Good Calories, Bad Calories

In 1926, Bernard’s concept was reinvented as homeostasis by the Harvard physiologist Walter Cannon, who coined the term to describe what he called more colloquially “the wisdom of the body.” “Somehow the unstable stuff of which we are composed,” Cannon wrote, “had learned the trick of maintaining stability.” Although “homeostasis” technically means “standing the same,” both Cannon and Bernard envisioned a concept more akin to what systems engineers call a dynamic equilibrium: biological systems change with time, and change in response to the forces acting on them, but always work to return to the same equilibrium point—the roughly 98.6°F of body temperature, for instance. The human body is perceived as a fantastically complex web of these interdependent homeostatic systems, maintaining such things as body temperature, blood pressure, mineral and electric-charge concentration (pH) in the blood, heartbeat, and respiration, all sufficiently stable so that we can sail through the moment-to-moment vicissitudes of the outside world. Anything that serves to disturb this harmonic ensemble will evoke instantaneous compensatory responses throughout that work to return us to dynamic equilibrium.

All homeostatic systems, as Bernard observed, must be amazingly interdependent to keep the body functioning properly. Maintaining a constant body temperature, for example, is critical because biochemical reactions are temperature-sensitive—they will proceed faster in hotter temperatures and slower in colder ones. But not all biochemical reactions are equally sensitive, so their rates of reaction will not change equally with changes in temperature. A biological system like ours that runs ideally at 98.6°F can spin out of control when this temperature changes and all the myriad biochemical reactions on which it depends now proceed at different rates. Our body temperature is the product of the heat released from the chemical reactions that constitute our metabolism. It is balanced in turn by the cooling of our skin in contact with the outside air. On cold days, we will metabolically compensate to generate more heat, and so more of the calories we consume go to warming our bodies than they would on hot days. Thus, the ambient temperature immediately affects, among other things, the regulation of blood-sugar and of carbohydrate and fat metabolism. Anything that increases body heat (like exercise or a hot summer day) will be balanced by a reduction of heat generated by the cells, and so there is a decrease in fuel use by the cells. It will also be balanced by dehydration, increased sweating, and the dilation of blood vessels near the surface of the skin. These, in turn, will affect blood pressure, so another set of homeostatic mechanisms must work, among other things, to maintain a stable concentration of salts, electric charge, and water volume. As the volume of water in and around the cells decreases in response to the water lost from sweating or dehydration, our bodies respond by limiting the amount of water the kidneys excrete as urine and inducing thirst, so we drink water and replenish what we’ve lost. And so it goes. Any change in any one homeostatic variable results in compensatory changes in all of them.

This whole-body homeostasis is orchestrated by a single, evolutionarily ancient region of the brain known as the hypothalamus, which sits at the base of the brain. It accomplishes this orchestral task through modulation of the nervous system—specifically, the autonomic nervous system, which controls involuntary functions—and the endocrine system, which is the system of hormones. The hormones control reproduction, regulate growth and development, maintain the internal environment—i.e., homeostasis—and regulate energy production, utilization, and storage. All four functions are interdependent, and the last one is fundamental to the success of the other three. For this reason, all hormones have some effect, directly or indirectly, on fuel utilization and what’s known technically as fuel partitioning, how fuel is used by the body in the short term and stored for the long term. Growth hormone, for example, will stimulate the mobilization of fat from fat cells to use as energy for cell repair and tissue growth.