|Energy and Protein Requirements, Proceedings of an IDECG workshop, November 1994, London, UK, Supplement of the European Journal of Clinical Nutrition (International Dietary Energy Consultative Group - IDECG, 1994, 198 pages)|
|Protein requirements of infants and children|
To better understand protein requirements of infants and children, more information is needed on a variety of topics. The following is a partial list of research priorities:
(1) There is a need for studies of protein requirements that include functional outcome measures (both short term and long-term), not just growth rate or nitrogen balance. The question 'protein requirements for what?' has not been adequately addressed. Such studies should include not only the potential consequences of marginal intakes, but also the long-term impact of moderately high protein intakes (such as later kidney function in formula-fed infants).
(2) Information on the efficiency of utilization of dietary protein at various ages and in children with different growth rates is needed. In particular, the efficiency of protein utilization in formula-fed vs breastfed infants should be studied to determine whether protein requirements differ by feeding mode in early infancy. Additional data on the utilization of non-protein nitrogen in human milk and infant formulas would also be useful.
(3) It is essential to determine the degree of intra-individual day-to-day variation in growth at various ages, and the nutrient needs (not just protein) required to permit the child to deposit new tissue rapidly during growth 'spurts'. Limited information (Lampl et al, 1992) suggests that spurts in linear growth may be of short duration (24-48 h), but it is not known whether the magnitude of growth during such episodes or the frequency of episodes can be affected by protein intake. Both animal models and human studies would be useful in answering these questions.
(4) Updated information on body composition during infancy and childhood, particularly the percentage of weight gain that is fat-free body mass, is important for accurate estimates of protein requirements using the factorial method.
(5) Better data on maintenance nitrogen requirements of infants, children and adolescents are needed, although ethical issues may limit the types of studies that can be carried out. Studies in children who were not previously malnourished, and data on the protein needs of female adolescents should be a high priority. In addition, long-term balance studies at protein intakes close to estimated requirements would provide key evidence to support or reject the adequacy of the estimates based on short-term balance studies.
(6) Amino acid requirements require further study at all ages, including children. Such research should consider not only amino acid needs for protein metabolism, but the other functions of amino acids as well. Requirements for the sulfur amino acids in particular require investigation. These, ironically are the amino acids for which calculated 'needs' are lower in the MIT pattern (see section 2.5) than in Rose's estimates. Yet data from rats, pigs and chicks suggest a high requirement for cysteine (as a proportion of total amino acid needs) at protein intakes close to maintenance levels, and recent data from the Rowett Research Institute (Fuller, personal communication) suggest that a significant proportion of this need appears to be devoted to the synthesis of taurine.
(7) To determine protein and amino acid needs for catch-up growth, data are needed on the maximal rate of growth that can be achieved without compromising body composition or function. The rates of catch-up growth achievable in non-hospital settings should be documented.
(8) In disadvantaged populations, community studies evaluating the impact of nutrition interventions that provide extra protein, controlling for the intakes of energy and other nutrients, would be particularly useful in determining whether protein is a key limiting nutrient for such children.
(9) The impact of infection on amino acid and protein needs requires further investigation. The impact of acute-phase protein synthesis on whole body protein economy and requirements is of particular interest. Data are needed to determine whether chronic stimulation of the immune system diverts essential nutrients from whole body growth in undernourished children, and, secondly, the dietary needs to prevent or compensate for this outcome. Several outcome variables would be useful: (a) the ratio of protein synthesis to protein breakdown under conditions of immunostimulation and refeeding; (b) the rate of oxidative loss of dietarily indispensable amino acids and (c) the rates of acute-phase protein synthesis, of transport protein synthesis, and the interactions between these and whole body protein metabolism. In studies of amino acid requirements in infected children, identification of the infective agent(s) should be attempted and concentrations of hormones known to mediate amino acid/protein metabolism during infection should be determined because these indices may facilitate predictions of the impact of infection in other populations.
Acknowledgements - Data for the pooled analysis of growth of breastfed infants came from seven studies conducted by the following investigators: KG Dewey, MJ Heinig, LA Nommsen and B Lonnerdal (USA); NF Krebs, CJ Reidinger and KM Hambidge (USA); KF Michaelsen and G Samuelson (Denmark); LA Persson and G Samuelson (Sweden); L Salmenpera, J Perheentupa and MA Siimes (Finland); AA Paul, TJ Cole, EA Ahmed and RG Whitehead (UK); and D Yeung (Canada). The analysis was funded by a grant from Wellstart International through the US Agency for International Development (AID), Bureau for Research and Development, Office of Health (Cooperative Agreement No. DPE-5966-A-00 1045-00) to the University of California, Davis. The contribution of Peter Reeds to this paper has been funded in part with federal funds from the US Department of Agriculture, Agricultural Research Service under Cooperative Agreement number 58-6250-1-003. The contents of this publication do not necessarily reflect the views or policies of the US Department of Agriculture, nor does mention of trade names, commercial products, or organizations imply endorsement from the US Government. We gratefully acknowledge comments on the initial draft from JC Waterlow and SJ Fomon.