Although not its primary role, protein can serve as a source of energy when insufficient carbo- hydrate and fat are available to meet the body’s needs. Proteins also form the major components of the hair and nails, as well as the structural framework of bones.
It is also important to note that non-protein nitrogenous compounds are produced from some of the amino acids. Glycine is used in haem, nucleic acid and bile acid synthesis. Other examples include the use of tryptophan to make nicotinic acid and tyrosine in catechol- amine synthesis.
Nitrogen balance
An overall indicator of protein metabolism in the body is the nitrogen balance, which is the difference between nitrogen intake and nitrogen output. When the balance is positive, protein is being retained in the body, indicating tissue synthesis. A negative nitrogen balance occurs when there is a net loss of protein from the body, either because there is catabolism or because protein or energy intakes are insufficient to meet the daily needs (see Figure 4.5).
When nitrogen intake and output are in equi- librium, then protein is neither being gained nor lost. Nitrogen balance studies do not, however, tell us where the protein is being stored or catab- olized, or what its functions are. Not all nitrogen
flux relates to protein: there is some metabolism of non-protein nitrogen taking place, and some retention of nitrogen may represent increases in this fraction rather than in protein content. However, it is generally assumed that the nitro- gen (N) in Western diets is largely of protein ori- gin, and the conversion factor of N 6.25 is used to convert between nitrogen and protein content. This implies that all proteins contain 16 per cent nitrogen; in reality the amount of nitro- gen in proteins varies between 15.7 per cent in milk and 19 per cent in nuts.
Losses of nitrogen can occur from the body via a number of routes. Faecal loss of nitrogen represents unabsorbed dietary nitrogen together with residual nitrogen from the digestive juices and mucosal cells shed into the tract. In health, these losses are small. Nitrogen is lost in the urine in the form of urea (mostly), ammonia, uric acid and creatinine. The urea content reflects dietary intake and, therefore, decreases as protein intake falls. Creatinine levels are rela- tively constant, as these are related to the muscle mass, and represent its daily turnover. In add- ition, nitrogen is lost daily in skin cells, hair, nails, sweat and saliva and, although it is possi- ble to measure these by meticulous study, in practice, a constant figure is used.
Balance studies carried out when the diet is devoid of protein are used to indicate the obliga- tory losses of nitrogen. Values obtained in such studies give an average obligatory protein loss of 0.34 g/kg body weight for adults (or 55 mg of nitrogen/kg body weight).
Nitrogen balance studies have been criticized as imprecise for a number of reasons. These include:
■ overestimation of intake;
■ incomplete estimation of losses – gaseous nitrogen loses are usually uncounted (breath and colonic gases);
■ uncertainty about adaptation to changes in protein intake.
However, despite these shortcomings, there is still a role for nitrogen balance data until newer methodologies are sufficiently reliable and robust. These include the use of tracer-labelled amino acid oxidation studies. The most com- monly used isotope is 13C, with measurement of labelled CO2excretion. These studies allow fac- torial assessments of metabolic demands, effi- ciency of utilization and requirement. However, it is still problematic to arrive at information relating to whole-body protein turnover.
Protein quality
If a protein is to be useful to the body, it should supply all of the indispensable amino acids in appropriate amounts. If this is not the case, any synthesis that is required can only take place by breaking down existing proteins. Alternatively, limited synthesis may take place until all of the amino acid present in least amounts has been used up. The body cannot synthesize incomplete proteins, therefore, synthesis is limited by this amino acid. Such an amino acid is termed ‘limit- ing’, and the protein from which it comes would be described as having low quality. How can this be quantified?
Milk or egg proteins have traditionally been used as the ‘reference proteins’, as their amino acid pattern most nearly conforms to that of total body protein. Most recently, the amino acid pat- tern of human milk has been set as the standard against which all other proteins can be judged for their efficiency of meeting human needs.
Different sources of protein have been shown to match the required amino acid pattern to varying extents, and combining different plant foods, for example, makes it possible to obtain the necessary amino acids from several sources and achieve an overall balance (Table 4.4).
PROTEIN QUALITY ❚
Populations have naturally been doing this for generations; there is nothing new about it and many traditional dishes reflect this.
In addition, it is possible to complement protein foods of plant origin with foods derived from animals to compensate for the limiting amino acid. In particular, milk and its products provide good complementary protein to partner the plant foods. Examples of such traditional mixtures include bread and cheese, macaroni cheese, rice pudding, cereal and milk.