Allostasis, Homeostasis, and the Costs of Physiological Adaptation (review)

For much of the past century the study of physiology has rested on the bedrock assumption that our bodies maintain a constant internal environment through the action of a network of homeostatic negative feedback loops. As one example, the central nervous system monitors onboard energy reserves through adipose-derived hormones, such as leptin, and adjusts energy intake and expenditure in a fashion that tends to defend a stable body weight. Similarly, when a sugary meal is consumed and blood glucose rises, the pancreas produces insulin, helping tissues remove the excess glucose, thereby maintaining blood sugar within narrow functional limits. Through the action of such regulatory processes our bodies preserve a stable internal environment against the backdrop of our ever-changing physical and social surroundings.

Or do they? If our physiologies are indeed self-correcting, as all physiology textbooks would have us believe, then how do we explain the current epidemic of obesity, the rising rates of hypertension or elevated fasting glucose, or the modern epidemic of psychopathologic states such as depression? The lion's share of contemporary health problems in countries such as the United States points to the tendency of our bodies to change their target state as environments change. These phenomena are not easily explained by traditional models of homeostasis.

To help account for such trends, the concept of allostasis, or "stability through change," has been proposed as an amendment to the homeostatic model (Sterling and Eyer 1988). In Allostasis, Homeostasis, and the Costs of Physiological Adaptation, edited by psychologist Jay Schulkin, the leaders of this emerging field make a strong case for taking the allostasis model seriously. The book's eight full-length chapters, introduction, and concluding commentary cover the history and theory of the allostasis model and its application in such fields as psychology, biomedicine, and aging research. After reading these contributions, one cannot help but realize that traditional models of physiologic regulation require updating and that allostasis holds promise as a way to move the field forward, even though the concept's advocates are not yet consistent in how they define it.

Following Schulkin's brief historical introduction, Peter Sterling—a coauthor of the original allostatic model—provides a wide-ranging discussion of allostasis, including an entertaining firsthand account of its history and a detailed consideration of how allostasis differs from homeostasis. To Sterling, an allostatic system is one in which the set points that govern internal states, such as the target range of blood pressure that the body attempts to protect, are not static but shift in response to both acute and sustained changes in demand. When systems are forced to operate outside their normal range, this is metabolically costly, but it also reduces the sensitivity of the system by pushing it to the fringes of its detection limits. When this deviation is sustained, the body compensates by recentering the system on this outlier state, a process of "continually rematching outputs to expected inputs" (p. 30). In Sterling's model of allostasis, then, systems are indeed self-correcting, but not in the classic sense of a homeostatic system defending a static internal state. Rather, regulatory systems adjust their structure to maximize their efficiency and sensitivity as the ranges under which they are forced to operate shift.

The allostatic model further differs from homeostasis in its emphasis on the brain-body interface, exemplified by the capacity of many systems to make adjustments that are predictive of future demand. For instance, the body's stress-induced mobilization of glucose is not a reaction to increased need, but a change made in anticipation of the increased demand for energy that predictably follows a stressful challenge. The central monitoring and regulation of many systems makes possible such feed-forward adaptive processes, and this anticipatory brain-body theme is an integral component of the allostatic model that differentiates allostasis from homeostasis.

After Sterling's overview, three chapters explore the health consequences of chronic overstimulation of allostatic systems, described as allostatic load...