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.