In life, not much is fair. With everything else being equal, a new study on obesity shows that African American women lost fewer pounds than their white counterparts, while adhering to the same dietary strictures and exercise regimens.

Leading the study at the University of Pittsburgh School of Medicine, investigator James DeLany says black women experience slower metabolisms, decreasing their daily expenditure of calories. "African-American women have a lower energy expenditure,” Delany told Health Day. “They're going to have to eat fewer calories than they would if they were Caucasian, and/or increase their physical activity more.”

In the study, Delany and his colleagues followed 39 African American women and 66 white women, all of whom were severely obese. All of the women were placed on 1,800-calorie daily diets, with identical exercise requirements.

The investigators measured weight changes and fluctuations in energy expenditure, as well as making objective assessments of physical activity and energy intake over the course of the six-month study. Within the study, investigators also made closer comparisons of 25 black women with 25 white women, matched for initial body weight.

Over the course of the study, African American women adhered to the dietary restrictions just as well as the white women. Yet, investigators neglected to account for metabolic differences between the two groups, which would require a greater exercise regimen for black women for similar results. “Therefore, to achieve similar weight loss in [African American] women, the prescribed caloric restriction cannot be based on weight alone, but must be lower than in Caucasians to account for lower energy requirements,” they wrote in a paper published Friday.

Mitch Roslin, chief of bariatric surgery at Lenox Hill Hospital in New York City, told Health Day that racial differences in health are not the work of what sociologists might call a human construct. "These things are real,” Roslin said. "However, this study advances our understanding by ruling out other explanations. It's not just related to socioeconomic class or access to care or environmental situations."

Although the racial imbalance has been known to science, Roslin says the new study helps to eliminate other possible explanations, such as socioeconomic status or environmental factors. Essentially, racial imbalances in obesity may stem from varying evolutionary pasts with regard to diet, with Americans of all races sharing a diet now based mostly on European culture.

Source: Delany, J.P., Jakicic, J.M., Lowery, J.B., Hames, K.C., Kelley, D.E., Goodpaster, B.H. African American Women Exhibit Similar Adherence To Intervention But Lose Less Weight Due To Lower Energy Requirements. International Journal of Obesity. 2013.