When you get right down to it, we humans have one big thing in common with the lowly worm: we are all comprised of living tissue wrapped around a tube that takes in food at one end and puts out waste at the other. That primal tube is literally the center of our existence.

Keeping the sources of energy flowing through that tube is so essential that much of our body's biochemical and nervous apparatus is organized primarily to find, consume, and digest food, and then to properly expel the waste products. The gut enslaves the mind by a variety of chemical, hormonal, and nervous whips that tell us when we're hungry and when we're not and when we'd better find a rest stop. And the gut succeeds where other slave drivers fail: it has convinced the mind to savor its slavery.

We eat because it is a pleasure to eat, whether or not we're hungry. A lot of the food we Westerners eat has little value other than to bring pleasure. We eat to bond with other people; indeed, we choose how much to eat in part to match how much our tablemates have consumed. We eat to kill boredom when we're restless, and to dull the ache of loneliness. Because food can serve all these functions, other than just filling the void in the gut, and because calorie-dense, fatty food is cheap and plentiful and the needs of life can be obtained without having to go out and expend energy in the effort to find, pick, kill, or even cook anything, non-nutritional eating and obesity are epidemic in developed societies.

Naturally, in a system as intricate as that which compels us to keep stuffing the input end of the tube, day after day, a lot can go wrong. If a person develops a genuine craving for something that's not food-like dirt and paint chips-we call this Pica-often it follows from some nutritional deficiency. But a person who swallows, say, coins or nuts and bolts in the belief that these metal objects will drive away the monster living in his intestines is motivated not by hunger but by a delusional belief.

These are strange problems for most people to contemplate. But we're all familiar with the struggle to limit what we take in, if not personally then for the dieters we know. Sometimes we just become conditioned to overeat because we have so many reasons beyond nutrition to eat. But some obesity is driven by brain chemistry. When a person gains weight after beginning a psychiatric medication, the person feels better and thus presumably has fewer psychological voids to fill with food, so one can see directly how simply resetting the neurotransmitters in charge of appetite can drive eating to dangerously insatiable levels.

With obesity so rampant, of course, the development of a safe and reliable weight loss drug remains the holy grail of behavioral psychopharmacology.

What about the other end of the tube? Is there really an appetite for voiding and evacuating, akin to the appetites we have for food, sex, and sleep? Well, sort of. On the one hand, these activities, along with other automatic activities like breathing and swallowing, are functions of the autonomic nervous system-thus beyond conscious control-but still regulated at key break points-like the anal sphincter-by voluntary muscle under the control of the cerebral cortex. So in a sense, pleasure is not essential for excretory functions, nor for that matter is consciousness, but for much of our lives we gain reward and satisfaction from the control of these functions, and shame from the lack of control.

Development of bowel and bladder control marks the beginning of sentient life, as the loss of control often signals its wane. The psychoanalysts seemed to be obsessed with bathroom functions. Some of them proposed that much human suffering, or neurosis, devolved from conflicts between parent and toddler over potty training. You still sometimes hear a particularly rigid person described as "anal-retentive" for this historical reason.

Concerns about excretory functions frequently arise in medical settings. Complaints of loose stools, bowel urgency, constipation, and so on are so common that they now comprise a diagnostic entity: irritable bowel syndrome. Similarly, symptoms of bladder discomfort and urgency, in the absence of a detectable pathological entity, comprise another set of loosely defined cystitis syndromes. Because both excretory systems respond to alterations in the quantity and quality of a person's intake of food and drink, and both are under the control of autonomic systems that may increase or decrease smooth muscle activity, getting one's bowels in an uproar, or one's panties in a bunch, or feeling "pissed off" vividly suggest how intimately emotional distress feeds into bathroom problems.

Which brings us back to the main point of this discussion-in this, and past, and future essays we have been exploring the ways the body arouses the mind to action, and thus the sources of motivation for almost any behavior you can imagine. Next time I'll try to tackle sex.

About the Author

Dean F. MacKinnon, M.D.

Dean F. MacKinnon, M.D. studies and treats affective disorders and teaches medical students at the Johns Hopkins University School of Medicine.

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