Indispensable amino acids -
water, body protein is
its second most plentiful constituent-nutrient.
Composed of amino acids, proteins are body's main tissue-building
material, as well as a source of building material for body's metabolic and
regulatory substances -
enzymes, hormones, antibodies, DNA, RNA, and
Usually regarded as the most important basic nutrient,
proteins are, again, second only to water from the standpoint of
body's most immediate nutritional necessities. When Dr. William Rose
pioneered protein research on humans back in early 1950s, he has
witnessed study participants suffering extreme
fatigue, irritability and loss of appetite after as little as two
days on insufficient intake of essential amino acids.
protein is broken down into its building blocks -
- which the body uses to build thousands and thousands of its own proteins. Body needs 22 amino acids; 8 of them that it cannot
synthesize are called essential, or
indispensable amino acids
- Isoleucine, Leucine,
Lysine, Methionine, Phenylalanine,
and Valine (with Histidine also being essential for infants and,
possibly, semi-essential for adults too). The body has to obtain them from food
as preformed amino acids.
Secondary body function of proteins is as a source
of energy. Each gram of protein burned for energy releases 4
calories. However, proteins are both, harder for the body to burn as
well as needed for synthesis, and it normally satisfy less than 10% of
its caloric needs by burning proteins. It will burn more proteins
only when its preferred energy source - carbohydrates and fats - are
Daily protein requirement
is hard to believe, but the debate on how much proteins - and with
which particular amino acid structure - your body needs for optimum
health, is still going on as of today. Considering the importance of
body protein in general, and essential amino acids in particular,
this is somewhat surprising. On the other hand, the fact is that the
science of nutrition is still in its infancy, and needs more time to
The difficulty of establishing
human protein needs arises from the great complexity of food digestion and
principal method used in studies and experiments is based on the measurement of intake
and excretion of
(specific component of proteins, making 16% of its weight). The assumption
is that a balance between nitrogen input and output - adjusted for known
interfering factors - indicates the level of protein intake needed to
maintain protein level required for body functions. Thus the amount of protein needed for
achieving protein balance would equal the minimum safe protein intake
for adults (children require extra protein for growth).
and incompleteness of (many) conducted tests, coupled with hard to assess
changes in protein metabolism due to a number of known and unknown variables
has prevented, thus far, arriving to a unanimous result and agreement on this
FAO/WHO protein intake recommendation
Based mainly on older studies using
nitrogen balance method (Rose, Levarton), the Food
and Agriculture Organization of the United Nations (FAO) has, in 1985, come up with
an average daily intake of 0.34g/kg of fully utilized (perfect) protein per kg
of lean body weight as the minimum needed to maintain body's protein level
for adults. When
individual deviations are taken into account (doubling the 15% standard
deviation), nearly 98% of healthy adults should be safe with
intake of fully utilized protein.
Since no protein is fully utilized, needed
maintenance protein intake is always higher. Applying net protein
utilization (NPU) of 0.6, which is roughly midway between the highest and lowest
protein NPU for foods considered to be significant protein sources, gives as
needed daily maintenance protein intake of
0.75g/kg (0.34g/pound) of lean body
Thus the FAO/WHO (World
Health Organization) recommended daily protein intake is
of protein per kg of lean body weight,
or 45g for an
average 60kg (133 pound) healthy adult female, and 56g for an average 75kg
(167 pound) male. The official US recommended protein intake is slightly higher, at
Research Council, 1989).
Hence, for an average moderately active
adult female, requiring 2400 calories a day, minimum safe protein intake
comes to 9% of total calories.
And for the average moderately active male requiring 2800 calories a
day, minimum safe protein intake makes
8% of the caloric total.
A little extra won't hurt, so we can round it off to about 10% of the
Much less research has been done on the subject of
excessive protein intake.
Results of a few studies suggest that, for most people, protein intake becomes excessive when it
15% of the total calories. That would put optimum protein intake
somewhere between 10% and 15% of total calories.
FAO protein recommendation for
pre-school children -
from 1.2 g/kg/day at age of 1, to 1g/kg/day at age 6 - are
based on somewhat obscured studies at the Institute of Nutrition of
Central America and Panama. Latter experimental results indicate lower
minimum requirements, so this subject is
in need of further research. Since children require extra protein for
growth, over the level securing nitrogen balance, their protein
requirements were estimated by combining the amount based on the
known average growth rates with the amount of protein estimated to
be deposited in
the newly formed tissues.
Similarly, protein requirements for
infants - based on
protein intake that seemed to be supporting normal growth in
observational studies - may be
significantly lower than the minimum FAO/WHO recommendation of 2g/kg/day
for the 1-3 month old.
Toward the opposite end, in the
elderly, protein needs
also seem to be higher than the minimum recommended intake level, mainly
due to lower utilization rate. This is indicated by some more recent
studies, and it is not yet officially recognized by the DRI.
Officially recommended protein intake can vary significantly from one
country to another, much more so than experimentally established protein
needs of different human races, or similar races in different world
Within the FAO/WHO, the subject of adequate
protein intake is, at least in general, agreed upon. What is much more in
dispute is the criteria of determining protein quality and essential amino
Alternative protein requirements concept
Since the late 1980s, almost immediately after it was
established, the FAO/WHO protein recommendation was
challenged based on new experimental results obtained using different test methodologies, primarily based on
tracing levels of
oxidation of the essential amino acids excess (amino acids not used for
protein synthesis are oxidized in the process of disposal). While these new results vary
from one study to another, what they have in common is that they tend to establish
higher total essential amino acids requirements,
also in somewhat different proportions
than the current standard. One noted
example of these studies (Young, Bier, Pellett ) arrived at the following
minimum safe intake of essential amino acids:
Essential amino acid
Young et al.
Phenylalanine + Tyrosine
Comparative amino acid
requirement for adults in mg/kg/day
(number in brackets is a relative fraction
of the amino acid in the total)
While these new values increase the adult methionine/cystine requirement by
a moderate 15%, nominal requirement for tryptophan is nearly doubled, and for
the rest of the essential amino acids, either doubled or more than doubled.
Total essential amino acid requirement is more than doubled.
The researchers emphasized that these figures are still only
approximations, but they do recommend new values as more reliable. This
illustrates well how significantly use of different methodologies - perhaps
the test subjects and
various circumstances as well - can influence the figures.
The new results haven't
been officially recognized, but likely did have a role in FAO/WHO switching to
its (confusing) latest model in which the recommended protein intake for pre-school
children (2 to 5 years of age) - with even higher protein requirement than
Young's recommendation - is
accepted as a formal standard for calculating protein "quality" indicator (PDCAAS)
for all, children and adults.
The fact that these new requirements roughly halve the relative
requirements for methionine/cysteine (for adults) gives to them certain
credibility. According to the present official adult requirement, our
effective protein usage for most major foods is in the 40%-60%
range, mainly due to the limit imposed by their methionine/cysteine content. It
is highly unlikely that the evolution would favor such a poorly adapted,
inefficient metabolic type through long periods of time when food
supply was rather scarce.
However, even if this significantly higher essential amino acid
requirement for adults would turn out to be correct,
would be insignificant,
assuming that the need for non-essential
amino acids would remain unchanged (Young's recommended pattern indicates even
somewhat lower nominal non-essential amino acids requirement).
Beginning with the safe minimum of 750mg/kg/day of
protein, 84mg of which are essential amino acids, the increase to 187mg
total of essential amino acids would take us to 853mg/kg of total protein (which
would have somewhat higher relative essential amino acid content). This is only
14% higher than the old requirement.
words, it would increase the minimum needed protein caloric share
from 8% to about 9% for
average adult female, and from 9% to
10%, or so, for adult male.
However, since the reduction of relative requirement for
methionine/cystine - which is the limiting amino acid in nearly all
important protein food sources except grains - would significantly
improve essential amino acid usage, the new relative caloric protein requirement
could be even lower than the old one.
In all, this new pattern for
indispensable amino acid requirement, if correct, still would fit
into the above rounded off average figure for the minimum safe protein
intake of about 10% of total
average adult protein intake in the US being around 15% of the caloric
total (Centers for Disease Control, 1999-2000), average American is more
than safe with respect to protein consumption. In fact, leaning toward protein over-consumption zone. This
doesn't necessarily mean everyone is getting enough proteins: those
with low caloric intake, inefficient protein metabolism, body injury
or disease are more exposed to that risk.
Indispensable amino acids