Overfeeding, autonomic regulation and metabolic consequences.

Scheurink AJ, Balkan B, Strubbe JH, van Dijk G, Steffens AB

Department of Animal Physiology, University of Groningen, Haren, The Netherlands.

The autonomic nervous system plays an important role in the regulation of body
processes in health and disease. Overfeeding and obesity (a disproportional
increase of the fat mass of the body) are often accompanied by alterations in
both sympathetic and parasympathetic autonomic functions. The
overfeeding-induced changes in autonomic outflow occur with typical symptoms
such as adiposity and hyperinsulinemia. There might be a causal relationship
between autonomic disturbances and the consequences of overfeeding and obesity.
Therefore studies were designed to investigate autonomic functioning in
experimentally and genetically hyperphagic rats. Special emphasis was given to
the processes that are involved in the regulation of peripheral energy
substrate homeostasis. The data revealed that overfeeding is accompanied by
increased parasympathetic outflow. Typical indices of vagal activity (such as
the cephalic insulin release during food ingestion) were increased in all our
rat models for hyperphagia. Overfeeding was also accompanied by increased
sympathetic tone, reflected by enhanced baseline plasma norepinephrine (NE)
levels in both VMH-lesioned animals and rats rendered obese by
hyperalimentation. Plasma levels of NE during exercise were, however, reduced
in these two groups of animals. This diminished increase in the
exercise-induced NE outflow could be normalized by prior food deprivation. It
was concluded from these experiments that overfeeding is associated with
increased parasympathetic and sympathetic tone. In models for hyperphagia that
display a continuously elevated nutrient intake such as the VMH-lesioned and
the overfed rat, this increased sympathetic tone was accompanied by a
diminished NE response to exercise. This attenuated outflow of NE was directly
related to the size of the fat reserves, indicating that the feedback mechanism
from the periphery to the central nervous system is altered in the overfed
state.