Obesity is a problem of modernity and specific cultures made up of people who didn't used to be obese generations ago. Are the populations around the world in which people aren't generally obese just filled with a large subset of the population constantly starving...or we agree that no such large subset is simply destined to obesity.
If we agree on this, then your argument isn't that people are destined to obesity, but rather that once they're obese, they have patterns of behavior that are extremely hard to break over a long term basis. That isn't the same as saying that weight loss is a futile endeavor, it's just saying that people who are obese are very often going to fail to stick to the patterns of behavior that resulted in weight loss.
