Researchers at Loyola University Health System and other centers compared African-American women in metropolitan Chicago with women in rural Nigeria. On average, the Chicago women weighed 184 pounds and the Nigerian women weighed 127 pounds.

The researchers studied 149 women from two rural Nigerian villages and 172 African American women from the west side of Chicago and suburban Maywood.

Adjusted for body size, the Chicago women burned an average of 760 calories per day in physical activity, while the Nigerian women burned 800 calories -- a difference not statistically significant, the researchers said.

"Decreased physical activity may not be the primary driver of the obesity epidemic," Loyola nutritionist Amy Luke said in a statement. "We would love to say that physical activity has a positive effect on weight control, but that does not appear to be the case."

Luke said diet may explain the difference between the women. The Nigerian diet was high in fiber and carbohydrates and low in fat and animal protein, but the Chicago diet is 40 percent to 45 percent fat and high in processed foods.

However, the Loyola researchers add that while those exercising burn more calories when they exercise, but they compensate by eating more.

The findings were published in the journal Obesity.