Protein-Energy Interactions (IDECG, 1991, 437 p.): The metabolic basis of amino acid requirements: Abstract

Protein-Energy Interactions (IDECG, 1991, 437 p.)

The metabolic basis of amino acid requirements

		(introduction...)
		Abstract
		1. Introduction: The nature of the problem
		2. Nutrient requirement models
		3. The Millward & Rivers requirement model: Qualitative aspects
		4. The variable extrinsic component of the maintenance requirement
		5. The anabolic drive
		6. Hormonal components of the anabolic drive
		7. Protein requirements: Formal statement
		8. The issue of protein quality
		9. Stable isotope studies
		10. Practical experience of biological values of dietary protein
		11. Urea salvage
		12. Indispensable amino acid requirements for the anabolic drive
		13. Conclusions
		References

Abstract

The Millward & Rivers model for protein requirements accounts for both functional intrinsic needs for amino acids (growth and obligatory amino acid catabolism), and for extrinsic responses to habitual protein intake in terms of oxidative losses of amino acids. These losses are part of homeostatic control, under acute and chronic regulation by dietary protein feeding, and reflect the potential toxicity of many indispensable amino acids (IAA). Because postabsorptive amino acid losses increase with increasing habitual protein intake, the diet must include sufficient protein to replace losses occurring in the previous postabsorptive state, i.e., the more you eat, the more you need for overall balance. Judgement of the value of any particular intake requires consideration of the regulatory influence of dietary amino acids on the organism: the anabolic drive. The optimal protein requirement comprises the sum of intrinsic needs for protein accretion, obligatory amino acid consumption, and sufficient amounts for an appropriate regulatory response or anabolic drive. Such a response might be an agreed rate of height growth or index of immunocompetence. While the issue of protein quality is relevant to each of these components, accretion accounts for so little in humans that no relationship should be expected between protein quality for growing animals and human needs. Transient accretion during diurnal cycling could involve some recycling of lysine and threonine, reducing their dietary needs.

The IAA requirement to cover obligatory amino acid needs cannot be predicted from first principles, but judging from pig studies is dominated by S-amino acids and threonine, with little lysine need. The IAA requirement for the anabolic drive is unknown. In any case, the emerging evidence that changes in the dietary amino acid composition can be achieved during urea salvage and bacterial amino acid synthesis in the lower gut means that the relationship between dietary need for protein and IAA and dietary protein quality becomes even more difficult to predict.