The Body: A Guide for Occupants

by Bill Bryson

A cooked potato, for instance, is about twenty times more digestible than a raw one.

the macronutrients: water, carbohydrates, fat, and protein—were

Vitamins are simply organic chemicals—that is, from things that are or were once alive, like plants and animals—while minerals are inorganic and come from soil or water.

“vitamines,” a contraction of “vital” and “amines” (amines being a type of organic compound).

The term describes thirteen chemical oddments that we need to function smoothly but are unable to manufacture for ourselves.

Vitamin D, one of the most vital of all vitamins, can both be made in the body (where it really is a hormone) or be ingested (which makes it a vitamin again).

Choline, for instance, is a micronutrient you have probably never heard of. It has a central role in making neurotransmitters and keeping your brain running smoothly, but that has only been known since 1998.

Vitamin A is needed for vision, for healthy skin, and for fighting infection, so it is vital to have it.

Iron similarly is vital for healthy red blood cells.

Curiously, too much or too little iron both provide the same symptom, lethargy. “Too much iron in the form of supplements can accumulate in our tissues causing our organs literally to rust,” Leo Zacharski of Dartmouth-Hitchcock Medical Center in New Hampshire told New Scientist in 2014. “It’s a far stronger risk factor than smoking for all sorts of clinical disorders,” he added.

a vital electrolyte, which is particularly alarming because potassium helps to keep your heart beating smoothly and your blood pressure within tolerable limits.

Thanks to Pauling, to this day many people believe that taking a lot of vitamin C will help to get rid of a cold. It won’t.

Of all the many things we take in with our foods (salts, water, minerals, and so on), just three need to be altered as they proceed through the digestive tract: proteins, carbohydrates, and fats.

In simplest terms, a protein is a chain of amino acids.

Why evolution has wedded us to such a small number of amino acids is one of the great mysteries of biology.

It is a slightly strange fact that we break down all the proteins we consume in order to reassemble them into new proteins, rather as if they were Lego toys.

Protein deficiency is almost never a problem for people who eat meat, but it can be for vegetarians because not all plants provide all the necessary amino acids.

CARBOHYDRATES ARE COMPOUNDS of carbon, hydrogen, and oxygen, which are bound together to form a variety of sugars—glucose, galactose, fructose, maltose, sucrose, deoxyribose (the stuff found in DNA), and so on.

Virtually all carbohydrates in the diet come from plants, with one conspicuous exception: lactose, from milk.

That means that a 150-gram serving of white rice or a small bowl of cornflakes will have the same effect on your blood glucose levels as nine teaspoons of sugar.

THE THIRD MEMBER of the trio, fats, are also made up of carbon, hydrogen, and oxygen, but in different proportions. This has the effect of making fat easier to store. When fats are broken down in the body, they are teamed up with cholesterol and proteins in a new molecule called lipoproteins, which travel through the body via the bloodstream. Lipoproteins come in two principal types: high density and low density. Low-density lipoproteins are the ones frequently referred to as “bad cholesterol” because they tend to form plaque deposits on the walls of blood vessels. Cholesterol is not as fundamentally evil as we tend to think it. Indeed, it is vital to a healthy life. Most of the cholesterol in your body is locked up in your cells, where it is doing useful work. Just a small part—about 7 percent—floats about in the bloodstream. Of that 7 percent, one-third is “good” cholesterol and two-thirds is “bad.”

So the trick with cholesterol is not to eliminate it but to maintain it at a healthy level. One way to do so is to eat a lot of fiber, or roughage. Fiber is the material in fruits, vegetables, and other plant foods that the body cannot fully break down. It contains no calories and no vitamins, but it helps to lower cholesterol and slows the rate at which sugar gets into the bloodstream and is then turned into fat by the liver, among many other benefits.

The upshot of all this is that the human body is designed to take in fuel, use what it needs, and store the rest to call on later as required.

Depending on where the fat ends up, it is known as subcutaneous (beneath the skin) or visceral (around the belly).

“Saturated fat” sounds greasy and unhealthy, but in fact it is a technical description of carbon-hydrogen bonds rather than how much of it runs down your chin when you bite into it.

Even more invidious are trans fats, an artificial form of fat made from vegetable oils.

Also known as hydrogenated oils, trans fats are much worse for your heart than any other kind of fat. They raise levels of bad cholesterol, lower levels of good cholesterol, and damage the liver.

“Trans fats are essentially a form of slow-acting poison.”

The conviction that we should all drink eight glasses of water a day is the most enduring of dietary misunderstandings.

One other enduring myth concerning water intake is the belief that caffeinated drinks are diuretics and make you pee out more than you have taken in.

Drinking too much water can actually be dangerous. Normally, your body manages fluid balance very well, but occasionally people take in so much water that the kidneys cannot get rid of it fast enough and they end up dangerously diluting the sodium levels in their blood, setting off a condition known as hyponatremia.

Keys found a direct correlation between levels of dietary fat and heart disease—a conclusion that is hardly surprising now but was revolutionary then.

Keys devoted himself to a Mediterranean-style diet long before anyone had heard of the term and lived to be a hundred. (He died in 2004.)

These days the most frequently cited culprit for dietary concern is sugar.

Heinz ketchup is almost one-quarter sugar. It has more sugar per unit of volume than Coca-Cola.

Coconut oil may be tasty, but it is no better for you than a big scoop of deep-fried butter.

Salt is vital to us. There is no question of that. We would die without it. That’s why we have taste buds devoted exclusively to it. Lack of salt is nearly as dangerous to us as lack of water. Because our bodies cannot produce salt, we must consume it in our diets. The problem is in determining how much is the right amount. Take too little and you grow lethargic and weak, and eventually you die. Take too much and your blood pressure soars and you run the risk of heart failure and stroke.

“In principle, it’s really pretty simple,” he says. “We should eat less added sugar, less refined grain, and more vegetables. It’s essentially a question of trying to eat mostly good things and avoiding mostly bad things. You don’t need a PhD for that.”

Just because you exercise regularly and eat a lot of salad doesn’t mean you have bought yourself a better life span. What you have bought is a better chance of having a better life span.

Roughly speaking, however, each meal you eat spends about four to six hours in the stomach, a further six to eight hours in the small intestine, where all that is nutritious (or fattening) is stripped away and dispatched to the rest of the body to be used or, alas, stored, and up to three days in the colon, which is essentially a large fermentation tank where billions and billions of bacteria pick over whatever the rest of the intestines couldn’t manage—fiber mostly. That’s why you are constantly told to eat more fiber: because it keeps your gut microbes happy and at the same time, for reasons not well understood, reduces the risk of heart disease, diabetes, bowel cancer, and indeed death of all types.

The stomach is less vital than you might think.

Many people have had their stomachs removed without serious consequence.

The rumblings of your gut, incidentally, come mostly from the large intestine, not the stomach.

One thing the stomach does do is kill off many microbes, by soaking them in hydrochloric acid. “Without your stomach, a lot more of what you ate would make you ill,”

“People tend to blame the last thing they ate, but it’s probably the thing before the last thing they ate.”

SLEEPING IS THE most mysterious thing we do. We know that it is vital; we just don’t know exactly why.

Sleep is clearly about more than just resting.

Hibernating is more like being concussed or anesthetized: the subject is unconscious but not actually asleep.

Whatever sleep gives us, it is more than just a period of recuperative inactivity.

“If sleep does not serve an absolutely vital function, then it is the biggest mistake the evolutionary process has ever made.”