The body, p.26
The Body, page 26
In nature, we actually starve pretty easily. We are incapable of deriving nutrition from most parts of most plants. In particular we cannot make use of cellulose, which is what plants primarily consist of. The few plants that we can eat are the ones we know as vegetables. Otherwise we are limited to eating a few botanical end products, such as seeds and fruits, and even many of those are poisonous to us. But we can benefit from a lot more foods by cooking them. A cooked potato, for instance, is about twenty times more digestible than a raw one.
Cooking frees up a lot of time for us. Other primates spend as many as seven hours a day just chewing. We don’t need to eat constantly to ensure our survival. Our tragedy, of course, is that we eat more or less constantly anyway.
The fundamental components of the human diet—the macronutrients: water, carbohydrates, fat, and protein—were recognized nearly two hundred years ago by an English chemist named William Prout, but it was even then clear that some other, more elusive elements were needed to produce a fully healthy diet. No one knew for the longest time exactly what these elements were, but it was evident that in their absence people were likely to suffer a deficiency disease like beriberi or scurvy.
We now know them, of course, as vitamins and minerals. 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. Altogether there are about forty of these little particles that we must get from our foods because we cannot manufacture them for ourselves.
Vitamins are a surprisingly recent concept. A little over four years after Wilbur Atwater died, a Polish émigré chemist in London, Casimir Funk, came up with the notion of vitamins, though he called them “vitamines,” a contraction of “vital” and “amines” (amines being a type of organic compound). As it turned out, only some vitamins are amines, so the name was later shortened. (Other names were also tried, among them nutramines, food hormones, and accessory food factors, but failed to catch on.) Funk didn’t discover vitamins but merely speculated, correctly, as to their existence. But because no one could produce these strange elements, many authorities refused to accept their reality. Sir James Barr, president of the British Medical Association, dismissed them as “a figment of the imagination.”
The discovery and naming of vitamins didn’t begin until almost the 1920s and has been a checkered affair, to put it mildly. In the beginning, vitamins were named in more or less strict alphabetical order—A, B, C, D, and so on—but then the system began to fall apart. Vitamin B was discovered to be not one vitamin but several, and these were renamed B1, B2, B3, and so on up to B12. Then it was decided that the B vitamins weren’t so diverse after all, so some were eliminated and others reclassified, so that today we are left with six semi-sequential B vitamins: B1, B2, B3, B5, B6, and B12. Other vitamins came and went, so that the scientific literature is filled with a lot of what might be called ghost vitamins—M, P, PP, S, U, and several others. In 1935, a researcher in Copenhagen, Henrik Dam, discovered a vitamin that was central to blood coagulation and called it vitamin K (for the Danish koagulere). The next year, some other researchers came up with vitamin P (for “permeability”). The process hasn’t entirely settled down yet. Biotin, for instance, was for a time called vitamin H, but then became B7. Today it is mostly just called biotin.
Although Funk coined the term “vitamines,” and is thus often given credit for their discovery, most of the real work of determining the chemical nature of vitamins was done by others, in particular Sir Frederick Hopkins, who was awarded the Nobel Prize for his work in 1929—a fact that left Funk permanently in one.
Even today vitamins are an ill-defined entity. The term describes thirteen chemical oddments that we need to function smoothly but are unable to manufacture for ourselves. Though we tend to think of them as closely related, they mostly have little in common apart from being useful to us. They are sometimes described as “hormones made outside the body,” which is a pretty good definition except that it is only partly true. 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).
A good deal of what we know about vitamins and their mineral cousins is surprisingly recent. 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. It is abundant in foods that we don’t generally eat a lot of—liver, Brussels sprouts, and lima beans, for instance—which doubtless explains why it is thought that some 90 percent of us have at least a moderate choline deficiency.
In the case of many micronutrients, scientists don’t know quite how much you need or even what they do for you when you get them. Bromine, for instance, is found throughout the body, but nobody is sure if it is there because the body needs it or is just a kind of accidental passenger. Arsenic is an essential trace element for some animals, but we don’t know if that includes humans. Chromium is definitely needed, but in such small amounts that it becomes toxic quite quickly. Chromium levels fall steadily as we age, but no one knows why they fall or what this indicates.
For nearly all vitamins and minerals, the risk of taking in too much is as great as the risk of getting too little. Vitamin A is needed for vision, for healthy skin, and for fighting infection, so it is vital to have it. Luckily, it is abundant in many common foods, like eggs and dairy products, so it’s easy to get more than enough. But there’s the rub. The recommended daily level is seven hundred micrograms for women and nine hundred for men; the upper limit for both is about three thousand micrograms, and exceeding that regularly can become risky. How many of us could begin to guess even roughly how close we are to getting the balance right? Iron similarly is vital for healthy red blood cells. Too little iron and you become anemic, but too much is toxic, and there are some authorities who believe that quite a number of people may be getting too much of it. 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.
In 2013, an editorial in the highly respected Annals of Internal Medicine, based on a study led by researchers at Johns Hopkins University, said that nearly everyone in high-income countries was sufficiently well nourished not to require vitamins or other health supplements and that we should stop wasting our money on them. The report came in for some swift and withering criticism, however.
Professor Meir Stampfer of the Harvard Medical School said it was regrettable that “such a poorly done paper would be published in a prominent journal.” According to the Centers for Disease Control, far from having plenty in our diet, some 90 percent of American adults don’t get the recommended daily dose of vitamins D and E and about half don’t get sufficient vitamin A. No less than 97 percent, according to the CDC, don’t get enough potassium, a vital electrolyte, which is particularly alarming because potassium helps to keep your heart beating smoothly and your blood pressure within tolerable limits. Having said that, there is often disagreement over what precisely we do need. In America, the daily recommended dose of vitamin E is fifteen milligrams, for instance, but in the U.K. it is three to four milligrams—a very considerable difference.
What can be said with some confidence is that many people have a faith in health supplements that goes some way beyond the fully rational. Americans can choose from among a truly staggering eighty-seven thousand different dietary supplements and we spend a no less impressive $40 billion a year on them.
The greatest of vitamin controversies was stirred up by the American chemist Linus Pauling (1901–94), who had the distinction of winning not one but two Nobel Prizes (for chemistry in 1954 and for peace eight years later). Pauling believed that massive doses of vitamin C were effective against colds, flu, and even some cancers. He took up to forty thousand milligrams of vitamin C daily (the recommended daily dose is sixty milligrams) and maintained that his large intake of vitamin C had kept his prostate cancer at bay for twenty years. He had no evidence for any of his claims, and all have been pretty well discredited by subsequent studies. 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.
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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. Let’s look at them in turn.
PROTEINS
PROTEINS ARE COMPLICATED molecules. About a fifth of our body weight is made up of them. In simplest terms, a protein is a chain of amino acids. About a million different proteins have been identified so far, and nobody knows how many more are to be found. They are all made from just twenty amino acids, even though hundreds of amino acids exist in nature that could do the job just as well. Why evolution has wedded us to such a small number of amino acids is one of the great mysteries of biology. For all their importance, proteins are surprisingly ill-defined. Although all proteins are made from amino acids, there is no accepted definition as to how many amino acids you need in a chain to qualify as a protein. All that can be said is that a small but unspecified number of amino acids strung together is a peptide. Ten or twelve strung together is a polypeptide. When a polypeptide begins to get bigger than that, it becomes, at some ineffable point, a protein.
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. Eight of the twenty amino acids cannot be made in the body and must be consumed in the diet.*2 If they are missing from the foods we eat, then certain vital proteins cannot be made. 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. It is interesting that most traditional diets in the world are based around combinations of plant products that do provide all the necessary amino acids. So people in Asia eat a lot of rice and soybeans, while indigenous Americans have long combined corn with black or pinto beans. This isn’t just a matter of taste, it seems, but an instinctive recognition of the need for a rounded diet.
CARBOHYDRATES
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. Some of these are chemically complex and known as polysaccharides, some are simple and known as monosaccharides, and some are in between and known as disaccharides. Although all are sugars, not all are sweet. Some, like the starches found in pasta and potatoes, are too big to activate the tongue’s sweet detectors. Virtually all carbohydrates in the diet come from plants, with one conspicuous exception: lactose, from milk.
We eat a lot of carbohydrates, but we use them up quickly, so the total amount in your body at any given time is modest—usually less than a pound. The main thing to bear in mind is that carbohydrates, upon being digested, are just more sugar—often quite a lot more. 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.
FATS
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.
Carbohydrates and fats are the principal fuel reserves of the body, but they are stored and used in different ways. When the body needs fuel, it tends to burn up the available carbohydrates and store any spare fat. The main point to bear in mind—and you are no doubt well aware of it each time you take your shirt off—is that the human body likes to hold on to its fat. It burns some of the fat we consume for energy, but a good deal of the rest is sent off to tens of billions of tiny storage terminals called adipocytes, which exist all over the body. 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. That makes it possible for us to be active for hours at a time without eating. Your body below the neck doesn’t do a lot of complicated thinking, and it is only too happy to hold on to any surplus fat you give it. It even rewards you for overeating with a lovely feeling of well-being.
Depending on where the fat ends up, it is known as subcutaneous (beneath the skin) or visceral (around the belly). For complex chemical reasons, visceral fat is much worse for you than the subcutaneous kind.
Fat comes in several varieties. “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. As a rule, animal fats tend to be saturated and vegetable fats to be unsaturated, but there are many exceptions, and you can’t tell by looking whether a food is high in saturated fat or not. Who would guess, for instance, that an avocado has five times as much saturated fat as a small bag of potato chips? Or that a large latte has more than almost any pastry? Or that coconut oil is almost nothing but saturated fat?
Even more invidious are trans fats, an artificial form of fat made from vegetable oils. Invented by a German chemist in 1902, they were long thought of as a healthy alternative to butter or animal fat, but we now know the opposite to be true. 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. As Daniel Lieberman has rather chillingly put it, “Trans fats are essentially a form of slow-acting poison.”
As early as the mid-1950s, Fred A. Kummerow, a biochemist at the University of Illinois, reported clear evidence of a link between high intake of trans fats and clogged coronary arteries, but his findings were widely dismissed, particularly with the influence of lobbying by the food processing industry. Not until 2004 did the American Heart Association finally accept that Kummerow was right, and not until 2015—almost sixty years after Kummerow first reported the dangers—did the Food and Drug Administration finally decree trans fats unsafe to eat. Despite their known dangers, it remained legal to add them to foods in America until July 2018.
Finally, we should say a word or two about the most vital of our macronutrients: water. We consume about two and a half quarts of water a day, though we are not generally aware of it because about half is contained within our foods. The conviction that we should all drink eight glasses of water a day is the most enduring of dietary misunderstandings. The idea has been traced to a 1945 paper from the U.S. Food and Nutrition Board, which noted that that was the amount that the average person consumed in a day. “What happened,” Dr. Stanley Goldfarb of the University of Pennsylvania told the BBC radio program More or Less in 2017, “was that people sort of confused the idea that this was the required intake. And the other confusion that occurred was then people said that it is not so much that you should take in eight ounces eight times a day, but that you should consume that in addition to whatever fluid you consume in association with your diet and your meals. And there was never any evidence for that.”
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. They may not be the most wholesome of options for liquid refreshment, but they do make a net contribution to your personal water balance. Thirst, curiously, is not a reliable indication of how much water you need. People allowed to drink all the water they want after getting very thirsty usually report feeling slaked after drinking only one-fifth the amount they have lost through perspiration.
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. In 2007, a young woman in California named Jennifer Strange died after drinking six quarts of water in three hours in a clearly ill-judged water-drinking competition held by a local radio station. Similarly in 2014, a high school football player in Georgia, complaining of cramps after practice, downed two gallons of water and two of Gatorade and soon afterward fell into a coma and died.
