7. Research suggestions

The concept of dietary protein quality seems at first sight to be self-evident. However, upon closer examination, a precise definition becomes more difficult. The capacity of a protein source to meet the amino acid and nitrogen requirements of the organism depends not only upon the amino acid composition and digestibility of the protein source or mixture, but also upon the composition and adequacy of the diet as a whole, and on the physiological, nutritional, and health status of the consumer. Among the dietary factors that might be included are level and source of carbohydrate and lipid intake, mineral and vitamin content, water intake, and size and frequency of meal ingestion. Nutritional status, age, and health factors interact in a complex way to modify the utilization of dietary protein. Studies of the effects of these various factors and their interactions are necessary in order to define adequately the variability and nutritional significance of dietary protein quality. However, the following suggestions for research are readily apparent.

Standard Proteins in Biological Assays

The biological assessment of protein quality depends upon an appropriate comparison with a suitable reference protein. In the past, casein has been used as the standard or "reference protein," and standards of protein quality in foods have been related most frequently to the nutritive value of casein. However, there are a number of reasons why casein does not provide the ideal reference standard. First, due to its limiting concentration of sulphur amino acids, it is not as efficiently utilized in meeting the nutritional needs of the rat, in comparison with proteins such as lactalbumin. Thus, it would appear more desirable to utiiize a protein of higher quality for assay purposes. Another difficulty is that even the high-nitrogen ANRC casein may not be as well standardized a product of constant nutritional quality as is commonly assumed. Therefore, it is recommended that research be focused on the development and standardization of a suitable reference protein. A defined mixture of L-amino acids should be explored as a possible reference protein of standard nutritive value. While lactalbumin has been used in slope-assay procedures, its availability and lack of standardization do not make this protein an ideal standard protein at the present time. ANRC casein supplemented with 1 per cent DL-methionine has proved to be of high nutritional value in the rat and is recommended for further evaluation as a possible standard protein for comparative purposes.

Choice of Animal Species in Bioassay

Bioassay procedures utilizing the laboratory rat require amounts of test protein that may be in excess of the amount readily available. Thus, it would be worthwhile to explore the use of other species for protein quality estimations. Another problem is the relevance of findings in the rat, or any other species, to quantitative aspects of protein quality in human subjects. Although some evidence indicates, as discussed earlier, that there is a close and quantitative relationship between studies in growing rats and nitrogen-balance indices in children who have recovered from malnutrition, the data are limited and it is not known whether results in children who have always been healthy would give similar close agreement. Furthermore, there are insufficient data in the literature to provide a critical comparison of protein quality in humans of different ages, although it is assumed that protein quality is of lesser significance in the adult. In order to develop appropriate bioassay procedures in rats that have relevance to human nutrition, these problems must be explored.

Assay Procedures

It might also be questioned whether any one of the common bioassay procedures is adequate or significantly better than other assays for all protein sources irrespective of the limiting amino acid. A particularly difficult problem is the appropriate choice of assay procedure with rats in the case of low-quality protein sources, especial)y those limited in lysine. Further critical examination of this problem is warranted.

Plasma Amino Acids

The earlier studies relating plasma amino acid levels and protein quality were carried out without full appreciation of all the various factors affecting plasma amino acid levels. With current information it may be possible to develop a more reliable and rapid protein quality assay with growing rats, using changes in plasma amino acid patterns. This approach may also be of potential value for the clinical evaluation of protein quality. Thus, it would be desirable to explore critically those experimental conditions under which the measurement of plasma amino acid levels would provide useful predictive data on protein nutritional quality in human feeding. This may be accomplished initially through examination of the plasma amino acid levels at the termination of standard rat bioassay procedures. An index of protein quality and identification of the limiting amino acid might be obtained through this approach.

Rapid Procedures

With the greater requirement for protein quality data for use in food labelling and other regulatory purposes, there is a tremendous need by both the food industry and the regulatory agencies for improved, rapid but reliable tests of protein quality. Animal bioassays are time-consuming and expensive, and they are not ideal for meeting the needs of the food industry for nutrition labelling of food products. The ideal test would be reproducible, rapid, inexpensive, and applicable to a wide variety of food products. Perhaps such a single test is unattainable. Because protein quality is not dependent on a single factor but rather is the result of an interaction of a complex set of variables and interrelated factors, more than one test may be needed. Such rapid tests should perhaps be concerned with the separate major components of quality - i.e., amino acid composition (score), processing damage, digestibility, and amino acid retention and utilization. Criteria might be devised for each of these categories rather than for a single quality measurement.

Finally, there is still a need to undertake collaborative studies of protein quality assay procedures, ranging from chemical and in vitro studies to rat and human bioassays. These collaborative investigations should involve laboratories both in developed and developing countries. It is hoped that the United Nations University will take the initiative for this research effort.