|Nutritional Evaluation of Protein Foods (UNU, 1980, 154 pages)|
Since the publication of Eva/uation of Protein Qua/ity in 1963 (1) there has been considerable work in the area of protein quality evaluation, not only with respect to defining nutritional requirements but also in relation to the practical needs to monitor and regulate protein quality in food production, processing, and marketing. In this regard, some government regulatory agencies have introduced protein quality criteria into their food standards in order to assure minimal protein values of processed foods and to provide standards of quality for commercial food products.
A new priority, which emphasizes the importance of protein quality and its evaluation, is the growing imbalance between world food supplies and population growth. When chronic food shortages are commonplace, clearly, there will be a need to utilize available food resources as efficiently as possible. At this time, it seems quite likely that developing countries, whose population expansion is putting increasing pressure on their food supplies, will have little opportunity to improve dietary protein quality through increased animal protein production. They will need to emphasize improvements of the protein content and quality in the basic plant foods, particularly cereals, food legumes, and root crops. Thus, considerations of protein quality are of importance in national and international food and nutrition policies.
Interest in protein quality has arisen also in the field of agricultural research (2, 3). This is particularly true in relation to considerations of improved nutritional value, particularly protein content and quality, in the breeding of food crops. Nearly all of the current efforts to produce improved varieties of cereals (rice, wheat, corn, and sorghum), legumes (common beans, cow peas, grams, pigeon peas, soy beans, and similar edible species), and root crops (potatoes and cassava) include evaluations of protein quality. Thus, the plant breeder has identified the need to screen for protein quality and quantity in large numbers of cultivars.
During the past decade, considerable research has been devoted to the approaches and methods used in assessing the nutritive value of food and feed protein sources.
Limitations of the methods that were in common use at the time of the previous publication are now recognized. In particular, the standardized biological assay procedures such as protein efficiency ratio (PER) and biological value (BV) are not now considered by many workers to provide satisfactory predictions of the capacity of proteins to fulfil nutritional needs. Re-evaluation of the methodology for assessing protein quality is therefore required, and in this revision modern developments are taken into full account in arriving at the recommendations for evaluation of protein quality.
The nutritional value of a dietary protein depends largely upon the pattern and concentration of essential amino acids that it provides for the synthesis of nitrogen-containing compounds within the body. This concept provides the fundamental basis for all methods that attempt to assess the protein nutritive value of a food. Thus, the quality of a protein may vary with the amount and pattern of amino acids required for the functions measured.
The comprehensive evaluation of protein quality begins with the determination of nitrogen content, identification of the principal nitrogenous constituents of the food, and assessment of nutritional values, including digestibility, by means of in vitro and in vivo assays. The final stage is determination of the capacity of the protein to meet nutritional requirements of human beings of various ages. The various chapters in part I of this report consider each phase in the evaluation of protein quality. The methods and procedures, as well as their significance and limitations in assessing the nutritional value of dietary protein sources, are discussed. Initially, a brief review is made of humans' protein and amino acid requirements as a basis for subsequent considerations of protein quality. Then the chemical, microbiological, and enzymic methods for the determination of protein and amino acids, and methods that are less direct but potentially useful in screening large numbers of samples for these constituents, are discussed. A chapter is devoted to the assessment of protein quality from amino-acid profile and digestibility data. This approach provides a relatively easy means of obtaining an approximation of the nutritive value of a protein or a mixture of proteins, and may be the only means of judging the potential of a protein source. Although chemical and in vitro methods provide initial information on the possible nutritive value of a protein, it is desirable and often essential to explore the nutritive value of proteins in vivo. Since food protein sources for direct human feeding require experimental evaluation of protein quality and, furthermore, the introduction of unconventional protein foods and changes in the usual dietary sources of protein intake make it important to assess the nutritional impact of protein quality in reference to humans, the biological assay of protein quality in both experimental animals and human subjects is discussed. A separate chapter covers the evaluation and choice of specific assay procedures.
Part 11, on the other hand, provides detailed methodology for workers concerned with various aspects of protein quality evaluation. The methods discussed range from relatively simple chemical procedures to suggested protocols for human studies. Inclusion of procedures in this report does not imply specific endorsement but is an attempt to make a wide range of relevant procedures available to investigators throughout the world whose facilities and requirements may vary greatly. The final chapter in part 11 outlines some of the statistical techniques for handling the data generated in such protein quality evaluation procedures.
Part 111 is a glossary of terms used in protein quality evaluation, in an attempt to define and standardize the meaning of terms and ratios as they are used not only in this publication but also on a worldwide basis.
Because there is a need to develop newer and improved crop varieties, consideration should be given to the sequence and methods of procedures to be followed in the monitoring and development of plant breeding programmes. A summary of the recommendations of the Protein-Calorie Advisory Group (PAG) of the United Nations system (4) is given in appendix A. Appendix B, in two parts, lists a selection of reviews and texts in the field of protein evaluation, and relevant documents from the PAG and from the United Nations University World Hunger Programme.
This report is not an exhaustive review of the problems and procedures applicable to a protein quality evaluation. Rather, it is intended to provide a broad overview and assessment of strengths and weaknesses of the various methods that may be used in the evaluation of dietary protein quality. However, an effort has been made to include the detailed descriptions of a number of established and new methods so that they may be applied by workers who may not have ready access to the original literature.
1. Committee on Protein Malnutrition, Food and Nutrition Board, National Research Council, Evaluation of Protein Quality, ed. P.L. Pellett, NAS-NRC Publ. 1100 (National Academy of Sciences, Washington, D.C., 1963). 2. J.H. Hulse, K.O. Rachie, and L.W. Billingsley, eds., Nutritional Standards and Methods of Evaluation for Food Legume Breeders, IDRC-TS7e (International Development Research Centre, Ottawa, Canada, 1977). Nutritional Eva/uation of Cereal Mutants, proceedings of an Advisory Group meeting, Vienna, July 1976, ST1/Pub. 444 (International Atomic Energy Agency, Vienna, Austria, 1977) 4. "Protein Methods for Cereal Breeders as Related to Human Nutritional Requirements," PAG Guideline No. 16,PAG Bulletin, V (2): 2248(1975).