Proteins and Amino Acids (Encyclopedia of Food & Culture)
PROTEINS AND AMINO ACIDS. The average human body, weighing 65 kilograms, contains about 11 kilograms of protein, 40 kilograms of water, and 9 kilograms of fat. The protein provides the "machinery" of the body, including not only the voluntary muscles and the heart muscles, but also the walls of the gut and the blood vessels, as well as the enzymes, the skin, and the hair. The word "protein" is used to describe a group of different compounds with varying propertiesoluble, insoluble, and so on. Originally they were classed together because, unlike fats and carbohydrates, proteins also contain nitrogen in addition to carbon, hydrogen, and oxygen. Now, we know that they are all composed of chains of "amino acids" linked together like enormous necklaces of thousands of individual beads. To continue the analogy, there are twenty different varieties of "bead" (or amino acid). The chemists' shorthand representation of the common formula for each amino acid is "H2N.CHR.COOH," where "wH2N" is the basic amino group, "COOH" is the organic acid group, and "R" the general symbol for whatever additional group is present for that particular "bead." Amino acids can form chains by reaction between the amino group of one molecule and the acidic group of another to give:
The body makes its own proteins and obtains amino acids from the digestion of the proteins in the foods that we eat. These large protein molecules cannot be absorbed through the gut walls into the bloodstream, but a series of digestive enzymes (which are proteins themselves, having these special digestive functions) break down the chains of amino acids into the individual amino acids, which are then absorbed. A small proportion of protein may remain undigested, especially with the more fibrous plant foods such as bran, where the cellulosic cell walls are not easily broken down in the tissues, but, for most mixed human diets, one can assume at least 90 percent protein digestibility.
The final nutrients that we obtain from our foods are thus amino acids rather than proteins, and we build our own protein "necklaces" up from the pool of free amino acids circulating in our body. This pool is also derived in part from body proteins that are continually being broken down and resynthesized. Our bodies are in a dynamic state and are constantly turning over.
Each protein in our body has a specific function and is made up of a predetermined succession of amino acids that, when the protein is being synthesized, are added one by one to the chain. If a particular amino acid is missing from the site of synthesis, the process stops. Of the different varieties of amino acids, some are termed "essential," which means that our bodies are unable to synthesize them by modification of another molecule. Four amino acids in this category are lysine, methionine, threonine, and tryptophan. Others, the "nonessential" amino acids, can be made in the body from other nitrogenous compounds.
Obviously, a growing body needs dietary protein in order to increase its own body tissues. This is particularly clear in species where the young grow quickly, such as calves and particularly piglets, and it is interesting that the dams' milk in these species is considerably higher in protein than is human breast milk, the natural food that meets the needs of much more slowly growing human infants. It is also obvious that older children need protein for their continued growth and that pregnant and lactating women need it to provide for the growing fetus and then for the suckling infant. The protein needs of other adults are not so obvious, except for the slow but continuous growth of hair and fingernails, the replacement of rubbed-off skin, and other minor losses. However, adults require much greater amounts of protein than would be expected to replace these lossesomething like 50 g/day.
It is true that living tissue is in a dynamic state, with protein being continually broken down to its constituent amino acids and resynthesized, but this should not increase requirements because the amino acids are fully available for reuse. However, there are enzymes in the liver whose function is to break down an excess of circulating amino acids. These enzymes allow the elimination of nitrogen in the form of urea and the utilization of the carbon-containing side-chains of the amino acids as energy sources for the tissues. It appears that what we might call the "idling rate" of these enzymes sets the requirement for amino acids to replace those lost in this way. In the course of evolution there has presumably been no advantage, in general, in selecting for individuals with lower "idling levels" because protein intake was not a limiting factor. However, evolutionary selection may explain the apparently lower protein requirements of natives in an area of Oceania with a traditionally low-protein diet.
At one time it was thought that the muscular contractions required in any kind of physical work consumed protein, as if the muscle used itself up or was its own fuel. We now know that this is not the case, and that carbohydrate and fat are the normal sources of the energy required for physical work. The erroneous belief that physical work uses up protein has, in the past, been of some practical import since it meant that traditional working-class families would give a large proportion of what meat was available to the "breadwinner" father because of his greater physical labor, so that little remained for the others even when the wife was pregnant and her requirement for more concentrated sources of nutrients was actually more critical.
Protein Levels in Foods
From the preceding discussion, it is obvious that meat from other animal species is a rich source of protein. The only qualification is that fat can seriously dilute the concentration of protein available. Lean muscle has a 3:1 ratio of water to protein, whereas fats are laid down without associated water. Thus, a cut of meat with 20 percent fat will also have only 20 percent of protein (that is, ¼ of 80 percent). Further, since fat provides 9 Calories of energy per gram and protein only approximately 4, it follows that 9/13, or approximately 70 percent, of the energy provided by the meat will be coming from animal fat. (Note that the dietitian's Calorie, spelled with a large "C," is one thousand times the standard "calorie" and is the heat needed to raise 1 kilogram of water by 1oC).
The working parts of plants, principally enzymes, also consist of proteins; and seeds always include protein that will be required for the synthesis of new tissues when the seed germinates. In general, the concentration of protein is lower than that in animal tissues because the structural components consist of fibrous carbohydrates and, for seeds or root stores, large amounts of starch and sugars are usually also present as a reserve of energy.
There are several different ways of comparing the relative richness of different foods in protein. The simplest way would be a straight comparison of the percentage of protein in each food. Thus, whole milk contains just over 3 percent while white bread contains 8 percent or a little over, that is, more than twice as much. Yet the dietitian's practice when considering adult-type menus is to compute the quantity provided by a typical "serving." Thus, for milk the standard serving is 1 "cup" (245 grams), which provides 8 grams of protein. If we take two slices (50 grams) as the standard serving for white bread, this provides only 4 grams of protein, that is, less. So, there is a contradiction. Milk, of course, is nearly 90 percent water. Dried whole milk contains 27 percent protein and dried bread only 13 percent. This is perhaps a fairer comparison since one would consume more water with a meal if one were not drinking fluid milk, and it leads to one finally regarding milk as being the richer source of protein.
Common refined cereals (white flour, white rice, and de-germed cornmeal) all contain about 10 g protein per 100 g dry matter, with wheat providing a little more and rice a little less, as do potatoes. Lean fish and meat have up to 60 percent protein in their dry matter and skim milk has 40 percent. Among plant products, the legume crops (peas and beans) have the highest values, commonly 20 to 25 percent of dry matter. Legumes that are rich in fat (the so-called oil-seeds) can be processed to remove the oil, and the residues have even higher protein values. Extracted soybean meal has 45 percent protein and materials of this type have been used to make vegetarian substitutes for meat.
One would expect that the wealthier countries, with their greater use of animal products of high protein concentration, would have overall diets of higher protein content, but this is not always the case. Wealthier population groups also consume higher levels of fat and sugar, much of them in fast foods and "cola-type" drinks. These dilute the protein concentration of the diet to the levels of those poorer countries whose diet is based on cereals with less in the way of supplementary foods of either high or low protein content.
A particular problem exists where a poor community has plantains and/or cassava (manioc) as its staple foods. These have only about 2 to 4 percent of their dry matter in the form of protein, that is, values much lower than the corresponding value for grains. In addition, these foods are so bulky that young children being reared on them are commonly unable to consume enough to meet even their energy needs. They are thus at constant risk of both protein and energy deficiencies and should be given more concentrated foods.
As mentioned above, if a human diet were to be totally deficient in one essential amino acid, a child would immediately cease to grow and even an adult would go into decline. In practice this never happens. Even though one can extract from some plants an individual protein that is of this kind (for example, the gluten present in wheat), the total mixture of proteins in any natural food contains some of each of the amino acids, though not usually in the exact ratio in which the human body will use them. This is not a problem so long as the essential amino acid is present at the lowest level that is adequate to meet the body's need for it. The remaining amino acids will conversely be present in excess, and, as explained in the previous discussion, mechanisms exist to metabolize these amino acids to provide an additional source of energy.
Providing fast-growing young rats with wheat flour as their sole source of protein in a diet that is otherwise well balanced, will result in relatively slow growth; rats fed on the same diet supplemented with a small quantity of the amino acid lysine will grow considerably faster. Under these experimental conditions, the protein of wheat flour is said to have a low "biological value" and its "limiting amino acid" to be lysine.
Mixtures of foods that have different limiting amino acids can supplement each other. Thus, if one protein source, such as a grain, is limited by its lysine content, but has methionine in excess, and another protein source, such as a legume, has a good level of lysine but is short of methionine, a mixture of the two will have a better balance than either alone. This phenomenon can be clearly demonstrated in experiments where young growing rats, for example, are fed a mixture of wheat flour, primarily deficient in lysine, and navy beans, primarily deficient in methionine. Young rats grow faster when they are fed with both wheat flour and navy beans, as opposed to being fed with either protein source separately, but still not as well as when they are fed with well balanced egg protein because the mixture is still partially deficient in a third amino acid, threonine.
There has been a great deal of research and controversy about how serious partial deficiencies of individual amino acids are in human diets. In poor communities in North Africa wheat has been the traditional staple food, with only small quantities of supplementary foods. When synthetic lysine became available at a reasonable cost, a trial was carried out to determine if fortification of the traditional diet with lysine at the time the grain was milled would result in improved vigor in people and in improved health among children. However, there was no evidence of any benefit. Nor was any benefit seen in comparable trials in Thailand.
It is clear that the young rat can be a misleading model for humans. A child grows much more slowly to its adult size then a small animal and, at its fastest rate of growth, adds no more than 3 grams of protein to its tissues in a day very small amount in relation to its protein intake, which would normally be ten times as great even with a diet containing no more than 10 PCal%. A more common problem than diet based on grain that is deficient in an essential amino acid, is the fact that people do not eat enough to cover their energy needsn adults, because of poverty in most cases; and in young children in the developing world because their food is too bulky for them to be able to consume all they need. In such cases adults can lose muscle power and general vigor and small children can develop kwashiorkor, a potentially fatal disease that appears to be a combined deficiency of energy and protein, because what protein is ingested has to be used as additional fuel.
It is clear from what has been written already that vegetarians can have an adequate intake of protein, particularly if they are willing to consume milk and eggs which provide good supplements to the protein of grains, and do not require animals to be killed. Vegans, who eliminate all animal products from their diets, commonly ensure that, in addition to consuming grains, they have a good intake of legumes as a supplementary source of proteins relatively rich in lysine. The legumes may not always be essential, but the supplementation provides a useful safety margin. The long-term problem for vegans is that they will become deficient in vitamin B12, unless they take a special vitamin supplement, because this vitamin occurs naturally only in animal tissues or from the fermentation of certain bacteria.
The recommended daily dietary allowance for an adult of average build is 0.8 grams of protein per kilogram of body weight, equivalent to 0.36 grams of protein per pound of body weight, regardless of sex. Since "average" men and women in the United States are assumed to weigh about 170 and 130 pounds, respectively, these standards give us an intake of 61 grams protein for the average man and 47 grams of protein for the woman, with a further addition of 15 grams of protein per day for women during pregnancy and lactation. These standards were designed to be adequate for diets containing protein of average quality and to contain some margin of safety. Virtually all American diets contain more than this amount of protein.
Despite children's need for growth, their requirement for protein as a proportion of their total food is lower than that of adults, because their energy expenditures in relation to their size are greater than those of adults, and their growth is so gradual. However, if a child's appetite is very poor and/or the diet is very bulky, the diet can become protein deficient because some of the small amount of protein consumed will be used as an energy source.
See also Assessment of Nutritional Status; Caloric Intake; Calorie; Disease; Lactation; Malnutrition; Nutrients; Nutrition.
Carpenter, Kenneth J. Protein and Energy. A Study of Changing Ideas in Nutrition. New York: Cambridge University Press, 1994.
World Health Organization. Energy and Protein Requirements. Technical Report Series No. 724. Geneva: World Health Organization, 1981.
Kenneth John Carpenter