After 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 others.

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.

During digestion, protein is broken down into its building blocks - amino acids - 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, Threonine, Tryptophan 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 not available.

Daily protein requirement

It 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 catch up.

The difficulty of establishing human protein needs arises from the great complexity of food digestion and utilization. The principal method used in studies and experiments is based on the measurement of intake and excretion of nitrogen (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).

However, limitations 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 subject.

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 0.45g/kg daily 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 weight.

Thus the FAO/WHO (World Health Organization) recommended daily protein intake is

0.75g 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 0.8g/kg (National 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 total calories.

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 exceeds about 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 also 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 regions.

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 acid requirements.

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

significantly 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 ▼	1985 FAO/WHO REQUIREMENT	Young et al.
Essential amino acid ▼	1985 FAO/WHO REQUIREMENT	Requirement	Ratio
Isoleucine	10 (0.12)	23 (0.12)	2.3
Leucine	14 (0.17)	39 (0.21)	2.1
Lysine	12 (0.14)	30 (0.16)	2.5
Methionine + Cystine	13 (0.16)	15 (0.08)	1.15
Phenylalanine + Tyrosine	14 (0.17)	39 (0.21)	2.1
Threonine	7 (0.08)	15 (0.08)	2.1
Tryptophan	3.5 (0.04)	6 (0.03)	1.7
Valine	10 (0.12)	20 (0.11)	2
TOTAL	83.5 (1.00)	187 (1.00)	2.2

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,

practical consequences 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.

In other 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 calories.

With the 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.

CONTINUES: Indispensable amino acids

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