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Plasticity of biologic set points

This is an excerpt from Biologic Regulation of Physical Activity by Thomas Rowland.

Biologic regulators that govern body physiologic functions are products of millions of years of evolutionary pressure. They should not, intuitively, be expected to be readily modifiable over time by extrinsic perturbations. However, clear examples of such shifts of homeostatic set points in response to environmental factors do exist.

Body temperature is normally tightly controlled by hypothalamic regulators to within a degree of 98.6 °F (37 °C) in respect for demands of thermal constancy of metabolic functions. Yet release of pyrogens in the bloodstream in response to infection and inflammation triggers the actions of prostaglandins in the brain, which shift the body temperature set point upward by as much as 4 °F (~2 °C). The resulting feveris considered to aid the body's immune system in combating the invading organisms. Antipyretics, drugs that are used to treat fever, such as aspirin, act by lowering prostaglandin levels, thereby bringing about a drop in the temperature set point.

As noted previously, all homeostatic set points in the body vary in a diurnal pattern. These circadian rhythms are intrinsic, with a periodicity of just over 24 hours. The pattern of these shifts in set points is altered, or entrained, by environmental influences, particularly light - dark cycles, which reset the rhythm periodicity to 24 hours. In this example, then, the level of biologic set points is affected by both intrinsic and extrinsic factors.

In the discussions in this book, it has been obvious that in the development of obesity, energy set points become readjusted to higher levels in response to a positive energy balance; that is, "obesity itself can often be viewed as a condition of body energy regulation at an elevated set point" (33, p. 1882S). A similar shift of set point may occur in control of blood pressure in individuals with hypertension, in whom physiologic regulators are adjusted to establish a steady state at higher levels.

Hibernation by animals is a classic example of biologic set points being not only modified but essentially eliminated altogether (69). During such periods, the metabolic rate of a bat has been reported to be 1% to 4% of that in the normal resting condition. At the same time, homeothermy - maintenance of body temperature - is virtually abandoned, with body temperature falling to close to 0 °C.

These examples illustrate that supposedly deterministic biologic controllers are not necessarily immutable. Such observations imply that efforts to improve the physical activity habits of the population by manipulating environmental factors might be successful even in the presence of a central nervous system controller of physical activity.

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