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Protein requirements of infants and children

1. Introduction
2. Protein requirements of infants
3. Protein requirements of children and adolescents
4. Protein needs during catch-up growth
5. protein needs associated with infection
6. Assessment of protein quality of weaning diets
7. Future research needs

KG Dewy1, G Beaton2, C Fjeld3, B Lönnerdal1 and P Reeds4 (with input from KH Brown1, MJ Heinig1, E Ziegler5, NCR Räihä6 and IEM Axelsson6)

1 Department of Nutrition and Program in International Nutrition, University of California, Davis, CA 95616-8669;
2 Department of Nutritional Sciences, Faculty of Medicine, University of Toronto, Toronto, Ontario, Canada M5S 1 A8;
3 Section of Nutritional and Health-Related Environmental Studies, International Atomic Energy Agency, PO Box 100, A1400 Vienna; 4 USDA/ARS Children's Nutrition Research Center, Department of Pediatrics, Baylor College of Medicine 1100 Bates Street, Houston, TX 77030; 5 Division of Nutrition, Department of Pediatrics, University of Iowa, Iowa City, Iowa 52242; 6 Department of Pediatrics, University of Lund, Malmo General Hospital, S-214 01 Malmo, Sweden

Descriptors: protein, amino acids, nutrition, nitrogen, catch-up growth, diet quality

1. Introduction

There are several approaches for estimating the protein requirements of infants and children. During early infancy, the intake of breastfed infants has been used as a model, under the assumption that protein requirements are satisfied by human milk alone. Alternatively, a factorial model can be used to calculate requirements, or a direct experimental approach can be taken whereby key outcomes are measured while subjects are fed varying levels of protein. Finally, an approach which has been called 'operational', based on protein-energy ratio, has been proposed (Waterlow, 1990). This variety of approaches has created a serious dilemma, as the estimates obtained in the past using these alternative models have differed considerably, partly because of confusion over which aspects of the actual distribution of protein requirements they were describing (Beaton, 1994). Furthermore, all the above approaches conceptualize requirements in terms of total protein, whereas recently there has been an increased focus on estimating requirements in terms of the needs for individual amino acids (Scrimshaw and Schürch 1991). Although there has been heated debate on this latter issue, and some data relevant to adults, there is a paucity of data for infants and children.

Another major dilemma is posed by the question: protein requirements for what? Historically, a satisfactory growth rate during infancy and childhood has been the 'litmus test' for the adequacy of protein intake. However, in recent years there has been greater attention to the need for assessing functional outcomes such as immune function and behavioral development. It is not clear whether the rate of growth is an adequate proxy for these other outcomes. Under conditions of nutrient deficiency, for example, it may be that growth falters only after other aspects of function, such as behavioral development, are compromised. Basing requirements on adequate growth might therefore underestimate the true need for optimal function. On the other hand, reports of differences in growth rates between breastfed and formula-fed infants (e.g. Dewey et al, 1992) have raised the issue of whether maximal growth is synonymous with optimal growth. It has been suggested that excessive protein intake may jeopardize certain physiological functions (see section 2.3). Therefore, it is theoretically possible that a protein intake that maximizes growth may be disadvantageous in other respects.

Note: assistance in preparing this report does not imply that all of the contributors agree with all of the conclusions and recommendations.
Correspondence: KG Dewey.

The task set out for this position paper - to evaluate whether the protein requirements for infants and children described in the 1985 FAO/WHO/UNU report on Energy and Protein Requirements should be revised - is thus very complex. Rather than attempting to cover all the issues in depth, the objective of this paper is to critically review the basis for the 1985 recommendations, to suggest which sections should be revised, and to identify topics requiring further research. The paper first reviews the protein needs of normal infants and children, followed by sections on protein requirements in situations of catch-up growth or in association with infections, methods for assessing the protein quality of weaning diets, and future research needs. The special needs of low birthweight infants are not covered.

It is useful to begin by clarifying the terminology that will be used in this paper. The literature on nutrient requirements in general and protein or amino acid requirements in particular is plagued by the fact that the term 'requirement' is used very loosely and is often confused with the notion of a recommended dietary allowance. In keeping with the 1985 report, this paper will use the word 'requirement' when discussing the true biological need for protein or amino acids (the lowest intake that will maintain functional needs of the individual). As recognized in the 1985 report, there is a distribution of such requirements among seemingly similar individuals, so the mean requirement is usually taken as the starting point. The 'safe level of intake' is defined as the amount that will meet or exceed the requirements of practically all individuals in a population, which is generally calculated as the mean requirement + 2 s.d. of the requirement. In the case of protein, this refers to a quantity of high-quality (or 'reference' protein). With mixed diets, the 'safe level' will need to be adjusted for digestibility and amino acid composition of the foods consumed in order to arrive at recommended intakes for a specific population. Understanding the distinction between biological requirements and recommended intakes is critical, particularly for infants whose total diet may be prescribed based on recommended allowances. When assessing the observed mean intake of a group, it should be kept in mind that this should be somewhat higher than the 'safe level' of intake. This is because the group mean intake required to ensure adequacy for all individuals must take into account both the distribution of requirements among individuals and the distribution of intakes among individuals (and the correlation between intake and requirement), whereas the conventional calculation of a 'safe level' considers only the distribution of requirements.

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