This study also concludes that, though the predominance of obesity is increasing in all socio-economic and ethnic groups, minority and low-income groups are disproportionately affected.

"Obesity is more prevalent in those living below the poverty line," Saunders says.

The National Health and Nutrition Examination Survey shows black women are the most overweight group; 66 percent of black women are overweight according to the survey. Mexican-American women are second at 65.9 percent. It is only among non-Hispanic whites that more men than women are overweight.

Americans' weight gain is attributed to biological, psychological and social causes, say experts.

The correlation between ethnicity and obesity is too complex to pin down, but Saunders concludes, "socio-economic status is definitely a factor."

Biological, psychological and social causes are all factors adding to Americans’ heft, adds Sharon Dalton, director of the graduate program in Nutrition and Food Studies at New York University.