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IV.C.2. - Proteins

The word "protein" was coined by Jöns Jakob Berzelius in 1838. For the previous 150 years, however, there had been the concept of an "animal substance," slight variants of which were thought to make up muscles, skin, and blood. In each form the substance was initially believed to be gluey. But it turned into hard, hornlike material when heated and became foul-smelling when kept under moist, warm conditions, giving off an alkaline vapor. This contrasted with the properties of starch and sugar and most whole plants that went to acid during damp, warm storage.

For people interested in nutrition, the obvious question was: "How does the animal kingdom, which as a whole lives on the plant kingdom, convert what it eats into the apparently very different animal substance?" Humans were, of course, included in the animal kingdom and assumed to have essentially the same nutritional system as animals. Some eighteenth-century discoveries threw light on the problem.

In 1728, the Italian scholar Jacopo Beccari announced that he had discovered the presence of a material with all the characteristics of "animal substance" in white wheat flour. When he wetted the flour to make a ball of dough, then washed and kneaded it in water, the fine, white starchy particles washed out. What remained was a sticky pellet of gluten, which, if its origin were unknown, would be judged animal in nature. Beccari concluded that the presence of this portion of preformed "animal substance" made wheat particularly nutritive. Wheat flour, as a whole, did not show animal properties because the greater quantity of starch overwhelmed the reactions of the gluten.

Nitrogen in Nutrition

Later in the eighteenth century, with the development of the new chemistry, the main elements were identified, and ammonia, the "volatile alkali," was shown to be a compound of nitrogen and hydrogen. Gluten was also found to contain nitrogen, in common with animal tissues, whereas starches, fat, and sugars did not.

At first it was thought that the process of animal digestion and nutrition must consist of the combining of nutrients in plant foods with atmospheric nitrogen in order to "animalize" them. In particular, it seemed that this theory might explain the slow digestion process and large storage stomachs in ruminant animals. However, further work in France made this appear less likely.

First, François Magendie reported in 1816 that dogs failed to survive for more than a few weeks on foods like fats and sugars that contained no nitrogen. Then, in the 1830s, Jean Boussingault showed that the nitrogen present in the hay and potatoes eaten by a cow was enough to balance the quantities present in the milk it secreted together with its regular daily nitrogen losses. There was, therefore, no need to suppose that atmospheric nitrogen was involved in animal nutrition. But because of the importance of nitrogen in nutrition, Boussingault concluded that plant foods should be valued in terms of their relative nitrogen contents. Thus, he believed that dry beans, with roughly twice the nitrogen content of grains, had twice their nutritional value.

By this time, further work on the composition of plants had shown that although they all contained nitrogenous compounds, most of them, unlike wheat gluten, were soluble in water, yet could be precipitated by heat or acid. In 1838, Gerritt Mulder, a Dutch physician who had taught himself chemical analysis, published a claim that all the important "animal substances" he had analyzed had the same basic formula, corresponding to 40 atoms of carbon, 62 of hydrogen, 10 of nitrogen and 12 of oxygen, which can be expressed more simply as C40H62N10O12. They differed in their properties only because they had different numbers of atoms of sulfur and/or phosphorus adhering to them. He sent his paper to the Swedish chemical authority, Jacob Berzelius, who replied that this was a most important discovery of the "fundamental or primary substance of animal nutrition" and that this substance deserved to be called "protein" after the Greek god Proteus.

The leading German organic chemist, Justus Liebig, confirmed Mulder’s finding and went on to argue that, from a chemical point of view, it was the plant kingdom alone that had the power of making protein. Animal digestion only loosened the association between their molecules to make them soluble and absorbable into the bloodstream and immediately ready for deposit into the animal system. The leading French scientists accepted this view but added that vegetable oils and carbohydrates were also required. Their combustion was needed within the animal body to maintain animal heat.

Protein as a Muscle Fuel

Liebig, although he had himself done no physiological work, developed a whole series of dogmatic statements as to the functions of nutrients in the body. He believed that the energy needed for the contraction of muscles came solely from the breakdown of some of their own protein, which was then immediately decomposed further, with the nitrogenous portion appearing as urea in the urine. A subject’s requirement for protein was, therefore, proportional to his or her performance of physical work. The role of fats and sugars was merely to protect living tissues (which reacted with oxygen that penetrated them) from the danger of oxygen damage. Consequently, protein was the only true nutrient.

Liebig’s views were accorded great weight, although there were many grounds on which they could be criticized. For example, in 1862, Edward Smith, a physician and physiologist who had been studying the health and diet of the inmates of London prisons, reported a study of factors influencing the daily output of urea. Prisoners who ate the same rations each day and engaged in hard labor three days per week were found to excrete almost the same quantity of urea on the day (and following night) of the labor as on the days when not laboring. However, the labor caused greatly elevated carbon dioxide output in the breath. The main factor influencing urea production appeared to be the amount of protein eaten in the previous 24 hours.

In 1865, Adolf Fick and Johannes Wislicenus, on the faculty of a Swiss university, followed up these findings. They put themselves on a protein-free diet for 24 hours and ascended almost 2,000 meters on a path to the summit of a convenient mountain. They calculated the amount of work done during the ascent and measured the amount of nitrogen in the urine they excreted. From this they calculated that they had each metabolized approximately 37 grams (g) of protein. Their friend in England, Edward Frankland, now calculated that the metabolism of protein yielded 4.37 kilocalories per gram.

By this time the principle of the "conservation of energy" had been accepted, and James Joules had estimated that 1 kilocalorie was equivalent to 423 kilogram-meters (kg-m) of mechanical work against the force of gravity. The energy released from the protein was, therefore, equivalent to some 68,000 kg-m. However, the net work required to lift each scientist up the mountain was approximately 140,000 kg-m, about twice as much. And further work has shown that muscles operate at something like 25 percent efficiency, so that four times the minimal theoretical amount of fuel is required. The conclusion was, therefore, that the energy required for muscular effort does not come primarily from protein but from dietary fats and carbohydrates.

Although Liebig’s grand scheme had been discredited, German workers, in particular, continued to maintain that a high-protein intake was desirable to maintain both physical and nervous energy. They argued this on the grounds that people from countries where the diet was largely vegetarian and low in protein lacked "get-up-and-go," and that wherever people were unrestrained by poverty and could eat what they wished, they chose a high-meat, high-protein diet. The first U.S. government standards, issued at the end of the 1800s by Wilbur Atwater, the Department of Agriculture’s nutrition specialist, followed the same line in recommending that physically active men should eat 125 g of protein per day.

Such a notion did not go unchallenged, however. From 1840 on, there had been a vegetarian "school" in the United States, which argued that eating meat was overstimulating and conducive first to debauchery and then to exhaustion of the irritated tissues. John Harvey Kellogg (cofounder of the family’s breakfast-food enterprise) believed that meat and other sources of excessive protein in the diet could putrefy in the large intestine, resulting in autointoxication. These ideas were regarded by the scientific establishment as unscientific and not meriting attention. However, in 1902 a serious challenge to the "high-protein" school was mounted by Russell Chittenden, professor of physiological chemistry at Yale.

Chittenden had six of his colleagues, a dozen army corpsmen, and a group of Yale’s athletes spend approximately six months on diets containing no more than one-half of the Atwater standard for protein. These men all remained healthy and vigorous in mind and body. Chittenden concluded, in his account published in 1904, that such diets were not only adequate but preferable because they put less strain on the kidney to cope with the excretion of both urea and the less soluble uric acid.

His findings stimulated an active debate among medical men. Most believed that Chittenden’s findings were still too limited to recommend wholesale dietary changes. For example, the subjects in his study had not been subjected to sudden stresses or to periods of inadequate feeding in which they had to rely on reserves. Moreover, experiments with dogs kept on low-protein diets had revealed that although they remained healthy and in nitrogen balance for a time, they eventually weakened and died. Chittenden, however, followed up this line of work with dogs in his own laboratory and concluded that it was not lack of protein that was responsible for long-term problems with some diets but a lack of one or more unknown trace nutrients. This conclusion constituted one of the stimuli for the work that dominated nutritional studies for the next 40 years and revealed the existence of the vitamins.

Amino Acids in Nutrition

By 1905, another question gaining prominence was whether the proteins that had now been isolated from many foods could all be considered equivalent in nutritional value. It had long been known that gelatin, obtained by autoclaving bones, would not support growth in dogs, even though it had the same nitrogen content as ordinary tissue proteins. But because it displayed some physical differences, such as remaining soluble in boiling water, it had been set aside from the "protein" classification.

Advances in the study of proteins required a better knowledge of their composition. That they did not diffuse through fine-pored membranes showed them to be large molecules. But during digestion, their physical properties changed dramatically. With the isolation of the digestive agent "pepsin" from the stomach walls of slaughtered animals, and then of "trypsin" from pancreatic juice, the process could be studied in more detail.

As early as the 1860s, workers had been surprised to discover the presence of "leucine" and "tyrosine" in digests of protein with pancreatic juice. These two compounds were already well known. They were recoverable from the product of boiling proteins with sulfuric acid and had been shown to be "amino acids" – meaning that they contained both an acid group and a basic one and were relatively small molecules, each with less than 25 atoms. However, the procedure yielding them seemed so severe as to have no relation to the mild conditions of the digestive system, and it was thought that the compounds might well have been produced by the hot and strong acid conditions.

Gradually, by in vitro digestion, and milder acid or alkali refluxing of proteins, a whole range of amino acids were recovered, and crude methods were developed to analyze the quantities of each that were present. Also, evidence accumulated that the compounds actually absorbed through the gut wall after digestion were simple amino acids. Early investigators had felt it unlikely that nature would employ such a system because it seemed extremely wasteful to break proteins down, only to rebuild the same compounds within the animal. Or were they the same compounds?

Some of the first experiments comparing the nutritional values of different proteins were carried out by Lafayette Mendel (from Chittenden’s group) and the plant chemist Thomas Osborne. They found that young rats would grow if given a diet of fat, carbohydrates, minerals, crude vitamin concentrates, and purified casein (a milk protein). However, with zein (a protein from corn) as the protein source, the rats did not grow unless the diet was fortified with both lysine and tryptophan. Chemical analysis had already indicated that zein lacked these two amino acids, and thus these two were characterized as "essential amino acids," meaning that they were essential or indispensable in the diet of growing rats. The results also indicated that animals used amino acids to build their own body proteins.

After 20 years of further experiments of this kind, W. C. Rose and his colleagues were able to obtain good growth in rats with diets containing no protein, but just a mixture of amino acids in its place. Table IV.C.2.1 summarizes their findings about the 20 amino acids present in animal proteins, some of which were indispensable ("essential") and some of which the rat could make for itself ("nonessential") if they were not supplied in its diet. Further work led to the development of values for the quantity of each indispensable amino acid that rats required for optimal growth.

Another group of studies compared the relative values of different protein sources (or of mixtures) for the support of growth in rats. The mixed proteins from individual vegetable foods (grains, beans, and so forth) all supported some growth, but not to quite the same extent as the mixed proteins in milk, meat, or eggs. The first limiting amino acid (meaning that this was the amino acid that increased growth when added as a single supplement) in most grains was lysine. This was to be expected in view of the growing rat’s known requirement for lysine and the low analytical value of lysine in grains. The corresponding first limiting amino acid in most beans and peas was found to be methionine. Because the two classes of materials had different deficiencies, one would expect a mixture of grains and legumes to support better growth in rats, and this has been confirmed.

Human Requirements

Rats, however, although useful as models, differ from humans (even in this context) in important ways. Humans spend most of their lives as adults, not growing at all but needing protein just for "maintenance." And in childhood, human growth is extremely slow compared to the growth of rats. Thus, we take something like six months to double our birth weight, which a young rat does in a few days. And at six months, the rat is fully matured, yet the child is still only one-tenth of its mature size. Moreover, although the tissue proteins of rats and humans are similar, hair protein is very different, and the rat has to synthesize proportionally more.

It was necessary, therefore, to discover whether humans needed to be supplied with the same essential amino acids as those needed by rats. But because it was neither practical (nor ethical) to keep young children on what might be inadequate experimental diets for long periods in order to compare their growth rates, the normal method of experimentation was to feed adult volunteers for periods of two weeks or so on diets in which there were mixtures of amino acids in place of protein.

If an essential amino acid were missing, the subject would, within a very few days, show a negative nitrogen balance, meaning that the amount of combined nitrogen found in urine and feces, plus the smaller estimated quantity rubbed off in skin and hair losses, had exceeded the daily nitrogen intake. Fortunately, no harm seems to come to humans in negative balance for a short period, and bodily reserves refill rapidly on resumption of a complete diet.

The first major finding from this work was that essential and nonessential amino acid needs are the same for humans as for the young rat. However, researchers were surprised to discover how low the quantitative need for each essential amino acid appeared to be in order to maintain nitrogen balance. In fact, the combined total of essential amino acids came to only 16 percent of the total protein requirement, even though they make up about 45 percent of our body proteins. Thus, it seemed that almost any mixture of foods that provided at least the minimum amount of total protein needed would automatically meet adult needs for each essential amino acid. For young children, however, it was felt safer to set a higher standard for amino acids, corresponding more or less to the composition of the proteins in breast milk. For older children, a compromise was adopted in official recommendations, with a pattern midway between that found to be needed for nitrogen balance in adults and that in human milk. These standards are summarized in Table IV.C.2.2.

There have been recent criticisms of the practice of basing standards solely on short-term nitrogen balance experiments, with V. R. Young (1986) and colleagues (1988, 1989) at the Massachusetts Institute of Technology (M.I.T.) arguing that the method itself has sources of error. These researchers have carried out sophisticated studies using diets based on amino acids, with a single essential amino acid labeled with an isotope so that its metabolism can be followed. They concluded that the levels at which the essential amino acids are required, in relation to total protein needs, are quite similar to the levels in which they occur in the body. Even after subjects have had time to adjust to lower intakes, the rate of renewal of body tissues is reduced, which may have adverse effects in a time of stress.

As Atwater suggested over a century ago, it is possible that intakes higher than those needed for nitrogen balance could confer some more subtle long-term benefits. However, there are as yet no studies of peoples living for long periods on diets borderline in protein but well served with all other nutrients that would clarify the situation. And in any event, it seems clear that even the higher amino acid levels proposed by the M.I.T. group are being fulfilled by the diets of most people, at least in the developed countries.

The Protein Contribution of Different Foods

The obvious way to express the level of protein in a food is as a percentage of the weight, like "g per 100g." But such a measurement can be deceptive. For example, it would show ordinary white bread to have nearly 3 times the protein content of cow’s milk because milk is 90 percent water, whereas bread is only about 34 percent water. Alternatively, one could compare the amounts of protein in equal weights of dry matter, but the common nutritional value of the great majority of the dry matter is its contribution of usable energy, whether from carbohydrate, fat, or protein. Thus, nutritionists have found it useful to compare the protein concentration of different foods in relation to their total calorie values. This could be expressed as "g per 100 kcalories," but it is easier (as protein itself has an average energy value of 4kcal/g) to express the concentration as "protein calories as a percent of total calories" (PCals%).

Although most of the time people have an instinct to eat enough food to meet their energy needs, there is a question of whether this quantity will also include enough protein.

Returning to the comparison of bread and milk, we can make the following comparisons:

Protein (g) Energy (kcal) PCals%

1 slice white bread (32 g) 3 96 12.5

1 cup whole milk (244 g) 8 150 21.3

1 cup skim milk (245 g) 8 86 37.2

In terms of PCals%, milk is richer in protein than bread, meaning that to get the same quantity of protein from bread as from a cup of milk, one would have to consume more total calories. Similarly, it can also be seen that although a cup of whole milk and one of skim milk (with the cream removed) have the same protein content, the PCals% values are very different, with the value for the skim milk being higher. There are equally large differences between different meat preparations, as can be seen in the comparison of a pork chop and a chicken breast:

Protein (g) Energy (kcal) PCals%

Pan-fried pork chop (89 g) 21 334 25

Roasted chicken breast without skin (86 g) 27 142 72

What this shows is that in a fried pork chop, for every 1 g protein (that is, 4 kcal) there are, in addition, 12 kcal from fat, whereas in the roasted chicken breast, 1 g protein is accompanied by only 1.6 kcal from fat. The PCals percentage values for a range of foods are set out in Table IV.C.2.3. These are "average" or "typical" values. Some animal carcasses are fatter than others, and the composition of plant foods can change significantly according to the environment in which the plants are grown, as well as the stage of harvesting. Wheats also differ significantly, with some strains being selected for high or low protein content according to the use for which the flour is marketed.

It is true that animal-product foods are generally higher in protein than plant products and also that people in the more affluent "Westernized" countries eat higher levels of animal products. However, the total protein intake in affluent cultures is not that much larger. The offsetting factor in these cultures is the higher consumption of sugars, fats, and alcoholic beverages, all of which contribute calories but no protein. Moreover, in many developing countries, some kind of beans forms a regular part of the day’s food, and they are a rich source of protein. Thus, calculations commonly indicate that diets in both rich and poor countries have mostly between 10.5 and 12.5 percent of their total calories in the form of protein.

The Food and Agriculture Organization of the United Nations (FAO) publishes estimates of the daily food supplies per head that are available in different countries. Here are three examples:

% of

Protein (g) protein

Total from Fat Sugars

kcal Veg. Animal animals PCals% (g) (g)

U.S.A. 3640 37 72 66 12.0 164 579

Romania 3330 58 44 43 12.3 95 295

Ghana 2200 33 13 28 8.4 43 64

In this comparison, based on recent data, we see that in Romania, a relatively poor European country, the average individual took in only about 60 percent as much animal protein as a counterpart in the United States, but the total protein supply was almost identical. This was because the Romanians ate much less fat and sugar and received correspondingly more calories from grains, which generally have 10 to 13 PCals%. This offsetting, however, breaks down when the staple energy food is not a grain but a starchy root with only 1–3 PCals%, as in West Africa, where cassava is a common staple. The data for Ghana illustrate this. Despite the low fat and sugar intakes, there is still only an overall 8.4 PCals% in the food supply estimated to be available for the average person. Of course, the first "red light" that we see upon looking at the data is the low total calorie intake, which is only 60 percent of the corresponding U.S. value. Not all the U.S. foods are actually consumed of course: There is a great deal of waste, with fat trimmed off meat and stale food thrown out. Conversely, there may be some unrecorded food sources in Ghana. But it is a general finding that, even if there is a good supply of starchy roots, their sheer bulkiness makes it difficult to consume enough to meet energy requirements, particularly for young children, so that neither energy nor protein needs are fully met.

Not surprisingly, West Africa is also the part of the world where the disease kwashiorkor was first studied. It strikes children 1 to 3 years old who appear bloated, though their muscles actually are shrunken. They often have ulcerated and peeling skin and are utterly miserable. Unless treated, they are likely to die. The condition is now thought to be due to a combination of undernutrition (in both protein and energy) with the stress of infections. Recovery can be rapid if the victims are given concentrated food, by stomach tube at first, if necessary. The food mix does not need to be high in protein; mixes with as little as 5 PCals% have proven successful.

Except for children subsisting on bulky and very low protein staples, there seems to be no problem of protein deficiency for people in any culture who can afford to satisfy their calorie needs, unless they are consuming extremely atypical diets. The Recommended Dietary Allowances (RDAs) for protein in the United States are summarized here for three groups, together with the estimated energy needs of individuals in those groups if moderately active:

Assumed Protein Energy

bodyweight RDA needs PCals%

Population group (kg) (g) (kcal) required

Children, ages 1–3 13 16 1,300 4.9

Women, ages 25–50 63 50 2,200 9.1

Men, ages 25–50 79 63 2,900 8.7

It is interesting to note that when one calculates the proportions of protein required (PCals%) for each class, the results are unexpected. Traditionally, wives have thought that men, as the "breadwinners" of the family, needed most of the meat, and children at least some extra dairy protein, but as can be seen, it is actually women who are estimated to need the highest proportion of protein in their diet. And for people involved in greater levels of physical activity, all the evidence indicates that their calorie needs increase greatly but not their protein needs, so that the resulting PCals% of their needs is decreased. Put another way, the extra food they need to meet their needs can be of quite low protein content. Similarly, although the protein needs per kg body weight of a 1- to 3-year-old child are 50 percent greater than for an adult, its energy requirement per kg is nearly 200 percent higher so that, once again, the PCals% of its needs are lower.

Returning to the estimated average food supplies in Ghana, it can be seen that the mix is just about at the lower limit for protein. However, the RDA for protein, as for all other nutrients except energy, does include a margin of safety, and the level of physical activity is always higher in developing countries where there is less mechanical transport.

For countries like the United States or Romania, the protein supplies are clearly well above the standard requirement levels. Thus, it follows that a high intake of meat cannot be justified because of the protein that it contributes. In fact, a major concern of late has been that the protein intake of affluent individuals may be undesirably high. Some have problems because their kidneys are inefficient in excreting the urea resulting from protein metabolism. And, even in healthy people, high-protein diets cause increasing urinary losses of calcium. Certainly, this effect is undesirable in a society whose growing percentages of older people contain more and more individuals whose bones have been weakened because of the loss of a considerable proportion of their mineral substance (mostly calcium phosphate). It is now recommended that we not consume more than twice our RDA for protein. This would mean an upper level of 100 g protein for a woman weighing 63 kg (139 lb.).

Kenneth J. Carpenter


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