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

Sapolsky uses a sexually-explicit excerpt written by D.H. Lawrence to elicit a stress-response in the reader and introduce how stress impacts the body. The nervous system is divided into the voluntary system and the autonomic, or involuntary, nervous system. The autonomic nervous system is further divided into the sympathetic and parasympathetic nervous systems. The sympathetic nervous system activates in response to alarm or excitement, while the parasympathetic nervous system manages calm events, like digestion. Certain hormones and neurotransmitters—chemicals that transmit information—are activated during a stress-response.

Hormone secretion is directed by the brain; the hypothalamus in the brain sends corticotropin releasing hormone (CRH) to the pituitary, which releases the hormone ACTH, which prompts the adrenal glands to secrete glucocorticoids. Epinephrine and norepinephrine are also released in response to the activation of the sympathetic nervous system. They cause physical arousal and alertness; glucocorticoids have a similar effect but work slower and for a longer duration. Other hormones that heighten during stress are glucagon, prolactin, and endorphins, while reproductive and growth hormones and insulin are suppressed.

Stress-responses are complex and variable. The concept of “fight-or-flight,” which has long stood as a synopsis for sympathetic nervous system activation, has been discovered to be limited. Researchers, including Shelly Taylor, argue that the so-called fight-or-flight response is a typical male response, while females experience less aggressive responses geared toward tending children and seeking social support, colloquially termed “tend and befriend” (33). Other complications include the variability in the features and effects of stress-responses, in the function of hormones like glucocorticoids, and in a person’s sensitivity to hormones.

The cardiovascular system is boosted during the stress-response, which evolved to help the individual fight or escape the stressor. Heart rate and blood pressure increase, vessels dilate, and blood is redirected away from nonessential functions like digestion.

The decrease in abdominal blood flow was discovered in the 1800s when white scientists put a tube in the abdomen of an Indigenous American who had been shot; when the man was stressed, his organs paled. Vasopressin, an antidiuretic hormone, is released during stress-responses, and it stops kidney function and promotes the reabsorption of water back into the circulatory system. Bladders involuntarily release urine, better enabling a human to fight or run. However, when the appropriate response to a stressor is stillness, such as in a gazelle hiding from a lion, the cardiovascular system slows, demonstrating the complexity of the stress-response.

Psychological stress activates an increase in the cardiovascular system, making it work harder. Chronic stress, which results in extra wear on the system, leads to a heightened risk of developing heart disease. Hypertension, or high blood pressure, is caused by the thickening of vessel walls. It causes thickening of the heart’s left ventricle and damage to blood vessels. Damaged blood vessels allow for the accumulation of plaque, or atherosclerosis. Scientists have found that vascular damage is a better predictor of heart disease than are high cholesterol levels, as plaque only builds up in damaged areas. Atherosclerosis and hypertension can cause cardiovascular disease or a thrombus—blood clot—which can result in a heart attack or stroke. Cardiovascular disease may cause myocardial ischemia, or the constriction rather than dilation of the heart’s blood vessels, and it reduces the effectiveness of the parasympathetic nervous system.

Strong negative emotions double the rise of a sudden cardiac event. Fibrillation, or irregular heartbeats, is likely the event leading to sudden cardiac death, and the heart is more susceptible to fibrillation after an individual experiences an intense emotion. Sudden cardiac events among females are rising and may be attributed to factors like obesity, smoking, and stress. Sapolsky notes that stress lowers estrogen, and estrogen has cardiac benefits. Studies in non-human primates show subordinate females have higher rates of atherosclerosis than dominant females, but dominant females who have their ovaries removed have a higher risk of the disease. Human studies of post-menopausal hormone replacement linked estrogen and progestin supplements with an increased risk of cardiovascular disease. This disparity, Sapolsky suggests, was due to the dosage or type of estrogen, the addition of progestin, or whether atherosclerosis had already developed.

Sapolsky discusses “voodoo death,” or a psychophysiological death, which occurs when someone dies after being hexed by a shaman. Two scientists, Wade Davis and Regis DeSilva, studied such deaths and found several potential causes. First, shamans might have a knack for detecting ill individuals; they may poison the hexed individual to maintain credibility; or the community may turn against the hexed individual, denying them food and water. Others may die from sudden cardiac events; however, there are some cases that cannot be fully explained, showing there is more to learn about cardiac risks. Researchers have also found a link between personality and heart disease, which is discussed later in the text.

The digestive system, or gastrointestinal tract, breaks down food into molecules that are then used to energize the body or are stored for later use—fatty acids are stored in fat cells as triglycerides, glucose is stored as glycogen in the liver and muscle tissues, and amino acids are stored as proteins in cells.

The storage process is stimulated by insulin. Energy storage is a function of the parasympathetic nervous system, so when the sympathetic nervous system is activated, insulin declines, stress hormones are secreted, and energy storage reverses. Stored energy is converted into useful molecules, like glucose, and sent to muscle tissues. Some diseases, like Addison’s disease and Shy-Drager syndrome, inhibit energy mobilization, which results in chronic fatigue syndrome.

Sapolsky compares energy storage to wealth management and argues that chronic stress depletes energy stores similar to how frequent withdrawal from a bank account depletes a person’s financial resources. A portion of energy is lost during energy mobilization, and chronic stress often results in general fatigue. It can also cause muscle atrophy, although it takes extreme stress for significant muscle deterioration to take place. Frequent energy mobilization from chronic stress also leads to more fatty acids and glucose circulating in the bloodstream, which then raises the risk of developing cardiovascular disease.

Those with diabetes may experience increased risks from chronic metabolic stress-responses. Juvenile, or Type-1, diabetes is an autoimmune disorder in which the body destroys the pancreatic cells that secrete insulin. Energy remains circulating, increasing the risk of cardiovascular diseases, which are further exacerbated by the metabolic stress-response. The body also becomes less sensitive to insulin, meaning that a diabetic person may have trouble regulating their insulin dosage. Research has shown that stress may speed up the onset of juvenile diabetes.

Type-2, or adult-onset, diabetes occurs when the body becomes resistant to insulin. Fat cells are finite, with the number of cells determined in adolescence; when fat cells reach maximum capacity, the body stops responding to insulin. Insulin resistance causes the pancreas to generate increasing levels of insulin until it burns out and stops producing insulin, resulting in Type-1 diabetes. Chronic stress increases the chances of developing Type-2 diabetes, as the stress-response lowers insulin sensitivity. The discussion on Type-2 diabetes is particularly relevant given the sharp rise of the disease’s prevalence among Americans starting in the 1990s. The disease is also rising in unindustrialized countries with increasingly westernized diets.

Metabolic syndrome, or Syndrome X, describes issues that incorporate both the digestive and cardiovascular systems, such as those discussed in Chapters 3 and 4. The diagnosis is applied to individuals who have two or more symptoms. Once two or more symptoms arise, it is likely that more will follow. As with other medical conditions, metabolic syndrome is exacerbated by stress.