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This introductory chapter expands on three important themes that form the backdrop of this book's study: nutrition touches and links all living things; nutrition is complex; and there have been benefits both from the highly specific and detailed approach of applied nutritional sciences and the simplified, general approaches adopted in the ecological sciences. At the most conspicuous level, nutrition is a primary factor defining the geographic distribution and temporal pattern of activity for many animals. Achieving nutritional homeostasis involves a complex interplay between multiple and changing nutrient needs and variable foods. The detailed studies of human and animal nutritionists have yielded a wealth of information for deriving dietary recommendations for human health, formulating animal feeds, designing dietary regimes for captive animals, supplementing the nutrition of free-ranging animals—and many other important practical applications.

This chapter discusses the Geometric Framework (GF) for nutrition. GF satisfies the multiple-food-components requirement using a simple device known as a nutrient space. A nutrient space is a geometric space built of two or more axes, where each axis represents a food component that is suspected to play a role in influencing the animal's responses to its environment. In most cases, these food components will be nutrients but this is not invariably the case. The nutrient space provides the common context in which to describe the pertinent aspects of the animal, its environment, the interactions between animal and environment, and the consequences of these interactions.

This chapter focuses on how animals are able to regulate their intake and use of multiple nutrients. To regulate the balance of nutrients eaten, an animal needs to assess the composition of available foods in relation to its nutritional requirements. Integration of information about food composition and nutritional state occurs both at the periphery, by nutrient-specific modulation of taste receptors, and more centrally as signals from systemic and peripheral sources converge onto the neural circuits that control feeding behavior. Meanwhile, postingestive regulatory responses can assist in rebalancing an imbalanced nutrient intake. Once digested and absorbed across the gut, nutrient supplies can be further rebalanced by differentially voiding excess nutrients and conserving nutrients that are in limited supply.