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short, they worked hard for what they ate and consequently ended up with bodies designed to do two somewhat contradictory things: to be active much of the time, but never to be more active than absolutely necessary. As Daniel Lieberman explains, “If you want to understand the human body, you have to understand that we evolved to be hunter-gatherers. That means being prepared to expend a lot of energy to acquire food, but not wasting energy when you don’t need to.” So exercise is important, but rest is vital, too. “For one thing,” Lieberman says, “you can’t digest food while
Because our ancient ancestors had to survive lean times as well as good, they evolved a tendency to store fat as a fuel reserve—a survival reflex that is now, all too often, killing us. The upshot is that millions of us spend our lives struggling to maintain a balance between paleolithically designed bodies and modern gustatory excess. It’s a battle too many of us are losing. Nowhere
Overweight is defined as a body mass index (BMI) of 25 to 30, and obesity as anything above that. BMI is a person’s weight in kilograms divided by the square of their height in meters. The Centers for Disease Control has a very handy BMI calculator, which allows you to determine your BMI instantaneously by
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?
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
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
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
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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.
Maintaining equilibrium within the body is called homeostasis.
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
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
includes a lot of things that we don’t usually think of in the context of immunity, like earwax, skin, and tears. Any invader that gets past these outer defenses—and comparatively few do—will
Interleukin-1, for instance, not only attacks pathogens but also plays a role in sleep, which may go some way to explaining why we are so often drowsy when unwell. By one calculation, we have some
that a very high proportion of the suffering we do is inflicted on us by our own defenses in the form of autoimmune diseases like multiple sclerosis, lupus, rheumatoid arthritis, Crohn’s disease, and many unappealing others. Altogether about 5 percent of us suffer from some form of autoimmune disease—a very high proportion for such an uncomfortable range of afflictions—and the numbers are growing faster than our abilities to treat them effectively. “You could look at it and
Lymphocytes are of two principal types: B cells and T cells. The B in B cells comes, a little oddly, from “bursa of Fabricius,” an appendix-like organ in birds where B cells were first seen.*1 Humans and
What Miller established was that the thymus is a nursery for T cells. T cells are a kind of elite corps in the immune system, and the dead cells found in the thymus were lymphocytes that had failed to pass muster because they were either not very good at identifying
cells subdivide into two further categories: helper T cells and killer T cells. Killer T cells, as the name suggests, kill cells that have been invaded by pathogens. Helper T cells help other immune cells act, including helping B cells produce antibodies. Memory T cells remember the details of earlier invaders and are therefore able to coordinate a swift response if the same pathogen shows up again—what is known as adaptive immunity. Memory T
Inflammation is essentially the heat of battle as the body defends itself from damage. Blood vessels in the vicinity of an injury dilate, allowing more blood to flow to the site, bringing with it white blood cells to fight off invaders. That causes the site to swell, increasing the pressure on surrounding nerves, resulting in tenderness. Unlike red blood cells, white blood cells can leave the circulatory system to pass through surrounding tissues, like an army patrol searching through jungle. When they encounter an invader,
It’s not the infection that makes you feel dreadful, but your body defending itself. The pus that seeps from a wound is simply dead white cells that have given their lives in defense of you. Inflammation is a tricky thing. Too much and it destroys neighboring tissues and can result in unnecessary pain, but too little and it fails to stop infection.
immune system gets so ramped up that it brings out all its defenses and fires all its missiles in what is known as a cytokine storm. That’s what kills you. Cytokine storms show up again and again in many pandemic
second mouse.
wasn’t until the development of the miracle drug cyclosporine from a soil sample fortuitously collected on a Norwegian holiday (as you will recall from chapter 7) that transplants could start to become
Today in the United States, of the 30,000 people who receive an organ transplant each year, over 95 percent are still alive twelve months later and 80 percent are alive five years later. The downside is that demand for replacement organs far outstrips supply. As of late 2018, 114,000 people were on transplant waiting lists in the United States. A new person joins the list every ten minutes,
on dialysis live an extra eight years on average, but that rises to twenty-three years with a transplant. About a third of kidney
types of autoimmune diseases, and the numbers are rising. Take Crohn’s disease, the increasingly common inflammatory bowel disease. Before 1932, when Burrill Crohn, a New York physician, described it in a paper in The Journal of the American Medical Association, it wasn’t even a recognized condition. At that time, Crohn’s affected one person in 50,000. Then it became one in 10,000, then one in 5,000. Today the proportion is one in 250 and still rising.*2 Why this has happened
Women are twice as likely as men to get multiple sclerosis, ten times more likely to get lupus, fifty times more likely to suffer a thyroid condition known as Hashimoto’s thyroiditis. Altogether, 80 percent of all autoimmune diseases occur in women. Hormones are the presumed culprit, but how exactly female hormones trip up the immune system when male hormones don’t is not at all clear. The
