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Recent Advances in Methods of Assessing Dietary Amino Acid Requirements for Adult Humans^1,2,

Gordon A. Zello^*, Linda J. Wykes^,**,3, Ronald O. Ball^**,, and Paul B. Pencharz^,**,,4

^* College of Pharmacy and Nutrition, University of Saskatchewan, Saskatoon, Saskatchewan, Canada S7N 5C9 The Research Institute, The Hospital for Sick Children ^** Department of Nutritional Sciences Department of Paediatrics, University of Toronto, Toronto, Ontario, Canada M5G 1X8 Department of Animal and Poultry Science, University of Guelph, Ontario, Canada N1G 2W1

The requirements for the indispensable amino acids have been determined by a number of different methods. Historically, descriptive or gross measures like growth and nitrogen balance have been used. However, technological advancements in recent years have resulted in the use of more precise and mechanistic metabolic approaches (i.e., plasma amino acid concentrations, amino acid oxidation, indicator amino acid oxidation) to examine requirement. Nevertheless, the current recommendations are still based on nitrogen balance studies. Requirement estimates based on other methodologies, such as plasma amino acid concentrations and direct amino acid oxidation, suggest that the requirement estimates derived from nitrogen balance experiments are too low. However, these higher estimates have also been criticized on conceptual and methodological grounds, resulting in considerable controversy in the area of indispensable amino acid requirements. A new technique, indicator amino acid oxidation, addresses many of the criticisms directed toward the alternative methods and the proposed higher requirement estimates. This paper reviews the current knowledge of amino acid requirements and makes recommendations in light of new information that has been provided from recent indicator amino acid oxidation research. It is concluded that the nitrogen balance-based estimates of amino acid requirement are too low.

KEY WORDS: • nitrogen balance • amino acids • oxidation • stable isotopes • humans

¹ Supported by the Medical Research Council of Canada (Grants MA10321 and MT5466) and the Natural Sciences and Engineering Research Council of Canada (Grant OGP0138042).

² The costs of publication of this article were defrayed in part by the payment of page charges. This article must therefore be hereby marked "advertisement" in accordance with 18 USC section 1734 solely to indicate this fact.

³ Present address: School of Dietetics and Human Nutrition, McGill University, Quebec, Canada H9X 3V9.

⁴ To whom correspondence should be addressed.

Manuscript received 21 December 1994. Revision accepted 11 September 1995.