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.
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
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.
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.
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.
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.
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.
leading German organic chemist, Justus Liebig, confirmed Mulders
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.
as a Muscle Fuel
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 subjects
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.
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.
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
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
Liebigs 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 Agricultures
nutrition specialist, followed the same line in recommending that
physically active men should eat 125 g of protein per day.
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 familys 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.
had six of his colleagues, a dozen army corpsmen, and a group
of Yales 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.
findings stimulated an active debate among medical men. Most believed
that Chittendens 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.
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.
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.
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.
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?
of the first experiments comparing the nutritional values of different
proteins were carried out by Lafayette Mendel (from Chittendens
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
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
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
rats 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.
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.
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.
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.
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.
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.
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.
Contribution of Different Foods
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 cows 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%).
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.
to the comparison of bread and milk, we can make the following
(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
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:
(g) Energy (kcal) PCals%
pork chop (89 g) 21 334 25
breast without skin (86 g) 27 142 72
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.
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 days 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.
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:
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
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 13 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.
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
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
group (kg) (g) (kcal) required
ages 13 13 16 1,300 4.9
2550 63 50 2,200 9.1
2550 79 63 2,900 8.7
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.
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
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.).
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