By Daniel Weintraub

Look at the health data for just about any collection of neighborhoods in California and one thing will soon become clear: Poor people are sicker and, on average, die younger than people with higher incomes.

The medical profession, social workers and health researchers have known this for a long time. But exactly why it is so remains, surprisingly, a mystery. Answering that question and then doing something about it would probably improve health more than all of the reforms advanced by President Barack Obama and so bitterly despised by his opponents.

The president’s reforms focus on providing health insurance and access to health care to more people. But even in countries where everyone has access to the same level of care, disparities still exist across income, racial and geographic lines. To narrow those disparities, you have to move upstream, closer to the source.

That source, it increasingly appears, is stress.

In small amounts, stress is good for us. It stimulates the mind and body, and keeps us sharp. It helps us avoid danger by triggering our “fight or flight” instincts. It helps us fight infection by unleashing our immune system.

But evidence suggests that the kind of unrelenting stress that comes with poverty can warp the way our bodies work and leave us more vulnerable to disease.

Nancy Adler, a professor and researcher at the University of California, San Francisco, School of Medicine, has studied the effects of socio-economic status on health for more than 15 years. She led a nationwide team of researchers funded by the MacArthur Foundation to explore the social and biological pathways that connect poverty and health.

Adler said researchers found only two things that would eliminate the effects of socioeconomic status on health.

One was a sense of control. People who feel they have the power to change their situation for the better tend to be more insulated from stress – and healthier than others of the same income or job status.

Think of two people in unsatisfying minimum-wage jobs, one who comes from a poor family and dropped out of high school, and another who is using the job to pay her way through college. While their incomes and job satisfaction might be identical, the college student almost certainly feels more in control of her condition than the high school dropout, and, it turns out, is more likely to be healthier.

“If you think about stress, it’s not just being exposed to challenges, it’s the sense that you don’t have the resources to deal with it,” Adler said. “People with a low sense of control are much more vulnerable to stress. They have this feeling of being overwhelmed.”

The other significant factor is a person’s own perception of where they stand on the socioeconomic ladder. People who believe they are at a higher level of status tend to be healthier, even if, measured objectively, that perception is inaccurate.

“When you feel you are just low on the social hierarchy, when you feel you are at the bottom of the heap, it adds to stress,” Adler said. “People at the same level, objectively, if they put themselves at the bottom it affects their health.”

That feeling of being trapped at the bottom seems to make people hyper-vigilant. Because they are in such a vulnerable state, struggling every day to survive, they are especially sensitive to threats to their well-being. Even the smallest thing – car trouble, a major appliance repair, an unexpected medical bill – can push them over the edge financially. Being constantly on guard for those threats prompts the release of the hormone cortisol, which tells the body to focus on the immediate danger and shut down non-essential functions.

This often leads to a lack of sleep and a rise in blood pressure, or fewer periods where blood pressure subsides, which it should after a stressful moment passes. And that can directly affect health, often leading to heart disease.

The second pathway is indirect, through the practice of behaviors that harm health, such as smoking, drinking or poor diets. But while these habits are commonly viewed as self-inflicted wounds, they, too, might be prompted by the chemical changes that stress triggers. There is some evidence, for example, that hormonal changes may alter the way the body processes food, causing cravings for sugar and fat while dampening the signals that tell a healthy person they are satiated and prompt them to stop eating.

While it may be a controversial idea, there is growing support in the scientific community for the notion that obesity is no more a matter of personal responsibility than, say, cancer. It is, rather, the result of the brain’s biochemistry gone awry because of chemical imbalances brought about by constant stress.

UC San Francisco researchers first observed that connection in laboratory rats. Then, in 2011 they published results of a study of a group of women who were interviewed about their levels of stress and then subjected to stressful tasks in the lab.

The researchers found that the women who reported the most stress in their lives were also the ones who ate more in response to their stress. The same women had more abdominal fat than women who did not experience a high level of chronic stress.

Bruce McEwen, a researcher at Rockefeller University in New York and author of “The End of Stress as We Know It,” has pioneered the exploration of the cumulative effects of stress on the body.

McEwen adapted the term “allostasis” to describe the generally healthy sequence of events that occurs whenever the body secretes hormones such as cortisol to re-balance itself under stress. He then coined the term “allostatic load” to describe the wear and tear that the body experiences after repeated cycles of stress – a much less healthy outcome.

An example: Researchers discovered that male monkeys fighting for dominance in an unstable hierarchy suffered from the formation of plaque in their arteries. The study concluded that the condition was caused by blood pressure surges brought on by the stress the monkeys experienced during these social confrontations, which elevated their levels of a hormone that accelerates hardening and narrowing of the arteries. The same might be true in humans living in poverty who feel as if they are constantly struggling for position in their own social hierarchy.

Other researchers have examined the role that stress plays in the body’s response to infectious disease through inflammation.

The symptoms of the common cold, for instance, are not caused directly by the viral infection but by the inflammation the immune system produces to rid the body of the infection. And studies have shown that people under a lot of enduring stress – such as the parents of children with cancer – lose their ability to regulate the production of the chemicals that cause inflammation, making them more likely to experience more frequent and severe colds.

A similar process – runaway inflammation caused by chronic stress – may also lead to more serious diseases. And in an especially alarming development, researchers have found that the bodies of children raised in stressful environments may undergo change that leaves them more vulnerable to disease later in life.

“They are set on physiological trajectories early in life that have a long-term impact on their health and well-being,” said Sheldon Cohen of Carnegie Mellon University. “There are facets of low social-economic status above and beyond behavioral ones, health behaviors, that have a major impact on health and well-being.”

Keeping people healthy, then, might be less about health insurance and more about social insurance – support that helps people with low incomes or education levels survive, and then climb the economic ladder. Doctors and hospitals will always be overwhelmed as long as so many people suffer from the chronic stress that comes from living on the edge.

Daniel Weintraub is the editor of the California Health Report at www.calhealthreport.org.