Asimov's New Guide to Science

by Isaac Asimov

  The human body is rather specialized (as organisms go) in its dietary needs. A plant can live on just carbon dioxide, water, and certain inorganic ions. Some of the microorganisms likewise get along without any organic food; they are called autotrophic (“self-feeding”), which means that they can grow in environments in which there is no other living thing. The bread mold Neurospora begins to get a little more complicated: in addition to inorganic substances, it has to have sugar and the vitamin biotin. And as the forms of life become more and more complex, they seem to become more and more dependent on their diet to supply the organic building blocks necessary for building living tissue. The reason is simply that they have lost some of the enzymes that primitive organisms possess. A green plant has a complete supply of enzymes for making all the necessary amino acids, proteins, fats, and carbohydrates from inorganic materials. Neurospora has all the enzymes except one or more of those needed to make sugar and biotin. By the time we get to humans, we find that they lack the enzymes required to make many of the amino acids, the vitamins, and various other necessities, and thus must get these ready-made in food.

  This may seem a kind of degeneration—a growing dependence on the environment which puts the organism at a disadvantage. Not so. If the environment supplies the building blocks, why should the cell carry the elaborate enzymatic machinery needed to make them? By dispensing with this machinery, the cell can use its energy and space for more refined and specialized purposes.

  For human beings (or other animals) to get the food they need, they must depend on physically ingesting other organisms. It is the organic constituents of those organisms that make up food. In the intestines of the eater, the small molecules of the eaten can be absorbed directly. The large molecules of starch, protein and so on are broken down, or digested, by means of enzyme action; and the fragments (amino acids, glucose, and so on) are absorbed. Within the eater’s body, those fragments are further broken down for the production of energy, or put together again to form the large molecules characteristic of the eater, rather than of the eaten. In a sense, animal life is an endless burglary-by-force.

  Some animals are carnivorous and eat only other animals. If all animals ate in this fashion, animal life would not long endure, for the transfer of energy and of tissue components from the eaten to the eater is very inefficient. As a general rule of thumb, it takes 10 pounds of the eaten to support 1 pound of the eater.

  There are animals that are herbivorous and eat plants. Plant life is much more common than animal life, so that the total mass of herbivorous animals is far higher than that of carnivorous animals, and the former can better support the latter. (Some animals—like human beings, bears, and swine—are omnivorous and eat both plants and animals.)

  The transfer of energy and of tissue components from plants to the animals that eat them is also highly inefficient, and life would soon dwindle to nothing if plants could not somehow renew themselves as fast as they were eaten. This they do by making use of solar energy in the process of photosynthesis (see chapter 12). In this way, plants live on the nonliving and keep virtually all of life going—and have been doing so for all the time they have existed.

  To be sure photosynthesis is even less efficient than the eating processes of animals. It is estimated that less than one-tenth of 1 percent of all the solar energy that bathes the earth is trapped by plants and converted into tissue, but this is still enough to produce between 150 billion and 200 billion tons of dry organic matter every year the world over. Naturally, this process can only last on Earth in its present form as long as the sun remains in essentially its present form—a matter of some billions of years.

  THE ORGANIC FOODS

  If energy were all that were required of food, we would not really need very much food. Half a pound of butter would supply me with all I would need for a day’s worth of energy at my sedentary occupation. However, food does not supply energy alone but is also a source of the building blocks I need to repair and rebuild my tissues, and these are found in a wide variety of places. Butter alone would not supply my needs in those respects.

  It was the English physician William Prout (the same Prout who was a century ahead of his time in suggesting that all the elements are built from hydrogen) who first suggested that the organic foods could be divided into three types of substances, later named carbohydrates, fats, and proteins.

  The chemists and biologists of the nineteenth century, notably Justus von Liebig of Germany, gradually worked out the nutritive properties of these foods. Protein, they found, is the most essential, and the organism could get along on it alone. The body cannot make protein from carbohydrate and fat, because these substances have no nitrogen, but it can make the necessary carbohydrates and fats from materials supplied by protein. Since protein is comparatively scarce in the environment, however, it would be wasteful to live on an all-protein diet—like stoking the fire with furniture when firewood is available.

  Throughout history, and in many places even today, people have difficulty getting enough food. Either there is literally insufficient food to go around, as during the famine that follows a bad harvest; or else there are flaws in the distribution, either physical or economic, so that there are people who cannot obtain the food or who cannot afford to buy it.

  Even when there seems to be enough food to chew on, the protein content may be too low, so that though there is no undernutrition in the broad sense of the word, there is malnutrition. Children, especially, may suffer from protein deficiency, since they need protein not merely for replacement but for the building of new tissue, for growth. In Africa, such protein deficiency is particularly common among children forced to exist on a monotonous diet of cornmeal. (Any monotonous diet is dangerous, for few items of food have everything one needs. In variety, there is safety.)

  There have always been a minority of people who can eat freely, and who therefore do and take in more of everything than they need. The body stores the excess as fat (the most economical way of packing calories into as little space as possible), and this is useful in many ways—as a store of calories to tide one over during a period when little food is available. If, however, there are no such periods, the fat remains, and a person is overweight or, in the extreme, obese. This state has its evils and discomforts and is associated with a greeater incidence of degenerative and metabolic diseases, such as diabetes, atherosclerosis, and so on. (And even overweight does not ensure against deficiencies in needed nutrients if one’s diet is not properly balanced.)

  In a country like the United States, where the standard of living is unusually high, and where fatness is considered unesthetic, overweight is a serious problem. The only rational way to prevent this is to cut down food intake or increase activity (or both), and people who refuse to do either are doomed to remain overweight, regardless of what tricks they try.

  PROTEINS

  On the whole, foods high in protein tend to be more expensive and in shorter supply (those two characteristics usually go together) than those low in protein. In general, animal food tends to be higher in protein than plant food does.

  This creates a problem for those human beings who choose to be vegetarians. While vegetarianism has its points, those practicing it have to try a little harder to make sure they maintain an adequate protein intake. It can be done, for as little as 2 ounces of protein per day might be enough for the average adult. Children and pregnant or nursing mothers need somewhat more.

  Of course, a lot depends on the proteins you choose. Nineteenth-century experimenters tried to find out whether the population could get along, in times of famine, on gelatin—a protein material obtained by heating bones, tendons, and other otherwise inedible parts of animals. But the French physiologist Francois Magendie demonstrated that dogs lost weight and died when gelatin was their sole source of protein. This does not mean there is anything wrong with gelatin as a food, but it simply does not supply all the necessary building blocks when it is the only protein in the diet. Again,
in variety lies safety.

  The key to the usefulness of a protein lies in the efficiency with which the body can use the nitrogen it supplies. In 1854, the English agriculturists John Bennet Lawes and Joseph Henry Gilbert fed pigs protein in two forms—lentil meal and barley meal. They found that the pigs retained much more of the nitrogen in barley than of that in lentils. These were the first nitrogen balance experiments.

  A growing organism gradually accumulates nitrogen from the food it ingests (positive nitrogen balance). If it is starving or suffering a wasting disease, and gelatin is the sole source of protein, the body continues to starve or waste away, from a nitrogen-balance standpoint (a situation called negative nitrogen balance). It keeps losing more nitrogen than it takes in, regardless of how much gelatin it is fed.

  Why so? The nineteenth-century chemists eventually discovered that gelatin is an unusually simple protein. It lacks tryptophan and other amino acids present in most proteins. Without these building blocks, the body cannot build the proteins it needs for its own substance. Therefore, unless it gets other protein in its food as well, the amino acids that do occur in the gelatin are useless and have to be excreted. It is as if housebuilders found themselves with plenty of lumber but no nails. Not only could they not build the house, but the lumber would just be in the way and eventually would have to be disposed of. Attempts were made in the 1890s to make gelatin a more efficient article of diet by adding some of those amino acids in which it was deficient, but without success. Better results were obtained with proteins not as drastically limited as gelatin.

  In 1906, the English biochemists Frederick Gowland Hopkins and Edith Gertrude Willcock fed mice a diet in which the only protein was zein, found in corn. They knew that this protein had very little of the amino acid tryptophan. The mice died in about fourteen days. (It is the lack of tryptophan that is the chief cause of the protein-deficiency disease Kwashiorkor, common among African children.) The experimenters then tried mice on zein plus tryptophan. This time the mice survived twice as long. It was the first hard evidence that amino acids, rather than protein, might be the essential components of the diet. (Although the mice still died prematurely, this was probably due mainly to a lack of certain vitamins not known at the time.)

  In the 1930s, the American nutritionist William Cumming Rose got to the bottom of the amino-acid problem. By that time the major vitamins were known, so he could supply the animals with those needs and focus on the amino acids. Rose fed rats a mixture of amino acids instead of protein. The rats did not live long on this diet. But when he fed rats on the milk protein casein, they did well. Apparently there was something in casein—some undiscovered amino acid, in all probability—which was not present in the amino-acid mixture he was using. Rose broke down the casein and tried adding various of its molecular fragments to his amino-acid mixture. In this way he tracked down the amino acid threonine, the last of the major amino acids to be discovered. When he added the threonine from casein to his amino-acid mixture, the rats grew satisfactorily, without any intact protein in the diet.

  Rose proceeded to remove the amino acids from their diet one at a time. By this method he eventually identified ten amino acids as indispensable items in the diet of the rat: lysine, tryptophan, histidine, phenylalanine, leucine, isoleucine, threonine, methionine, valine, and arginine. If supplied with ample quantities of these, the rat could manufacture all it needed of the others, such as glycine, proline, aspartic acid, alanine, and so on.

  In the 1940s, Rose turned his attention to human requirements of amino acids. He persuaded graduate students to submit to controlled diets in which a mixture of amino acids was the only source of nitrogen. By 1949, he was able to announce that the adult male required only eight amino acids in the diet: phenylalanine, leucine, isoleucine, methionine, valine, lysine, tryptophan, and threonine. Since arginine and histidine, indispensable to the rat, are dispensable in the human diet, it would seem that in this respect the human being is less specialized than the rat, or, indeed, than any other mammal that has been tested in detail.

  Potentially a person could get along on the eight dietarily essential amino acids; given enough of these, he can make not only all the other amino acids he needs but also all the carbohydrates and fats. Actually a diet made up only of amino acids would be much too expensive, to say nothing of its flatness and monotony. But it is enormously helpful to have a complete blueprint of our amino-acid needs so that we can reinforce natural proteins when necessary for maximum efficiency in absorbing and utilizing nitrogen.

  FATS

  Fats, too, can be broken down to simpler building blocks, of which the chief are fatty acids. Fatty acids can be saturated, with the molecules containing all the hydrogen atoms they can carry; or unsaturated, with one or more pairs of hydrogen atoms missing. If more than one pair are missing, they are polyunsaturated.

  Fats containing unsaturated fatty acids tend to melt at lower temperatures than those containing saturated fatty acids. In the organism, it is desirable to have fat in a liquid state; thus, plants and cold-blooded animals tend to have fat that is more unsaturated than that fat in birds and mammals, which are warm-blooded. The human body cannot make polyunsaturated fats out of saturated ones, and so the polyunsaturated fatty acids are essential fatty acids. In this respect, vegetarians have an advantage and are less likely to suffer a deficiency.

  Vitamins

  Food fads and superstitions unhappily still delude too many people—and spawn too many cure-everything best sellers—even in these enlightened times. In fact, it is perhaps because these times are enlightened that food faddism is possible. Through most of human history, people’s food consisted of whatever could be produced in the vicinity, of which there usually was not very much. It was eat what there was to eat or starve; no one could afford to be picky, and without pickiness there can be no food faddism.

  Modern transportation has made it possible to ship food from any part of the earth to any other, particularly with the use of large-scale refrigeration, and thus reduced the threat of famine. Before modern times, famine was invariably local; neighboring provinces could be loaded with food that could not be transported to the famine area.

  Home storage of a variety of foods became possible as early humans learned to preserve foods by drying, salting, increasing the sugar content, fermenting, and so on. It became possible to preserve food in states closer to the original when methods of storing cooked food in vacuum were developed. (The cooking kills microorganisms, and the vacuum prevents others from growing and reproducing.) Vacuum storage was first made practical by a French chef, François Appert, who developed the technique in response to a prize offered by Napoleon I for a way of preserving food for his armies. Appert made use of glass jars; but nowadays, tin-lined steel cans (inappropriately called tin cans or, in Great Britain, just tins) are used for the purpose. Since the Second World War, fresh-frozen food has become popular, and the growing number of home freezers has further increased the general availability and variety of fresh foods. Each broadening of food availability has increased the practicality of food faddism.

  DEFICIENCY DISEASES

  All this is not to say that a shrewd choice of food may not be useful. There are certain cases in which specific foods will definitely cure a particular disease. In every instance, these are deficiency diseases—diseases that occur when food, and even protein, is ample. They are produced by the lack in the diet of some substance essential to the body’s chemical machinery in tiny amounts—yet where these tiny amounts are not present in the diet. Such deficiency diseases arise almost invariably when a person is deprived of a normal, balanced diet—one containing a wide variety of foods.

  To be sure, the value of a balanced and variegated diet was understood by a number of medical practitioners of the nineteenth century and before, when the chemistry of food was still a mystery. A famous example is that of Florence Nightingale, the heroic English nurse of the Crimean War who pioneered the adequate feeding of soldie
rs, as well as decent medical care. And yet dietetics (the systematic study of diet) had to await the end of the century and the discovery of trace substances in food, essential to life.

  The ancient world was well acquainted with scurvy, a disease in which the capillaries become increasingly fragile, gums bleed and teeth loosen, wounds heal with difficulty if at all, and the patient grows weak and eventually dies. It was particularly prevalent in besieged cities and On long ocean voyages. (It first made its appearance on shipboard during Vasco da Gama’s voyage around Africa to India in 1497; and Magellan’s crew, during the first circumnavigation of the world a generation later, suffered more from scurvy than from general undernourishment.) Ships on long voyages, lacking refrigeration, had to carry nonspoilable food, which meant hardtack and salt pork. Nevertheless, physicians for many centuries failed to connect scurvy with diet.

  In 1536, while the French explorer Jacques Cartier was wintering in Canada, 110 of his men were stricken with scurvy. The native Indians knew and suggested a remedy: drinking water in which pine needles had been soaked. Cartier’s men in desperation followed this seemingly childish suggestion. It cured them of their scurvy.

  Two centuries later, in 1747, the Scottish physician James Lind took note of several incidents of this kind and experimented with fresh fruits and vegetables as a cure. Trying his treatments on scurvy-ridden sailors, he found that oranges and lemons brought about improvement most quickly. Captain Cook, on a voyage of exploration across the Pacific from 1772 to 1775, kept his crew scurvy-free by enforcing the regular eating of sauerkraut. Nevertheless, it was not until 1795 that the brass hats of the British navy were sufficiently impressed by Lind’s experiments (and by the fact that a scurvy-ridden flotilla could lose a naval engagement with scarcely a fight) to order daily rations of lime juice for British sailors. (They have been called limeys ever since, and the Thames area in London where the crates of limes were stored is still called Limehouse.) Thanks to the lime juice, scurvy disappeared from the British navy.