – many scientists think that the Younger Dryas helped to spur the collapse of hunter-gatherer societies and the first development of agriculture. But what about biological adaptation and natural selection? Scientists believe some animals perfected their natural ability to survive cold spells during this period – notably the wood frog, which we’ll return to later. So why not humans? Just as the European population may have “selected” the hemochromatosis gene because it helped its carriers withstand the plague, might some other genetic trait have provided its carriers with superior ability to withstand the cold? To answer that, let’s take a look at the effect of cold on humans.

      Immediately upon his death in July 2002, baseball legend Ted Williams was flown to a spa in Scottsdale, Arizona, checked in, and given a haircut, a shave, and a cold plunge. Of course, this wasn’t your typical Arizona spa – this was the Alcor Life Extension cryonics lab, and Williams was checking in for the foreseeable future. According to his son, he hoped that future medical science might be able to restore him to life.

      Alcor separated Williams’s head from his body, drilled a couple of dime-size holes in it, and froze it in a bucket of liquid nitrogen at minus 320 degrees Fahrenheit. (His body got its own cold storage container.) Alcor brochures suggest that “mature nanotechnology” might be able to reanimate frozen bodies “perhaps by the mid-21st century,” but they also note that cryonics is a “last-in-first-out process wherein the first-in may have to wait a very long time.”

      Make that a very, very long time, like … never. Unfortunately for Williams and the other sixty-six superchilled cadavers at Alcor, human tissue doesn’t react well to freezing. When water is frozen, it expands into sharp little crystals. When humans are frozen, the water in our blood freezes, and the ice shards cut blood cells and cause capillaries to burst. It’s not dissimilar to the way a pipe bursts when the water’s left on in an unheated house – except no repairman can fix it.

      Of course, just because we can’t survive a true deep freeze doesn’t mean our bodies haven’t evolved many ways to manage the cold. They have. Not only is your body keenly aware of the danger cold poses, it’s got a whole arsenal of natural defenses. Think back to some time when you were absolutely freezing – standing still for hours on a frigid winter morning watching a parade, riding a ski lift with the wind whipping across the mountain. You start to shiver. That’s your body’s first move. When you shiver, the increased muscle activity burns the sugar stored in your muscles and creates heat. What happens next is less obvious, but you’ve felt the effect. Remember the uncomfortable combination of tingling and numbness in your fingers and toes? That’s your body’s next move.

      As soon as the body senses cold, it constricts the thin web of capillaries in your extremities, first your fingers and toes, then farther up your arms and legs. As your capillary walls close in, blood is squeezed out and driven toward your torso, where it essentially provides a warm bath for your vital organs, keeping them at a safe temperature, even if it means the risk of frostbite for your extremities. It’s natural triage – lose the finger, spare the liver.

      In people whose ancestors lived in particularly cold climates – like Norwegian fishermen or Inuit hunters – this autonomic response to cold has evolved with a further refinement. After some time in the cold, the constricted capillaries in your hands will dilate briefly, sending a rush of warm blood into your numbed fingers and toes before constricting again to drive the blood back into your core. This intermittent cycle of constriction and release is called the Lewis wave or “hunter’s response,” and it can provide enough warmth to protect your extremities from real injury, while still ensuring that your vital organs are safe and warm. Inuit hunters can raise the temperature in the skin of their hands from near freezing to fifty degrees in a mater of minutes; for most people it takes much longer. On the other hand, people descended from warm-weather populations don’t seem to have this natural ability to protect their limbs and their core at the same time. During the frigid cold of the Korean War, African American soldiers were much more prone to frostbite than other soldiers.

      Shivering and blood vessel constriction aren’t the only ways the body generates and preserves heat. A portion of the fat in newborns and some adults is specialized heat-generating tissue called brown fat, which is activated when the body is exposed to cold. When blood sugar is delivered to a brown fat cell, instead of being stored for future energy as it is in a regular fat cell, the brown fat cell converts it to heat on the spot. (For someone acclimated to very cold temperatures, brown fat can burn up to 70 percent more fat.) Scientists call the brown fat process nonshivering thermogenesis, because it’s heat creation without muscle movement. Shivering, of course, is only good for a few hours; once you exhaust the blood sugar stores in your muscles and fatigue sets in, it doesn’t work anymore. Brown fat, on the other hand, can go on generating heat for as long as it’s fed, and unlike most other tissues, it doesn’t need insulin to bring sugar into cells.

      Nobody’s written the Brown Fat Diet Book yet because it requires more than your usual lifestyle change. Adults who don’t live in extreme cold don’t really have much, if any, brown fat. To accumulate brown fat and get it really working, you need to live in extreme cold for a few weeks. We’re talking North Pole cold. And that’s not all – you’ve got to stay there. Once you stop sleeping in your igloo, your brown fat stops working.

      The body has one more response to the cold that’s not completely understood – but you’ve probably experienced it. When most people are exposed to cold for a while, they need to pee. This response has puzzled medical researchers for hundreds of years. It was first noted by one Dr. Sutherland, in 1764, who was trying to document the benefits of submersing patients in the supposedly healing – but cold – waters of Bath and Bristol, England. After immersing a patient who suffered from “dropsy, jaundice, palsy, rheumatism and inveterate pain in his back,” Sutherland noted that the patient was “pissing more than he drank.” Sutherland chalked the reaction up to external water pressure, figuring (quite wrongly) that fluid was simply being squeezed out of his patient, and it wasn’t until 1909 that researchers connected increased urine flow, or diuresis, to cold exposure.

      The leading explanation for cold diuresis – the need to pee when it’s cold – is still pressure; СКАЧАТЬ