A new survey finds that one in three homeless people in Boston are clinically obese, a number that casts in relief the strange reality of food in the 21st century United States.

Not long ago, malnourishment was embodied by emaciation. Now it's far more likely to be hidden in folds of fat.

"This study suggests that obesity may be the new malnutrition of the homeless in the United States," wrote the researchers, led by Harvard Medical School student Katherine Koh, in an upcoming Journal of Urban Health study.

The findings are the latest and most dramatic illustration of what's called the "hunger-obesity paradox," a term coined in 2005 by neurophysiologist Lawrence Scheier to describe the simultaneous presence of hunger and obesity.

Around that time, a vernacular sea change occurred, with "hunger" and its connotations of starvation replaced by "food insecure," a term more descriptive of people who might consume enough raw calories but not enough nutrients.

The paradox fit with a general modern relationship in the United States between weight and wealth. Whereas obesity was once a sign of wealth, it now tracks with poverty. The poorer and less food-secure people are, the more likely they are to be overweight or obese.

Whether this pattern also extended to homeless people, the most food-insecure population of all, wasn't known. A few studies suggested as much, but they were small and methodologically limited. Koh drew on the medical records of 5,632 men and women seen in 2007 and 2008 by doctors from the Boston Health Care for the Homeless Program, the primary caregiver for Boston's 11,000 homeless residents.

By the metric of body mass index, just 1.6 percent were underweight. Roughly one-third fell in what's considered a normal weight range. Some 65.7 percent were overweight, of which half – 32.3 percent of the study's total homeless population – were formally obese.

The numbers generally matched obesity rates expected among the general population, a surprising finding among people ostensibly too poor to buy enough food.

"I thought we'd see a lot more people who were malnourished or underweight," said Jim O'Connell, president of the Boston Health Care for the Homeless Program and co-author of the study.

The seemingly paradoxical existence of homeless obesity likely has multiple causes, some of them physiological.

"People who are homeless are under a lot of stress, and stress causes higher cortisol levels. Higher cortisol levels lead to weight gain," said nutritionist Sherry Tanumihardjo of the University of Wisconsin, who has studied the hunger-obesity paradox but was not involved in this research.

Linkage between stress, weight gain and a more-efficient conversion of calories to fat is probably an ancient physiological adaptation designed to store energy in times of uncertainty. It's a double-whammy when combined with the natural instinct of a food-insecure person to eat as much food as possible when given the opportunity.

However, homeless people in developing countries share the same stress responses, and obesity isn't a problem for them. Something more is needed: a food system in which low-nutrient, high-calorie foods are especially inexpensive.

It may not be quite as difficult as sometimes portrayed to eat healthy on a budget, but homeless people certainly don't have the money. Potato chips and soda are relatively affordable, accessible and filling.

"You can buy calories very cheaply, but it's hard to buy nutrition cheaply," said Andrew Rundle, a Columbia University obesity researcher who was also not involved in the study.

Obesity-related diseases among America's 3 million homeless are a looming problem, said O'Connell. Diabetes is typically managed with diet and exercise, neither of which are easily controlled on their lifestyle. Insulin is also problematic because needles are often not permitted inside shelters.

"This is going to be a national issue," said O'Connell, who described the homeless situation as anticipating trends in the mainstream U.S. population. Project current health trends into the future, and one in three U.S. adults will be diabetic in 2050.

"The homeless will show it to us early," O'Connell continued. "Because they're living on the edge of survival, they frequently point out to us the weaknesses of our mainstream systems."

Citation: "The Hunger-Obesity Paradox: Obesity in the Homeless." By Katherine Koh, Jessica Hoy, James O'Connell and Paul Montgomery. Journal of Urban Health, forthcoming.