They may rely on cheap, fattening foods to fill up, researchers say.

(HealthDay) -- One-third of homeless people in the United States are obese, about the same rate as the general population, a new study finds.

It might seem that hunger and lack of food would put homeless people at risk for weighing too little, according to the researchers at Harvard Medical School in Boston and the University of Oxford in the United Kingdom.

But, the high rate of obesity among homeless people may be due to their reliance on cheap foods that contain high levels of fat and sugar. Another possible explanation could be physiological -- chronic food shortages cause the body to adapt by storing fat reserves.

Other factors may include little physical activity, sleep deprivation and stress.

The researchers examined the body-mass index (a measure of body fat based on height and weight) in more than 5,600 homeless men and women in Boston and found that 32 percent were obese, just less than 6 percent were morbidly obese and just less than 2 percent were underweight.

The overall obesity rate among homeless people was almost as high as among the general population (about 34 percent). Homeless women, however, were much more likely to be obese than non-homeless women -- 43 percent vs. 35 percent.

The findings, which will appear in the Journal of Urban Health, suggest that obesity may have replaced underweight as the new malnutrition of the homeless, the researchers concluded.

"To our knowledge, this is the first study to rigorously evaluate whether obesity is a problem among the homeless in the U.S., as very little research has been done in this area," study co-author Paul Montgomery, a professor of psycho-social interventions at the University of Oxford, said in a journal news release.

"This study highlights the importance of the quality, as well as the quantity, of food that the homeless are consuming," Montgomery said. "Interventions aimed at reducing obesity in the homeless, such as improving nutritional standards in shelters or educational efforts at clinical sites, should be considered in light of these findings."

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