Nutrition and Metabolism, Second Edition, edited by S. A. Lanham-New, I. A. Macdonald and H. M. Roche. © 2011 The Nutrition Society. 1.1 Introduction This textbook on nutrition and metabolism covers macronutrient aspects of nutrition in an integrated fashion. Thus, rather than considering the macronu- trients separately, this book brings together informa- tion on macronutrients and energy in relation to specific states or topics (e.g. undernutrition, overnu- trition, cardiovascular disease). Before considering these topics in detail it is necessary to outline the core concepts that underlie nutritional metabolism. The core concepts to be covered in this chapter are nutri- ent balance, turnover and flux, metabolic pools, and adaptation to altered nutrient supply. 1.2 Balance As discussed in Chapter 3, nutrient balance must be considered separately from the concepts of metabolic equilibrium or steady state. In this chapter, the concept of balance is considered in the context of the classical meaning of that term, the long-term sum of all the forces of metabolic equilibrium for a given nutrient. The concept of nutrient balance essentially restates the law of conservation of mass in terms of nutrient exchange in the body. It has become com- mon practice to refer to the content of the nutrient within the body as a ‘store’ but in many cases this is not appropriate and the term ‘reserve’ is better. Thus, the idea of nutrient balance is summarised by the equation: nutrient nutrient change in body intake utilisation nutrient reserves é ù é ù é ù - = ê ú ê ú ê ú ë û ë û ë û The above equation can have three outcomes: ● zero balance (or nutrient balance): intake matches utilisation and reserves remain constant ● positive balance (or positive imbalance): intake exceeds utilisation and reserves expand ● negative balance (or negative imbalance): utilisa- tion exceeds intake and reserves become depleted. In relation to macronutrient metabolism, the con- cept of balance is most often applied to protein (nitrogen) and to energy. However, many research 1 Core Concepts of Nutrition Ian A Macdonald and Michael J Gibney Key messages The change in body reserves or stores of a nutrient is the difference • between the intake of that nutrient and the body’s utilisation of that nutrient. The time-frame necessary to assess the body’s bal- ance of a particular nutrient varies from one nutrient to another. The concept of turnover can be applied at various levels within • the body (molecular, cellular, tissue/organs, whole body). The flux of a nutrient through a metabolic pathway is a measure • of the rate of activity of the pathway. Flux is not necessarily related to the size of the pool or pathway through which the nutrient or metabolite flows. Nutrients and metabolites are present in several pools in the • body. The size of these metabolic pools varies substantially for different nutrients/metabolites, and a knowledge of how these pools are interconnected greatly helps us to understand nutrition and metabolism. Darwinian theory of evolution implies a capacity to adapt to • adverse conditions, including adverse dietary conditions. Many such examples can be cited. Some allow for long-term adapta- tion and others buy time until better conditions arrive. COPYRIGHTED MATERIAL