A new study dispels the myth that all homeless people living on the streets are starving and underweight.
New research by Oxford University and Harvard Medical School has found that obesity is just as common among the homeless as it is among the general non-homeless population.
The study, to be published in a forthcoming issue of the Journal of Urban Health, suggests this could be because cheap foods that are instantly satisfying often contain high levels of fats and sugars.
Another reason could be that bodies experiencing chronic food shortages adapt by storing fat reserves, a release from the University of Oxford said.
Researchers examined the body mass index (BMI) data of 5,632 homeless men and women in Boston, and found that nearly one-third of them were obese.
They used the medical electronic records at 80 hospital and shelter sites for the homeless in Boston, using data from the Boston Health Care for the Homeless Program, one of the largest adult homeless study populations reported to date.
They found that just 1.6 per cent of the homeless in the sample could be classed as "underweight".
Morbid obesity - where people are 50 per cent-100 per cent above their ideal body weight - was three times more common with 5.6 per cent of homeless adults classed as morbidly obese.
The study authors also compared the BMI of the US homeless adults with 5,555 non-homeless adults, using data from the National Health and Nutrition Examination Survey.
They found that obesity amongst the homeless (32.3 per cent) was almost as high as among the general population (33.7 per cent).
The mean BMI among homeless adults was 28.4 kg per metre squared compared with 28.6 kg per metre squared among the general non-homeless population.
However, homeless women had a significantly higher percentage of obesity (42.8 per cent) than non-homeless women (35.3 per cent).
Factors associated with being homeless, such as a largely sedentary lifestyle, sleep debt, and stress may also contribute to the high prevalence of obesity, the study suggests.
The researchers express caution about what the precise risk factors and causes of obesity in the homeless might be.
Lead study author Katherine Koh, who carried out the research at the University of Oxford but is now at Harvard Medical School, said: "The recently described 'hunger-obesity paradox', which describes the co-existence of hunger and obesity in the same person, may help explain these findings".
Obesity among the homeless population could be due to the tendency to buy cheap, low-nutrient