How stress affects human health

by Sally Colby

One portion of this year’s Dairy Nutrition Workshop, held recently in Hershey, PA, addressed an important concern in the dairy industry: stress. A preconference symposium focused on human stress, the effect of the stress response on dairy cattle, social stress in dairy cattle, heat stress in dairy cattle, and managing heat stress in dairy cattle.

Dr. Michael Bailey, physician at The Ohio State University College of Medicine, discussed how stress affects human health, noting that there’s debate about what constitutes stress. “Some people refer to stress as the stimuli that are placed on the body,” he said, “other people define stress as the body’s response to those stimuli. I conceptualize stress as a process in which a stimulus, or stressor, induces some sort of response in the body.”

Everyone responds to stress differently. “Some stressors are stressful to nearly everyone,” said Bailey, using the example of being chased by a cheetah. “But there are other stimulators with a lot of variability – something that is terrifying for one person might be exhilarating for another person.”

Stressors have three characteristics: they’re threatening, unpredictable and uncontrollable. Bailey described 1960s experiments in which participants were divided into groups, given a learning task and told they would receive a shock when they made a mistake. One group was told if they responded rapidly, the intensity of the shock would be reduced. “That was deceptive,” said Bailey. “It didn’t matter if they responded quickly, everyone received the magnitude shock. Interestingly, the people who were told the shock would be less severe if they responded rapidly had a significantly lower stress response.”

Bailey said this experiment proves that the ability to control or predict a stressor has a significant affect on whether or not a stress response will result.

To explain the physiological effects of stress on the body, Bailey discussed the workings of the nervous system. “During a stressful experience, there are two portions of the nervous system that are almost always activated – the sympathetic branch of the autonomic nervous system and the hypothalamic pituitary adrenal axis,” he said. “The sympathetic nervous system gives us the fight or flight response – the energy to run away from the cheetah.”

The fight or flight response becomes activated almost instantly after a stressor. “The sympathetic nervous system responds so rapidly because it’s a hard-wired response,” said Bailey. “The stress response is characterized by rapid heart rate, dilation of pupils, skeletal muscle contraction and increased respiration. In general, the primary purpose of the stress response is to divert blood flow from the periphery to the central, vital organs – the lungs, heart and brain. It’s consistent with readying the body for the fight or flight response.”

About five to 10 minutes after the initial stressor is presented, an endocrine response occurs. Bailey explained that this response results in glucocorticoid (cortisol) release, which increases glucose levels in the body to support the fight or flight response.

Although past thinking classified all stress as negative, Bailey said that isn’t the case. “Performance on an exam tends to be low when stress levels are low,” he said. “Performance on an exam tends to be highest with a low level of stress, but as stress levels increase or become prolonged, performance starts to decrease.”

When is stress healthy, and how much stress too much? Bailey began answering that question by explaining that glucocorticoids follow a natural diurnal rhythm, with low levels at night and rising levels as morning approaches. Problems occur when the stress response becomes prolonged, which disrupts the diurnal rhythm, resulting in high cortisol levels throughout the day. “People with chronic stress or high levels of depression have a disruption in cortisol rhythm and also a disruption in the sleep cycle,” said Bailey. “They have problems falling asleep, and once they do fall asleep, they have trouble waking up.”

Bailey said certain areas of the brain are responsible for the production of new neurons. However, during of periods of chronic stress, new neuron formation is reduced. “We know glucocorticoids can also affect the immune system,” he said. “Virtually every immune cell in the body has receptors for glucocorticoids, and also receptors for norepinephrine and epinephrine. Activation of these receptors will significantly change the functioning of immune cells.”

Science has also proven that immune cells recognize a pathogenic virus or bacteria through activation of specific receptors in those cells. During a stress response, high levels of glucocorticoids can repress the activation of key transcription factors, and these cells no longer produce an inflammatory response. As a result, activation of the immune system becomes skewed and that skewing leads to a downstream effect.

Numerous studies have examined the effects of stress on the immune system and real-life consequences. Bailey said those who have high levels of chronic stress are more likely to get a common cold, and the severity tends to be more pronounced. “This is thought to be directly due to disregulation of the immune response,” he said. “If we compare individuals with high levels of chronic stress in their life versus low levels of chronic stress and induce an immune response (such as through vaccination), individuals with chronic stress tend to have a lower immune response to the vaccine.”

Chronic stress leads to increased susceptibility to a number of infectious diseases, and autoimmune diseases can become worse during stress. “Stressors for humans include caring for a spouse with a chronic illness, high levels of family dysfunction, low levels of social support, lower socio-economic status or economic instability,” said Bailey. “Research is still trying to understand how this works to determine what we can do to reduce the effect of stress.”

Bailey said one new area of interest is the role of stress in relation to the gut microbiome, and how the stress response might increase immune system activity. He explained that during a stress response, beneficial bacteria that naturally reside in the intestines begin to translocate from the intestine throughout the body. That translocation stimulates the immune system and leads to a low-grade inflammatory response, which under the right circumstances and in the right people, can lead to an immune response that can damage tissue.

“The reason I’m pointing out that stress can increase immune system activity and enhance inflammatory response is because if that inflammation becomes severe enough, inflammation in the periphery can start to affect the brain, leading to neuroinflammation,” said Bailey. “Neuroinflammation can affect cognition, the ability to learn and remember new tasks, and also affects anxiety and depression. We’re beginning to realize how some clinically severe cases of depression and anxiety are resistant to standard anti-depression and anti-anxiety medication.”