Protein Requirement Recommendation

The requirement indicated by the meta-analysis (a median requirement of 105 mg nitrogen/kg per day or 0.66 g/kg per day of protein) can be accepted as the best estimate of a population average requirement for healthy adults.

General recommendation

For adults, the protein requirement per kg body weight is considered to be the same for both sexes, at all ages, and for all body weights within the acceptable range. The value accepted for the safe level of intake is 0.83 g/kg per day, for proteins with a protein digestibility-corrected amino acid score value of 1.0. No safe upper limit has been identified... (p. 242)

Amino Acid Requirements of Adults

Protein Requirement Definition

Protein requirement can be defined as: the lowest level of dietary protein intake that will balance the losses of nitrogen from the body, and thus maintain the body protein mass, in persons at energy balance with modest levels of physical activity, plus, in children or in pregnant or lactating women, the needs associated with the deposition of tissues or the secretion of milk at rates consistent with good health.

To satisfy the metabolic demand, the dietary protein must contain adequate and digestible amounts of nutritionally indispensable amino acids (histidine, isoleucine, leucine, lysine, methionine, phenylalanine, threonine, tryptophan and valine), and amino acids that can become indispensable under specific physiological or pathological conditions (conditionally indispensable: e.g. cysteine, tyrosine, taurine, glycine,arginine, glutamine and proline), plus sufficient total amino acid nitrogen, which can be supplied from any of the above amino acids, from dispensable amino acids (aspartic acid, asparagine, glutamic acid, alanine and serine) or from other sources of non-essential nitrogen. 

At present, no method is entirely reliable for determining the dietary requirement for indispensable amino acids. The available nitrogen balance data have been shown to yield greatly differing estimates according to the assumptions about unmeasured nitrogen losses and the statistical method employed to analyse the balance data.

Additional Citation

A more active person expends greater amounts of energy, consumes greater amounts of food, and hence has a higher absolute level of protein consumption. Since, with increasing activity, the demand for amino acids and nitrogen increases to a much lesser extent (if at all) than energy demands, it becomes easier to satisfy nitrogen demands, and the amino acid pattern of the diet becomes of lesser importance. In contrast, as activity levels fall, food consumption falls and hence absolute protein intake falls, so any relative imbalance between the pattern of amino acids provided by the diet and the pattern required by the body will become more evident. Thus, at lower levels of food consumption, a diet that might have been adequate for protein at high levels of activity, may no longer be adequate at lower levels of activity.

The pathways of amino acid metabolism and interchange are critically dependent upon an adequate micronutrient status, and hence upon the amount and quality of food consumed.

While activity can increase the demand for protein, the extent of this may be minimized by training and by adequate and appropriate energy intake.

The dietary supply of nitrogen-containing compounds into the system is predominantly protein, but also includes free amino acids, nucleotides and creatine, each of which may be important for health. 

 ...There is significant secretion of proteins associated with the processes of digestion and absorption. Most proteins are digested and the resulting amino acids or peptides absorbed. 

...The “efficiency” of utilization of indispensable amino acids depends upon the total nitrogen and the form of nitrogen in the diet. The higher the total nitrogen in the diet, the lower the consumption of indispensable amino acids to achieve nitrogen balance.

 ...The requirement for indispensable amino acids is not an absolute value, but can be expressed only in relation to the total nitrogen intake. The demonstration that the consumption of any form of dispensable nitrogen reduces the need for indispensable nitrogen implies that at lower levels of total nitrogen consumption, indispensable amino acids are being used inefficiently as a source of nitrogen for the formation of dispensable amino acids.

The human organism can and does tolerate a wide range of dietary protein concentrations at no obvious cost. The difficulty for defining nutritional requirements for protein and amino acids lies in identifying the lower and upper limits of this intake range, beyond which any further adaptation may involve costs of one sort or another.