LITMIR.BIZ

Скачать книгу

imagine that some small group of people had a different response to the cold. Faced with year-round frigid temperatures, their insulin supply slowed, allowing their blood sugar to rise somewhat. As in the wood frog, this would have lowered the freezing point of their blood. They urinated frequently, to keep internal water levels low. (A recent U.S. Army study shows there is very little harm caused by dehydration in cold weather.) Suppose these people used their brown fat to burn that over-supply of sugar in their blood to create heat. Perhaps they even produced additional clotting factor to repair tissue damage caused by particularly deep cold snaps. It’s not hard to imagine that these people might have had enough of an advantage over other humans, especially if, like the wood frog, the spike in sugar was only temporary, to make it more likely that they would survive long enough to reach reproductive age.

      There are tantalizing bits of evidence to bolster the theory.

      When rats are exposed to freezing temperatures, their bodies become resistant to their own insulin. Essentially, they become what we would call diabetic in response to the cold.

      In areas with cold weather, more diabetics are diagnosed in colder months; in the Northern Hemisphere, that means more diabetics are diagnosed between November and February than between June and September.

      Children are most often diagnosed with Type 1 diabetes when temperatures start to drop in late fall.

      Fibrinogen, the clotting factor that repairs ice-damaged tissue in the wood frog, also mysteriously peaks in humans during winter months. (Researchers are taking note – that may mean that cold weather is an important, but underappreciated, risk factor for stroke.)

      A study of 285,705 American veterans with diabetes measured seasonal differences in their blood sugar levels. Sure enough, the veterans’ blood sugar levels climbed dramatically in the colder months and bottomed out during the summer. More telling, the contrast between summer and winter was even more pronounced in those who lived in colder climates, with greater differences in seasonal temperature. Diabetes, it seems, has some deep connection to the cold.

      We don’t know enough today to state with certainty that the predisposition to Type 1 or Type 2 diabetes is related to human cold response. But we do know that some genetic traits that are potentially harmful today clearly helped our ancestors to survive and reproduce (hemochromatosis and the plague, for example). So while it’s tempting simply to question how a condition that can cause early death today could ever confer a benefit, that doesn’t look at the whole picture.

      Remember, evolution is amazing – but it isn’t perfect. Just about every adaptation is a compromise of sorts, an improvement in some circumstances, a liability in others. A peacock’s brilliant tail feathers make him more attractive to females – and attract more attention from predators. Human skeletal structure allows us to walk upright and gives us large skulls filled with big brains – and the combination means an infant’s head can barely make it through its mother’s birth canal. When natural selection goes to work, it doesn’t favor adaptations that make a given plant or animal “better” – just whatever it takes for it to increase the chances for survival in its current environment. And when there’s a sudden change in circumstances that threatens to wipe out a population – a new infectious disease, a new predator, or a new ice age – natural selection will make a beeline for any trait that improves the chance of survival.

      “Are they kidding?” said one doctor when told of the diabetes theory by a reporter. “Type 1 diabetes would result in severe ketoacidosis and early death.”

      Sure – today.

      But what if a temporary diabetes-like condition occurred in a person who had significant brown fat living in an ice age environment? Food would probably be limited, so dietary bloodsugar load would already be low, and brown fat would convert most of that to heat, so the ice age “diabetic’s” blood sugar, even with less insulin, might never reach dangerous levels. Modern-day diabetics, on the other hand, with little or no brown fat, and little or no exposure to constant cold, have no use – and thus no outlet – for the sugar that accumulates in their blood. In fact, without enough insulin the body of a severe diabetic starves no matter how much he or she eats.

      The Canadian Diabetes Association has helped to fund Ken Storey’s study of the incredible freezing frog. It understands that just because we haven’t definitively linked diabetes and the Younger Dryas doesn’t mean we shouldn’t explore biological solutions to high blood sugar found elsewhere in nature. Coldtolerant animals like the wood frog exploit the antifreezing properties of high blood sugar to survive. Perhaps the mechanisms they use to manage the complications of high blood sugar will help lead us to new treatments for diabetes. Plants and microbes adapted to extreme cold might produce molecules that could do the same.

      Instead of dismissing connections, we need to have the curiosity to pursue them. And in the case of diabetes, sugar, water, and cold, there are clearly plenty of connections to pursue.

       Chapter Three THE CHOLESTEROL ALSO RISES

      Everybody knows that humanity’s relationship with the sun is multifaceted. As we all learned in primary school, almost the entire global ecology of our planet depends on sufficient sunlight – beginning with the production of oxygen by plants through photosynthesis, without which we wouldn’t have food to eat or air to breathe. And as we all have learned more and more over the last couple of decades, too much sun can be a bad thing on a global level and an individual one, throwing our environment into chaos by causing drought or causing deadly skin cancer.

      But most people don’t know that the sun is just as important on an individual, biochemical level – and the relationship is just as two-sided. Natural sunlight simultaneously helps your body to create vitamin D and destroys your body’s reserves of folic acid – both of which are essential to your health. To manage this can’t-live-with-you-can’t-live-without-you relationship, different populations have evolved a combination of adaptations that, together, help to protect folic acid and ensure sufficient vitamin D production.

      Vitamin D is a critical component of human biochemistry, especially to ensure the growth of healthy bones in children and the maintenance of healthy bones in adults. It ensures that our blood has sufficient levels of calcium and phosphorus. New research is discovering that it’s also crucial to the proper function of the heart, the nervous system, the clotting process, and the immune system.

      Without enough vitamin D, adults are prone to osteoporosis and children are prone to a disease called rickets that results in improper bone growth and deformity. Vitamin D deficiencies have also been shown to play a role in the development of dozens of diseases – everything from many different cancers to diabetes, heart disease, arthritis, psoriasis, and mental illness. Once the link between vitamin D and rickets was established early in the twentieth century, American milk was fortified with vitamin D, all but eliminating the disease in America.

      We don’t have to rely on fortified milk for vitamin D, however. Unlike most vitamins, vitamin D can be made by the body itself. (Generally speaking, a vitamin is an organic compound that an animal needs to survive but can usually obtain only from outside the body.) We make vitamin D by converting something else that, like the sun, has been getting a bad rap lately, but is 100 percent necessary for survival – cholesterol.

      Cholesterol is required to make and maintain cell membranes. It helps the brain to send messages and the immune system to protect us against cancer and other diseases. It’s a key building block in the production of estrogen and testosterone and other hormones. And it is the essential component in our manufacture of vitamin D through a chemical process that is similar to photosynthesis in its dependence on the sun.

      When we are exposed to the right kind of sunlight, our skin converts cholesterol to vitamin D. The sunlight necessary for this process is ultraviolet B, or UVB, which typically is strongest when the sun is more or less directly overhead – for a few hours every day beginning around noon. In parts of the world that are farther from the equator, very little UVB reaches the earth during winter months. Fortunately, the body is so efficient

Скачать книгу