Still, where do sedentary lifestyles and "obesogenic," Super-Size-Me environments come into play?
Environment plays a role, but it's unclear exactly how it interacts with a person's genetics. In fact, Bouchard said epigenetics, or measuring what changes gene expression, is the mysterious frontier of obesity research.
Until scientists can piece together nature's effects on nurture, it will be tough to be sure what exactly causes obesity -- and even more difficult to create medical interventions to include into health care policy.
Refusing to treat obesity like other metabolic diseases seems prejudiced, Bouchard said.
"When we put obesity in another category, I always feel it is purely a consequence of looking at it from a discriminatory point of view," he said.
Some people may think that calling obesity a disease rids people of personal responsibility to watch their weight. Other research suggests that people who cannot properly produce a compound called leptin are more likely to become obese, too, said Judith Stern, a distinguished professor of nutrition at the University of California-Davis. Other theories put forth that viruses or infections might factor into a person's risk.