Metabolic adaptation to decreases in energy intake due to changes in the energy cost of low energy expenditure regimen.
(1) The energy content in food is used in the human body for three main purposes. The first is to maintain the dissipative structures. Most of the structures of the body are of this kind, i.e. they represent stationary non-equilibrium states, or (generalized) stationary potentials, and are inherently unstable. The second is to maintain a body temperature independent of and usually higher than that of the surroundings. The third is to provide energy for performance of external work. The functional structure of the system providing these results consists of a large number of coupled processes (chemical reactions and translocations), in series and in parallel, whose general nature is well understood but whose quantitative extents are mainly unknown. The coupled processes are driven by the spontaneous reaction of the main substrates with oxygen. Energy flows through the system and is converted to heat (and external work) with simultaneous creation of stationary generalized potentials. For each potential there is an associated flow of energy and the relation between the two is an expression of the efficiency with which the potential is maintained. The processes giving rise to the potentials are likely to be controlled with respect to the efficiency with which the potentials are maintained. The control is partly provided through feedback from the potentials themselves: the potentials are regulated. In this way, the system can respond in a non-linear fashion to perturbations in the energy intake (or energy expenditure): the potentials are maintained at constant, or nearly constant, values. The concept of metabolic adaptation implies that control of the efficiency by feedback from the potentials is an important element in the overall regulation of the potentials, including that of the body temperature. (2) The concept of metabolic adaptation can be framed in such a way that it becomes operational. Quantities such as maintained potentials and efficiency can be revealed in terms of their external manifestations: performance of tasks and associated costs (energy expenditure). A change in efficiency is defined as a change in cost for given tasks. Performances can be defined such that they are both reproducible and of physiological relevance and costs can be measured. There are several complications associated with this approach to operational definition, the most important being the effect on energy expenditure of changes (or differences) in body composition. (3) The FAO approach to estimation of prevalence of undernutrition is based on measurements of energy expenditure for given tasks. This approach requires knowledge of the extent of metabolic adaptation.(ABSTRACT TRUNCATED AT 400 WORDS)
Garby
L
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Department of Physiology, University of Odense, Denmark.
D000222:Adaptation, Physiological / Q000502:physiology*; D002149:Energy Intake / Q000502:physiology*; D004734:Energy Metabolism / Q000502:physiology*; D006801:Humans