Meyers realizes that sedentariness is one of the many ills that afflict the poor to a greater degree than the rich. People earning less than $36,000 are far less likely to exercise than those earning $80,000 or more. Low-income people may live in dangerous areas, have little free time, lack access to parks, or some combination.

The bike program is one example of the various ways physicians are attacking a vexing problem that’s not in any medical handbook: Poor patients are sicker, and their poverty actually makes them sick.

How ‘Toxic Stress’ Damages the Brain

One in every six Americans lives in poverty–for an individual, that means earning less than $11,670 per year. The immediate lifestyle implications of such an income are clear: It’s not enough to buy a decent one-bedroom apartment in most cities, let alone a gym membership, fresh produce, or access to high-end medical care. A healthy diet, as one study determined last year, costs $1.50 more per day than an unhealthy one.

And it’s well known that low-income people aren’t as healthy. People of a lower socioeconomic status have a 50 percent higher risk of developing heart disease, for example. Writing in the New York Times, Annie Lowrey found that though Virginia’s Fairfax County and West Virginia’s McDowell County are separated by just 350 miles, men in the richer Fairfax County have “a life expectancy of 82 years and women, 85, about the same as in Sweden. In McDowell, the averages are 64 and 73, about the same as in Iraq.”

But a growing body of evidence suggests that the very condition of living with no money, in a tumultuous environment, and amid stark inequality can alter individuals’ gene expression. What’s more, the pressure of being poor sometimes weighs so heavily that the body pumps out more stress hormones, which ravage the immune system over time.

Poor nutrition, trying times, and environmental toxins in childhood can turn certain genes “on” or “off.” Even poor children who seemingly overcome the hardships of poverty—by making good grades and adapting socially—tend to have higher levels of stress hormones, blood pressure, and body mass index than their wealthier peers.

"Exposure to stress over time gets under the skin of children and adolescents, which makes them more vulnerable to disease later in life," says Gene Brody, founder and director of the University of Georgia Center for Family Research.

Child-rearing problems that are more prevalent among poor households, such as chronic neglect or a parent's incarceration, compound on money woes and congeal into something known as “toxic stress.” These “adverse childhood experiences” jab at the brain at critical moments in its development, changing the architecture of key brain structures and setting the stage for long-term anxiety and mood-control issues.