We have presented and tested in experiments with insects a new framework which integrates functional, mechanistic, ontogenetic and comparative aspects of nutrition. The framework firstly identifies local optima (nutritional, intake and growth `targets') within a multi-dimensional nutritional space, where each functionally relevant nutrient forms a single dimension. The positions of these targets are located experimentally through studies of feeding behaviour and physiology and their functional significance is tested using independent performance criteria. Functional rules employed by animals to either reach these targets or, if that is not possible, find a point of best compromise, are then investigated by reference to the geometry of arrays of intake and growth across a range of foods. Changes in the position of the targets with ontogeny are considered, as are the nature of underlying homeostatic mechanisms. We also show how comparative analyses of a range of species can be used to investigate the influence of non-nutritional, ecological factors on the location of the targets.