Why Zebras Don't Get Ulcers: The Acclaimed Guide to Stress, Stress-Related Diseases, and Coping

by Robert M. Sapolsky

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You’re the zebra running for your life, don’t think about lunch. That’s the reason why we lose our appetites when we’re stressed. Except for those of us who, when stressed, eat everything in sight, in a mindless mechanical way. And those who claim they’re not hungry, are too stressed to eat a thing, and just happen to nibble 3,000 calories’ worth of food a day. And those of us who really can’t eat a thing. Except for chocolate-chocolate chip hot fudge sundaes. With whipped cream and nuts.

stress makes about two-thirds of people hyperphagic (eating more) and the rest hypophagic.

It turns out that there are ways to explain why some of us become hyper- and others hypophagic during stress.

During the stressor, appetite and energy storage were suppressed, and stored energy was mobilized. Thus, what’s the logic during the post-stress period? Obvious—recover from that, reverse those processes. Block the energy mobilization, store the nutrients in your bloodstream, and get more of them. Appetite goes up.

glucocorticoids don’t just stimulate appetite—they stimulate it preferentially for foods that are starchy, sugary, or full of fat—and we reach for the Oreos and not the celery sticks.

Thus, we appear to have a problem here. CRH inhibits appetite, glucocorticoids do the opposite.* Yet they are both hormones secreted during stress. Timing turns out to be critical. When a stressful event occurs, there is a burst of CRH secretion within a few seconds. ACTH levels take about fifteen seconds to go up, while it takes many minutes for glucocorticoid levels to surge in the bloodstream (depending on the species). Thus, CRH is the fastest wave of the adrenal cascade, glucocorticoids the slowest.

This difference in time course is also seen in the speed at which these hormones work on various parts of the body. CRH makes its effects felt within seconds, while glucocorticoids take minutes to hours to exert their actions. Finally, when the stressful event is over, it takes mere seconds for CRH to be cleare...

if there are large amounts of CRH and glucocorticoids in the bloodstream, you are probably in the middle of a sustained stressor. Also a good time to have appetite suppressed. You can pull this off only if the appetite-suppressing effects of CRH are stronger than the appetite-stimulating effects of glucocorticoids. And that’s exactly how it works.

if there are substantial amounts of glucocorticoids in the circulation but little CRH, you have probably started the recovery period. That’s exactly when digestion starts up again and your body can begin to replenish those stores of energy consumed in that mad dash across the savanna. Appetite is stimulated. In chapter 4, we saw how glucocorticoids help to empty out the bank account of stored energy during a stressor. In this case, glucocorticoids would not so much serve as the mediator of the stress-response, but as the means of recovering from the stress-response.

Suppose that something truly stressful occurs, and a maximal signal to secrete CRH, ACTH, and glucocorticoids is initiated. If the stressor ends after, say, ten minutes, there will cumulatively be perhaps a twelve-minute burst of CRH exposure (ten minutes during the stressor, plus the seconds it takes to clear the CRH afterward) and a two-hour burst of exposure to glucocorticoids (the roughly eight minutes of secretion during the stressor plus the much longer time to clear the glucocorticoids). So the period where glucocorticoid levels are high and those of CRH are low is much longer than the period of CRH levels being high. A situation that winds up stimulating appetite.

In contrast, suppose the stressor lasts for days, nonstop. In other words, days of elevated CRH and glucocorticoids, followed by a few hours of high glucocorticoids and low CRH, as the system recovers. The sort of setting where the most likely outcome is suppression of appetite.

The type of stressor is key to whether the net result is hy...

So a big reason why most of us become hyperphagic during stress is our westernized human capacity to have intermittent psychological stressors throughout the day. The type of stressor is a big factor.

Another variable that helps predict hyperphagia or hypophagia during stress is how your body responds to a particular stressor. Put a bunch of subjects through the same experimental stressor (for example, a session on an exercise bicycle, a time-pressured set of math questions, or having to speak in public) and, not surprisingly, not everyone secretes the exact same amount of glucocorticoids. Furthermore, at the end of the stressor, everyone’s glucocorticoid levels don’t return to baseline at the same rate.

Differences can also arise from physiology—one person’s liver may be pokier at breaking down glucocorticoids than the next person’s.

when given an array of foods to choose from during the post-stress period, they also atypically crave sweets. This is an effect that is specific to stress.

Restrained eaters are actively restricting their food intake. What the studies consistently show is that during stress, people who are normally restrained eaters are more likely than others to become hyperphagic. This makes lots of sense. Things are a bit stressful—corporate thugs have looted your retirement savings, there’s anthrax in the mail, and you’ve realized that you hate how your hair looks. That’s exactly the time when most people decide that, as a coping device, as a means of being nice to themselves during a tough time, they need to ease up on something about which they’re normally pretty regimented.

So we differ as to whether stress stimulates or inhibits our appetite, and this has something to do with the type and pattern of stressors, how reactive our glucocorticoid system is to stress, and whether eating is normally something that we keep a tight, superegoish lid on. It turns out that we also differ as to how readily we store food away after a stressor. And where in the body we store it.

Glucocorticoids not only increase appetite but, as an additional means to recover from the stress-response, also increase the storage of that ingested food. Mobilize all that energy during that mad dash across the savanna, and you’re going to have to do a lot of energy storage during your recovery period.

glucocorticoids trigger fat cells to make an enzyme that breaks down the circulating nutrients into their storage forms, ideal for storing them for next winter.

fat cells located in your abdominal area, around your belly, are known as “visceral” fat. Fill up those fat cells with fat, without depositing much fat elsewhere in your body, and you take on an “apple” shape.

In contrast, fat cells around your rear end form “gluteal” fat. Fill those up preferentially with fat and you take on a “pear” shape, being round-bottomed.

Apples have waists that are bigger than hips, producing a “waist-hip ratio” (WHR) that is bigger than 1.0, while pears have hips that are bigger than waists, producing a WHR that is less than 1.0.

It turns out that when glucocorticoids stimulate fat deposition, they do it preferentially in the abdomen, promoting apple-shaped obesity. This even occurs in monkeys. The pattern arises because abdominal fat cells are more sensitive to glucocorticoids than are gluteal fat cells; the former have more receptors that respond to glucocorticoids by activating those fat-storing enzymes.

High glucocorticoids and high insulin? This happens during the recovery phase.

Match them for weight, and it’s the apples who are at risk for metabolic and cardiovascular disease.

So with lots of stress, you get cravings for starchy comfort food and you pack it in the abdomen.

You expect your gut to magically convert all that into a filtrate of nutrients in your bloodstream? It takes energy, huge amounts of it. Muscular work. Your stomach not only breaks down food chemically, it does so mechanically as well. It undergoes systolic contractions: the muscle walls contract violently on one side of your stomach, and hunks of food are flung against the far wall, breaking them down in a cauldron of acids and enzymes. Your small intestines do a snake dance of peristalsis (directional contraction), contracting the muscular walls at the top end in order to squeeze the food downstream in time for the next stretch of muscle to contract. After that, your bowels do the same, and you’re destined for the bathroom soon.

Circular muscles called sphincters located at the beginning and end of each organ open and close, serving as locks to make sure that things don’t move to the next level in the system until the previous stage of digestion is complete, a process no less complicated than shuttling ships through the locks of the Panama Canal.

At your mouth, stomach, and small intestines, water has to be poured into the system to keep everything in solution, to make sure that the sweet potato pie, or what’s left of it, doesn’t turn into a dry plug. By this time, the action has moved to your large intestines, which have to extract the water and return it to your bloodst...

All told, your run-of-the-mill mammals, including us, expend 10 to 20 percent of t...

if you are that zebra being pursued by a lion, you can’t waste energy on your stomach walls doing a rumba. There isn’t time to get any nutritional benefits from digestion. And if you are that lion running after a meal, you...

Digestion is quickly shut down during stress. We all know the first step in that process. If you get nervous, you stop secreting saliva and your mouth gets dry. Your stomach grinds to a halt, contractions stop, enzymes and digestive acids are no longer se...

blood flow to your stomach and gut is decreased so that the blood-borne oxygen and glucose can be delivered elsewhere, where they’re needed.

The parasympathetic nervous system, perfect for all that calm, vegetative physiology, normally mediates the actions of digestion. Along comes stress: turn off the parasympathetic, turn on the sympathetic, and forget about digestion.

End of stress; switch gears again, and the digestive...

small intestines, in turn, are responsible for absorbing nutrients out of this mess and delivering them to the bloodstream. As is apparent to most of us, not much of what we eat is actually nutritious, and a large percentage of what we consume is left over after the small intestines pick through it. In the large intestines, the leftovers are converted to feces and eventually exit stage left. Yet again, you sprint across the veld. All that stuff sitting in your large intestines, from which the nutritive potential has already been absorbed, is just dead weight. You have the choice of sprinting for your life with or without a couple of pounds of excess baggage in your bowels. Empty them.

Relatively large amounts of water are needed for digestion, to keep your food in solution as you break it down so that it will be easy to absorb into the circulation when digestion is done. As noted, a job of the large intestine is to get that water back, and that’s why your bowels have to be so long—the leftovers slowly inch their way through the large intestine, starting as a soupy gruel and ending up, ideally, as reasonably dry stool. Disaster strikes, run for your life, increase that large intestinal motility, and everything gets pushed through too fast for the water to be absorbed optimally. Diarrhea, simple

Broadly, there are two types of gastrointestinal disorders. In the first, you feel terrible, something isn’t working right, and the doctors find something wrong. These are “organic” GI disorders.

But suppose you feel terrible, something isn’t working right, and the docs can’t find a thing wrong. Congratulations, you now have a “functional” GI disorder. These are immensely sensitive to stress.

The most common functional GI disorder, which will be considered here, is irritable bowel syndrome (IBS), which involves abdominal pain (particularly just after a meal) that is relieved by defecating and symptoms such as diarrhea or constipation, passage of mucus, bloating, and abdominal distention.

As we saw, what stress does is increase the contractions in the colon, getting rid of that dead weight. And IBS—also known as “spastic colon”—involves the colon being too contractile, an excellent way of producing diarrhea.